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2454 | craig | 1 | |
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3 | little cms Engine |
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4 | http://www.littlecms.com |
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5 | |||
6 | lprof - A profile construction set library |
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7 | |||
8 | How to use the the sample profilers |
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9 | |||
10 | by Marti Maria |
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11 | |||
12 | Ver 1.09 |
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13 | |||
14 | --------------------------------------------------- |
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15 | |||
16 | |||
17 | Welcome to this small guided tour on littlecms sample profilers. |
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18 | The goal of this document is to show how these programs can be |
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19 | used to generate ICC profiles for your monitor, scanner, digital |
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20 | camera, film projector or input device. It has also some advice |
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21 | on color management in general, as well as some guidelines on |
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22 | how to handle several littlecms utilities. |
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23 | |||
24 | |||
25 | Since the best way to see how it works seems to be by example, we |
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26 | will step by step make a whole calibration procedure for monitor |
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27 | and scanner. |
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28 | |||
29 | In all this tour, we will make extensive use of the measurement |
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30 | and proof utility: qtMeasurementTool. You will also need a IT8 |
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31 | target, available from several vendors. If you plan to do scanner |
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32 | profiling, make sure to get one of these. |
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33 | |||
34 | qtMeasurementTool is a "virtual colorimeter" that used in |
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35 | conjunction with a proper scanner profile can be used as a cheap |
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36 | replacement of expensive hardware. It can also be used to proof |
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37 | an Image across two profiles. The most simple way to check if |
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38 | all is going fine is by visual inspection. You can load a Image |
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39 | in the measurement tool, and apply a transform between your |
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40 | input profile (AKA, scanner or camera) and your monitor profile. |
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41 | Since right now we have no profiles to check, we should first |
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42 | create a profile for our monitor. |
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43 | |||
44 | This is done by qtMonitorProfiler. This program can work in two |
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45 | different modes. The "advanced" mode, that gives accurate |
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46 | profiles by using hardware devices like X-Rite DP92 or EyeOne, |
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47 | and the "simple" mode, that allow to build reasonably accurate |
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48 | profiles without any additional hardware. |
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49 | |||
50 | |||
51 | Building a monitor profile: a coarse approximation |
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52 | ================================================== |
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53 | |||
54 | So, let's build our first monitor profile to check how all this |
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55 | stuff works. We don't need right now a big accuracy. We want to |
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56 | calibrate coarsely, just to visually check if is all going ok. |
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57 | |||
58 | After installing the package, type |
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59 | |||
60 | ./qtmonitorprofiler |
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61 | |||
62 | This will start the monitor profiler application. As said, we |
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63 | need only a coarse profile, so, we select the radio button |
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64 | labeled: |
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65 | |||
66 | "I want to build a coarse profile, giving approximate values" |
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67 | |||
68 | This is selected by default. We need not any measurement right |
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69 | now, only a few visual adjusts. Press the button labeled: |
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70 | |||
71 | "Enter monitor values" |
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72 | |||
73 | You will see the monitor values page. In order to obtain maximum |
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74 | performance, we need to adjust first the physical monitor |
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75 | controls. |
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76 | |||
77 | Set contrast to near maximum, (yes, near maximum. This is the |
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78 | gain of monitor and we need as many as monitor can deliver) and |
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79 | then adjust brightness until you can see all grays of a |
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80 | grayscale. Leave the monitor alone, and return to monitor |
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81 | profiler program. |
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82 | |||
83 | Now we will adjust gamma correction. This is by far the most |
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84 | important setting. On our first approach, we will try with only |
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85 | one gray channel (make sure "Link channels" is checked). |
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86 | |||
87 | Move the slide until the central frame seems to "fade" on background. |
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88 | Don't spend too much time adjust it, gamma varies with many factors. |
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89 | A coarse adjust will be enough right now. |
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90 | |||
91 | You should obtain a typical value of 2.1-2.4, in some cases it |
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92 | could be even lower. |
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93 | |||
94 | Now we go on the white point stuff. Some monitors refers white |
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95 | point as "temperature". Most users does have white point set to |
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96 | D93 in order to get more bright, unfortunately this restricts |
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97 | the gamut and adds a big quantity of blue, so my advice would be |
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98 | to select something lower. |
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99 | |||
100 | If you can switch temperature of your monitor, D65 (near 6500K) |
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101 | could be a good choice for multiuse. For proofing devices, D50 |
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102 | is almost a must, but it adds too yellow for a unadapted used. |
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103 | D50 requires a dim or dark room to give the best results. If you |
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104 | don't know which temperature your monitor has, select D65 or |
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105 | D93. On our fist approach both these will give reasonable |
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106 | results. |
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107 | |||
108 | |||
109 | |||
110 | If your monitor has any other white point, you can select "User |
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111 | defined", the last option. You can then select your particular |
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112 | white point as a temperature in Kelvin degree. |
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113 | |||
114 | Don't use CIE illuminants. They are intended for workspace |
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115 | construction, and will be discussed latter. |
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116 | |||
117 | Next adjust is primaries set. For the primaries, select the |
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118 | default ITU/R BT.709 if you are profiling a CRT monitor. There |
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119 | are primaries for Samsung TFT and LCD, but these devices are not |
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120 | fully supported by the profiler. The reason to not support them |
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121 | is the angular dependence, which makes them unusable for color |
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122 | management tasks. |
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123 | |||
124 | Other primaries are for building workspaces. If you don't know |
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125 | the primaries, don't worry. Use the default ITU/R BT.709. There |
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126 | are 6 or 7 CRT tube manufacturers in the world, and all them |
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127 | does use this set. |
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128 | |||
129 | Now you can fill the info about your monitor. Although this is |
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130 | not strictly required, it could prevent a lot of pain when you |
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131 | have more than a couple of profiles. For filling the info field, |
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132 | select the "profile identification" tab, and be creative on your |
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133 | comments! |
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134 | |||
135 | Now we are ready to generate our profile. We must specify the |
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136 | output filename, so, select the "Main" tab, and press the "..." |
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137 | button on right side of the "output profile file" edit. |
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138 | |||
139 | Select "profiles" directory. This is where other tools will look |
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140 | for monitor profile. There are a couple of profiles there, leave |
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141 | them alone. For the name, we could use "CoarseMonitor.icm" So, |
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142 | type in the name and then proceed to generate the profile by |
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143 | pressing the "GO!" button. If all is ok, the status console will |
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144 | show "Profile DONE!" |
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145 | |||
146 | Let's now check how well is working our profile by eye. For |
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147 | that, we need another tool. So, close the monitor profile and |
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148 | start the measurement tool by typing |
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149 | |||
150 | ./qtMeasurementTool |
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151 | |||
152 | This utility has many usage. One would be to "proof" a color |
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153 | transform. Let's first configure the measurement tool. Select |
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154 | the "Options" tab. Adjust the default settings as following: |
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155 | |||
156 | On "Pick template" --> "Empty Picker" |
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157 | |||
158 | On "Input profile" --> "Tutorial Workspace" |
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159 | |||
160 | On "Monitor profile" should appear your profile. If you filled |
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161 | the info fields, here will be your description. If you didn't |
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162 | enter description it will be"(unknown) - Little cms construction |
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163 | set", which is the default string. Now you know why to fill |
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164 | information fields! |
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165 | |||
166 | |||
167 | |||
168 | We are going to check your profile with a highly tweaked sample |
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169 | Image This is a counterexample on what a good workspace should |
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170 | be. NEVER use this profile for practical purposes! |
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171 | |||
172 | Ok, let's see how a Image looks in this tweaked colorspace. |
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173 | Select the "Image" tab, and press "Load Image" button. |
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174 | |||
175 | Select "pics" directory. There is a "party_4s.png" Load it. You |
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176 | will see the raw, non color managed picture. Since the encoding |
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177 | space of this Image is quite different of any monitor, the |
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178 | images looks "flat" and "washed out" Now press the "Proof" |
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179 | button. The Image is shown using color management. Color |
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180 | changes should be noticeable, despite gamma of image is |
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181 | near gamma of monitor. |
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182 | |||
183 | |||
184 | Building scanner profile |
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185 | ======================== |
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186 | |||
187 | We have now our monitor calibrated, and will try to generate a |
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188 | scanner profile. For this, we will also need the measurement |
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189 | tool. |
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190 | |||
191 | Scanner profiles are built by using a thing known as "IT8 |
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192 | target". IT8 targets are just a photo with a set of color |
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193 | patches. There are known colorimetric measurements for each |
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194 | of these patches. There is also a standard way to specify |
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195 | these measurements, the IT8/CGATS format. |
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196 | |||
197 | |||
198 | IT8/CGATS is a way to encode data that is both, human and |
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199 | machine readable. You can open any of these IT8 reference sheets |
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200 | with your text editor to see it. You can even modify the |
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201 | contents. Since it is a standard, most color management systems |
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202 | does accept it. For these reasons, seems a good way to store |
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203 | measurement data. |
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204 | |||
205 | |||
206 | A IT8 file does contain measurements for a set of color patches. |
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207 | The measurement of each patch can be the device colorant values |
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208 | (the RGB or CMYK in 0..255 range) the tristimulous values (XYZ |
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209 | or Lab) and some statistic data. |
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210 | |||
211 | |||
212 | A IT8 sheet looks like: |
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213 | |||
214 | ----------------------------------------------------------- |
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215 | IT8.7/2 |
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216 | ORIGINATOR "Wolf Faust" |
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217 | DESCRIPTOR "L* a* b* Batch average data (light D50, viewing angle 2)" |
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218 | MANUFACTURER "Wolf Faust - http://www.coloraid.de" |
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219 | CREATED "January 07, 2002" |
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220 | PROD_DATE "2001:12" |
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221 | SERIAL "DIN A4 R012020" |
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222 | MATERIAL "Kodak Professional Digital 3 - Non-Glossy" |
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223 | NUMBER_OF_FIELDS 9 |
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224 | BEGIN_DATA_FORMAT |
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225 | SAMPLE_ID XYZ_X XYZ_Y XYZ_Z LAB_L LAB_A LAB_B LAB_C LAB_H |
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226 | END_DATA_FORMAT |
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227 | NUMBER_OF_SETS 288 |
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228 | BEGIN_DATA |
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229 | A1 3.25 2.71 1.94 18.86 11.18 2.87 11.55 14.41 |
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230 | A2 4.07 2.73 1.60 18.91 23.59 6.54 24.49 15.48 |
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231 | A3 5.79 3.42 1.75 21.67 33.38 9.60 34.73 16.05 |
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232 | A4 4.28 2.66 1.46 18.62 27.73 7.59 28.75 15.32 |
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233 | A5 11.97 10.30 7.42 38.38 15.04 4.16 15.61 15.47 |
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234 | .... etc ... |
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235 | ------------------------------------------------------------ |
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236 | |||
237 | |||
238 | The target manufacturer does provide a IT8 file containing these |
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239 | measurements for the target. So, the target is giving to us a set |
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240 | of KNOWN colors. We must then, tell to the profiler which RGB |
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241 | values our scanner returns for each of these patches. |
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242 | |||
243 | Measurement tool can generate sheets with the RGB values of each |
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244 | patch grabbing them from a Image. In order to generate a |
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245 | profile, we need the RGB and Lab values for each patch. |
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246 | |||
247 | let's outline the scanner profiling procedure: |
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248 | |||
249 | * First (and very important!) adjust controls of scanner to |
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250 | obtain a proper exchange space. |
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251 | |||
252 | * Scan by means of you scanning software a Image of your |
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253 | target. |
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254 | |||
255 | * By using measurement tool, grab the RGB values of each |
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256 | patch. Store them in a IT8 file |
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257 | |||
258 | * The scanner profile then will compare the RGB |
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259 | obtained by the scanner, with the known colorimetric |
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260 | measurement of target, as stored in vendor's reference |
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261 | sheet. |
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262 | |||
263 | * The scanner profiler will create a model for the |
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264 | translation RGB -> Lab or RGB -> XYZ and will store it in |
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265 | a profile. |
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266 | |||
267 | |||
268 | The math behind this model is not trivial. There is a big |
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269 | quantity of factors taken into account. Don't worry for the |
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270 | apparent missing of data. Yes, there are colors not present in |
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271 | target, however the scanner profiler is capable to "guess" or |
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272 | extrapolate them, and should produce a profile handling all |
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273 | gradients between these colors in a smooth and pleasant way. |
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274 | |||
275 | Lets' discuss each step in detail: |
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276 | |||
277 | * Step 1: Adjust controls of scanner. |
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278 | |||
279 | You will need 24 bits per pixel at least. Gray profiles |
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280 | are NOT supported in this revision. |
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281 | |||
282 | There is a big amount of controversy about how to set the |
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283 | scanner controls. Some people does advice to use linear gamma |
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284 | (gamma=1.0), which is a wrong advice unless you are working with |
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285 | 16 bits per sample (48 bits per pixel) |
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286 | |||
287 | If you are going to scan in 8 bits per sample (24 or 32 bits) |
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288 | your best choice is a gamma between 2.2 and 3.0 |
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289 | |||
290 | Gamma 2.2 has the additional benefit of being close to sRGB, and |
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291 | this means the uncorrected Image will "look nice" on average |
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292 | monitors. It is also near to perceptual gamma. |
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293 | |||
294 | Gamma 2.4 has the additional benefit of being closest to |
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295 | perceptual space, and this is a very good reason to use this |
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296 | value. |
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297 | |||
298 | Less that 2.2 (and of course the infamous 1.0) can generate huge |
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299 | loss of detail in shadows, only to give a slight bettering of |
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300 | highlights. Don't use this unless your are using 16 bits per |
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301 | sample, and even in such case, don't do it unless you know what |
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302 | are you doing! |
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303 | |||
304 | More that 2.4 (up to 3.0) drops too much highlight to be safe, |
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305 | despite you will still see the Image quite correct. |
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306 | |||
307 | More on controls: Turn off all color management, color enhancing |
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308 | and tweaking on scanner driver. Set brightness contrast and hue |
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309 | (if available) to default values. It is supposed this will be your |
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310 | working scanning mode, so set all to reasonable defaults. |
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311 | |||
312 | IMPORTANT: The profile will only be valid with those settings! |
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313 | |||
314 | |||
315 | * Step 2: Scan the IT8 target. |
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316 | |||
317 | Store it in PNG, TIFF, BMP on depending of supported formats of |
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318 | your platform. DON'T use JPEG! Its lousy nature makes it |
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319 | unsuitable for this purpose. You could use JPEG on final images, |
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320 | but not in profiling process. |
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321 | |||
322 | * Step 3: Extract the RGB values for each patch. |
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323 | |||
324 | Without help, this could be a huge task. There are plenty of |
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325 | patches and taking each sample by hand would be tedious. With |
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326 | the help of measurement tool, this is a easy task. |
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327 | |||
328 | Start the measurement tool by typing |
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329 | |||
330 | ./qtMeasurementTool |
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331 | |||
332 | Press "Load Image" button. Select the Image of target you scanned |
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333 | in step 2. I will use the scandmo.png image sited on "pics" |
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334 | directory as sample. This is a Kodak film target, scanned with a |
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335 | scanner that adds a huge violet cast. Don't worry, the profile |
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336 | will turn this one to a nice looking Image. |
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337 | |||
338 | Since this is a Kodak target that has some additional patches |
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339 | (those on I,J,K,L - 20,21,22) we can choose a picker template |
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340 | according the nature of target. Press "Options" tab, and in the |
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341 | combo labeled "Pick template" select: |
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342 | |||
343 | "IT8.7 19+3 (Kodak) column picker" |
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344 | |||
345 | Press "Image" tab. |
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346 | |||
347 | You will see a lot of green rectangles. Using the mouse you |
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348 | must place the pick template over the patches. Use left button to |
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349 | move whole template and right button to resize it. Since this |
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350 | Image is not perfectly horizontal, you may want to "unshear" the |
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351 | layout: You can do it by pressing "Ctrl" key while dragging with |
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352 | left mouse button. |
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353 | |||
354 | The "hot area" is marked with green rectangle. Assure all green |
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355 | rectangles are inside patches. Failure to do so could result in |
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356 | wrong profile! You can also adjust the relative size of "hot |
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357 | areas" by setting the % in "safe frame" on "Options" tab. |
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358 | Normally there is no need to touch anything. |
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359 | |||
360 | Once you were happy with the location of patch grabber zones, |
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361 | press the "Pick" button, placed on left side of window. |
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362 | |||
363 | The measurement tool will build a IT8 sheet containing RGB |
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364 | values of each of your patches. Now you can save this sheet by |
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365 | pressing the "Save IT8 sheet" button. For our tutorial I will |
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366 | name this measurement "scandmo.it8" and will place it on |
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367 | "pics" directory. (There is already one scandmo.it8, that you |
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368 | can overwrite) |
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369 | |||
370 | Our work with measurement tool is now done. We have right now |
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371 | the reference sheet of target, which is given by Kodak, and the |
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372 | RGB values obtained by our scanner. Now we can compare these |
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373 | files to create a profile. |
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374 | |||
375 | * Step 4: Close the measurement tool, and launch the |
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376 | scanner profiler by typing: |
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377 | |||
378 | ./qtScannerProfiler |
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379 | |||
380 | From here is quite easy. First we must select the target |
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381 | reference. In this tutorial, we have used a film target of |
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382 | reference 1996-08, so, press the "Select target & vendor type" |
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383 | button and select "targets", "Kodak" folder, then "Emissive" |
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384 | folder (since it is film). Press Ok. The left combo will be |
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385 | filled with all target references. Then select 1996-08 (is on |
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386 | last part of list) |
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387 | |||
388 | Once selected, you need to specify your measurement. Press the |
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389 | "..." button placed on right side of "Measurement" edit. Select |
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390 | "picts" folder and "scandmo.it8" file. |
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391 | |||
392 | Now we have to specify the profile filename. Press the "..." button |
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393 | placed on right of "output profile file", select the "profiles" |
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394 | folder. The name could be "scandmo.icm" |
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395 | |||
396 | You can now, as we did in the monitor profile, fill some info to |
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397 | help us to locate the profile latter. Press the "profile |
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398 | identification" tab, and fill in the fields. |
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399 | |||
400 | You are ready to generate the profile. Press "GO!" button to |
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401 | launch the profiler process. After a few seconds, you will have |
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402 | a small profile for your scanner. In the demo, the obtained dE is |
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403 | about 1.24, which is reasonably. Now, we can better a lot the |
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404 | error level, but this will take more time to generate the profile. |
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405 | Press the "Parameters and tweaking" tab, and select on |
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406 | "Resolution" group "33 points". Then check the checkbox labeled |
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407 | "local convergence analysis". Press the "GO!" button. Now the |
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408 | generation will take a while, but the obtained profile has |
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409 | bettered a lot... maximum dE is about 0.7 and the peak value is |
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410 | as small as 3!!! |
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411 | |||
412 | * Step 5: Let's check how this profile does affect the |
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413 | original Image. |
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414 | |||
415 | Start the measurement tool, select "options" tab and make sure |
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416 | picker template is set to "empty picker", "input profile" should |
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417 | be set to this newly created scanner profile, and "monitor |
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418 | profile" should be the profile we have made for monitor. Press |
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419 | "Image" tab, press "Load Image" button, load the scanned Image |
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420 | (scandmo.png in our sample) and then press the "proof" button to |
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421 | see the results. The girl's Image is dark, this is normal. |
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422 | However, rest of patches are hugely modified, the grayscale |
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423 | should appear smooth and without any cast, as well as all violet |
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424 | tint should disappear. |
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425 | |||
426 | |||
427 | |||
428 | Making workspace profiles |
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429 | ========================= |
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430 | |||
431 | Some applications, for example Adobe Photoshop, does use what is |
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432 | called "Workspaces". These are RGB color spaces, which are not |
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433 | intended to be representing any device, nor storage but a |
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434 | "working space". That is, Image processing operations are |
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435 | performed in such space, and also can be used as a way to |
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436 | interchange image data between applications. |
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437 | |||
438 | |||
439 | The monitor profiler, despite its name, can be effectively used |
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440 | to build such profiles. |
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441 | |||
442 | Just select the gamma, primaries set and white point. Press GO! |
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443 | and the workspace profile will be generated. |
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444 | |||
445 | |||
446 | Building accurate monitor profiles by hardware measurement devices |
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447 | =================================================================== |
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448 | |||
449 | To measure your monitor, you need to fill a IT8 sheet, |
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450 | containing XYZ measurements for several RGB combinations. A |
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451 | sample sheet of this kind can be found in "pics" directory. The |
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452 | file is called "sRGB.IT8" and it measures an ideal sRGB monitor. |
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453 | |||
454 | You need to change the XYZ values by those read by your |
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455 | measurement device. Don't put any Lab value, just RGB and XYZ. |
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456 | |||
457 | Please keep the RGB values the sRGB.IT8 has. Optionally, you can |
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458 | use more patches to assure consistent color reproduction, but make |
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459 | sure to include at least that ones. |
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460 | |||
461 | Note the big amount of gray patches needed. You can also add as |
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462 | many as you wish, on more gray patches, better linearization |
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463 | curves control. You must specify at least 16 gray patches. |
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464 | |||
465 | Now we will try this procedure, no matter you have not any |
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466 | hardware device. As we will see below, this method could be used |
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467 | to dissect profiles and do some tweaking. |
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468 | |||
469 | We are going to generate a sRGB profile, by using the sRGB.IT8 |
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470 | measurement sheet. For that, we will use monitor profiler, so |
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471 | start it by typing: |
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472 | |||
473 | ./qtMonitorProfiler |
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474 | |||
475 | Now, we are going to use it in advanced mode. Select the radio |
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476 | button labeled: |
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477 | |||
478 | "I want to build accurate profile from measurement sheet" |
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479 | |||
480 | |||
481 | Then press the "..." button located at right of edit. Select in |
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482 | "pics" directory the file called "sRGB.IT8". In the "Output |
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483 | profile file", select a new profile in "profiles" directory. |
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484 | Call it "my_sRGB.icm". |
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485 | |||
486 | I will use this sample to present another utility, the profile |
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487 | checker. Since the profile checker can understand advanced tags, |
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488 | we want to include all information in the profile, so select the |
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489 | "parameters and tweaking" tab, and make sure to select "Verbose, |
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490 | store anything" in the "profile verbosity level" group. |
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491 | |||
492 | Now, fill the info fields if you want (again, I recommend to do |
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493 | that) and press the GO button. A new "my_srgb.icm" profile will |
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494 | be created. |
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495 | |||
496 | We will check now how accurate is this profile. Start the |
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497 | profile checker program by typing |
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498 | |||
499 | ./qtProfileChecker |
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500 | |||
501 | The profile checker is a tool for inspecting some of profile |
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502 | internals. In our case, we want to check the accurancy. For |
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503 | selecting the profile, press the ".." button on the right side |
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504 | of the "profile" edit. Select "pics" directory, and choose |
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505 | "my_srgb.icm" profile. Press "GO!" button. |
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506 | |||
507 | On "dE report tab" you will see the error statistics. (unless |
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508 | you forgot to set the profile verbosity to "store anything") |
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509 | Our sample scored 0.18, which is a incredibly low value, not |
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510 | to be even reached in real world cases, but this could give |
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511 | an idea of the precision the profiler can give with only a |
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512 | very few measurements. |
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513 | |||
514 | |||
515 | The profile checker has also the capability of draw chromaticity |
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516 | diagram, to show primaries, color displacements, to guess the |
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517 | gamma of the profile and many other functionality. Feel free to |
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518 | experiment, the profile checker will never write on your |
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519 | profile, so you can play with it in a safe way. |
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520 | |||
521 | Additional utilities |
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522 | ==================== |
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523 | |||
524 | Finally, there is a additional command-line utility that can be |
||
525 | used in conjunction of monitor profiler. It is called icc2it8 |
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526 | and is placed on ICCtoIT8 directory. This utility is capable of |
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527 | generate IT8 measurements from profiles, so, it can be used as a |
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528 | "profile tweacker". You can generate a IT8 sheet from a given |
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529 | profile, then edit manually XYZ values and then build a new |
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530 | profile using the modified sheet. This right now is limited to |
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531 | matrix-shaper profiles, but even with this small limitation, |
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532 | it can become a valuable tool. |
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533 | |||
534 | Other useful tools are included in littlecms CMM package. |
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535 | You have TIFFICC that can be used to apply profile transforms |
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536 | to TIFF files, and JPEGICC that does same on JPEG. Also, |
||
537 | you have ICCTRANS, that can be used to check profiles |
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538 | by numbers. See the littlecms documentation for more details. |
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539 | |||
540 | |||
541 | That's all for now. There are still many options that this small |
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542 | introduction don't cover, like the CIECAM97s appearance model, |
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543 | and a more advanced usage. However, you have now a basic |
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544 | knowledge of how this stuff works. There is a mailing list on |
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545 | littlecms for asking and sharing your tricks on how get the best |
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546 | from littlecms profilers. Feel free to ask. |
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547 | |||
548 | http://lists.sourceforge.net/lists/listinfo/lcms-user |
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549 | |||
550 | |||
551 | Enjoy! |