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--- doc:appunti:software:color_management [2019/12/27 09:10] – [The ColorChecker Chart] niccolo
+++ doc:appunti:software:color_management [2020/01/02 16:31] – niccolo
@@ Line 39: / Line 39: @@
 To **convert** the stored **numerical values** back to **light intensity**, an **exponent** of **2.2** must be applied. Notice that a **linear color space** can be seen as a non-linear space where the exponent **gamma is 1.0**.
-So the **sRGG** (non linear) color space is widely used; that model is often called **perceptual gamma**. But if you work with high quality images (**16 bits per channel**) and if you want to **apply filters** that work on light and shadow, you should use a linear (RBG) workspace. The **GIMP 2.10** program internally uses linear channels data when higher than 8-bit per channel precision is used.
+So the **sRGB** (non linear) color space is widely used; that model is often called **perceptual gamma**. But if you work with high quality images (**16 bits per channel**) and if you want to **apply filters** that work on light and shadow, you should use a linear (RBG) workspace. The **GIMP 2.10** program internally uses linear channels data when higher than 8-bit per channel precision is used.
   * **[[https://www.kinematicsoup.com/news/2016/6/15/gamma-and-linear-space-what-they-are-how-they-differ|Gamma and Linear Space - What They Are and How They Differ]]**
@@ Line 82: / Line 82: @@
 ===== The GIMP =====
-The **GIMP** version **2.10** is required for **color management** and to handle images with **16 bit** per color channel.
+The **GIMP** version **2.10** is required for **color management** and to handle **TIFF images** with **16 bit** per color channel.
 {{.:color_management:color-picker-cielab.png?120 |GIMP Color Picker with CIE LAB values}}
 To view **CIE LAB** values in **Color Picker**: select the Color Picker tool, then keep the **shift key** pressed while clicking over the image.
-===== The ColorChecker Chart =====
+===== Other Interesting Pages =====
-The **[[wp>ColorChecker]]** is a registered trademark of a [[wp>Color chart|color chart]] introduced in 1976 and manufactured by **Macbeth**, now **X-Rite**; it features **24 squared patches** of various colors. The colors were choosen quite arbitrarly: six **shades of gray**, some **primary colors**  (red, green, blue, cyan, magenta, and yellow) and other colors of natural objects, like skin tones, foliage, etc. There are **several manufacturers** that sell similar and **more or less compatible color charts**, the price and quality vary widely. Durability of the materials, uniformity and consistency of the colors and printing method are the distinctive features.
+  * **[[colorchecker_clones]]**
+  * **[[imagemagick_color_management]]**
-The most inexpensive color charts are obtained by **[[wp>Offset printing|offset printing]]**, the color patches are not made from solid colors, but obtained using **dithering** and **halftoning** of the primary printing colors. The following close-up are taken from the **The 2 in 1** checker, manufactured by **[[https://www.greywhitebalancecolourcard.co.uk/|Grey White Balance Colour Cards]]**; you can see how the patches are obtained with dithering, in particular the second photo shows the **grey patches** on the left: they are made by a **white background with black dots** over it:
-{{.:color_management:colorchart-dithering-1.jpg?380|Color chart in offset printing}}
-{{.:color_management:colorchart-dithering-2.jpg?380|Color chart in offset printing}}
-Having a **reference picture** of the ColorChecker can be very useful. A ColorChecker reference image can be used as a comparison with images acquired using a scanner or with photos taken in some light condition: just include the actual ColorChecker into the scan or into the photo and calculate the transformation functions to obtain the same color tones of the refrence picture with a suitable software (e.g. //Darktable//).
-Surprisingly enough, it is not easy to find or to create such a picture.
-X-Rite publishes the document [[https://xritephoto.com/documents/literature/en/ColorData-1p_EN.pdf|ColorData-1p_EN.pdf]] (here a {{.:color_management:colordata-1p_en.pdf|local copy}}), where the reference colors are defined with enough precision in the **%%CIE L*a*b*%%** color coordinates; unfortunately the **RGB** equivalence is expressed with only 8 bits per channel. On the internet there is also available a nice paper witht the title **[[http://www.babelcolor.com/index_htm_files/RGB%20Coordinates%20of%20the%20Macbeth%20ColorChecker.pdf|RGB coordinates of the Macbeth ColorChecker]]** (here a {{.:color_management:rgb_coordinates_of_the_macbeth_colorchecker.pdf|local copy}}); it reports the **official RGB values expressed with 16 bit per channel**, it reports also the **measures of the actual color tones** (avaraged on several color checkers) producing the same RGB 16 x 3 bit values.
-Beware that the above data refers to ColorChecker **manufactured before November 2014**, after that data some changes were introduced, perhaps due the necessity to comply with toxicity law in color pigments. For X-Rite ColorChecker manufactured after november 2014 there is the **{{.:color_management:colorchecker24_after_nov2014.txt|official color coordinates provided by X-Rite}}** released in October **2015**, but they are only in **%%L*a*b*%%** coordinates.
-Another problem in creating ColorChecker images is the **poor support** for L*a*b* color space and 16 bit/color in image manipulation applications. Using the **GIMP** you need at least **version 2.10** to open 16 bit TIFF images; you will have a color picker that shows you color coordinates in **sRGB 16 bit integers** and **%%L*a*b* floating points%%**, but you are still limited to enter color coordinates using **0-255** or **0-100** ranges.
-So here there are **three reference images** (ColorChecker simulation), created from three different datasets. They were downloaded from the page [[http://www.babelcolor.com/colorchecker-2.htm#xl_CCP2_images|The ColorChecker Pages, ColorChecker images]], which contains all the details, plus other images in different color spaces:
-  * **{{.:color_management:colorchecker_srgb_from_lab_16bit.zip|ColorChecker sRGB image Macbeth Lab}}**\\ Created from GretagMacbeth L*a*b* D50 data, for charts manufactured before Nov. 2014.
-  * **{{.:color_management:colorchecker_srgb_from_avg_16bit.zip|ColorChecker sRGB image BabelColorAvg}}**\\ Created from BabelColor data, average of 30 charts manufactured before Nov. 2014.
-  * **{{.:color_management:colorchecker_srgb_from_lab_16bit_afternov2014.zip|ColorChecker sRGB image X-Rite AfterNov2014}}**\\ Created from X-Rite L*a*b* D50 data, for chars manufactured after Nov. 2014.
-===== Web References =====
-  * **[[http://www.babelcolor.com/colorchecker.htm|The ColorChecker Pages]]** by BabelColor.
-  * **{{.:color_management:rgb_coordinates_of_the_macbeth_colorchecker.pdf|RGB coordinates of the Macbeth ColorChecker}}**, June 2006 update (downloaded from [[http://www.babelcolor.com/index_htm_files/RGB%20Coordinates%20of%20the%20Macbeth%20ColorChecker.pdf|here]]).
-  * **{{.:color_management:colorchecker_rgb_and_spectra.zip|ColorChecker RGB and spectra}}**, April 2012 update (downloaded from [[http://www.babelcolor.com/index_htm_files/ColorChecker_RGB_and_spectra.zip|here]]).
-  * **{{.:color_management:colorchecker24_after_nov2014.txt|ColorChecker24 After Nov2014}}**, October 2015 (downloaded from [[http://www.babelcolor.com/index_htm_files/ColorChecker24_After_Nov2014.txt|here]]).