Sorry for the delay in replying; I wanted to take the time to properly look at everything you've said. Yes, Elle, you're right. I take what I said back; we should be using the adapted values. If anyone is still doubting this, you can try playing around with this demo program I created: http://ix.io/8ho It creates lcms profiles for the babl RGB and Y formats, and then uses lcms to convert from RGB to Y. Here's the output: (R, G, B) -> Y (1.000000, 0.000000, 0.000000) -> 0.222492 (0.000000, 1.000000, 0.000000) -> 0.716902 (0.000000, 0.000000, 1.000000) -> 0.060606 (0.500000, 0.500000, 0.500000) -> 0.500000 On Mon, Sep 23, 2013 at 5:46 PM, Elle Stone <l.elle.stone@xxxxxxxxx> wrote: > A technically correct luminance-based conversion to black and white is > done using this formula: > > L = R*Yr + G*Yg + B*Yb > > where R, G, and B are the image RGB values for any given pixel and Yr, > Yg, and Yb are the Y values from the image's actual ICC profile. If > the profile white point isn't D50, the Y values are always adapted Y > values. The image must be in a linear gamma version of the color > space. > > So far I'm repeating what I've already said. Below are concrete > examples showing that using the **un**adapted sRGB Y values results in > the wrong Luminance values: > > Consider an image with a color block of the sRGB "reddest possible > red" 16-bit integer color: (65535, 0, 0). > > Convert the sRGB image to a different ICC profile using relative > colorimetric intent and the RGB values will change but the **meaning** > of the RGB values will be the same. In other words, the new RGB values > will point to the same XYZ coordinates as the previous sRGB RGB values > pointed to. Here are some examples: > > According to the argyllcms xicclu utility, sRGB values (65535, 0, 0) > are the same as XYZ (0.435837, 0.222366, 0.013916). > > Convert the sRGB image to WideGamut: > Widegamut RGB values (38832, 6228, 766) are the same as XYZ (0.435838 > 0.222362 0.013910) > > Convert the sRGB image to ProPhoto: > ProPhoto RGB values (34671, 6442, 1105) are the same as XYZ (0.435843 > 0.222371 0.013910) > > Convert the sRGB image to the Identity profile: > Identity RGB values (29623 14573 1106) are the same as XYZ (0.435837 > 0.222370 0.013921) > > Notice that for each profile's equivalent-to-sRGB-reddedst-red, the > XYZ "Y" value is almost identical to the sRGB ADAPTED Y value. The > color's coordinates in XYZ space don't change upon converting the > image from one color space to another using relative colorimetric > intent (unless there is gamut clipping, but then only the clipped > values are affected; I didn't mention black point compensation because > for working spaces with 0 black points, using or not using blackpoint > compensation makes no difference, and if it does, there' a problem in > the code). > > The "Y" in a color's equivalent XYZ values **is** a color's > Luminance. If your method of calculating Luminance gives something > other than the color's "Y" value in XYZ space, your method of > calculating Luminance is wrong. > > Back to the formula for calculating Luminance: > > sRGB-unadapted: 65535 * 0.21265600 + 0 * 0.71515800 + 0 * 0.07218600 = 13936.4 > > sRGB-adapted: 65535 * 0.22236633 + 0 * 0.71704102 + 0 * 0.06059265 = 14572.8 > > WideGamut: 38832 * 0.25871277 + 6228 * 0.72470093 + 766 * 0.01658630 = 14572.5 > > ProPhoto: 34671 * 0.28804016 + 6442 * 0.71195984 + 1105 * 0.00000000 = 14573.1 > > Identity: 29623 * 0.00000000 + 14573 * 1.00000000 + 1106 * 0.00000000 = 14573.0 > > In the above equations, the RGB values for the color of red specified > by the reddest possible sRGB red were determined by eye droppering a > red color block after doing the appropriate ICC profile conversions. > As you can see, the "Luminance" value calculated by the UNadapted sRGB > "Y" values is out of line with the other four calculated values. > Notice that 14573/65535 is 0.22237, the ADAPTED red Y value for sRGB. > If you did the same calculations for bluest sRGB blue and greenest > sRGB green, you'd get the ADAPTED sRGB blue and green Y values. > > The actual eye droppered grayscale values for my four color test image > (reddest sRGB red, bluest sRGB blue, and greenest sRGB green against > an 18% gray background), after converting to black and white using > Luminance, are as follows (ICC profile conversions done at 32-bit > floating point, which is how I did the conversions, are more accurate > than working with 16-bit values as given above): > > Profile WtPt Red block Green block Blue > block 18% Gray > sRGB D65-unadapted 13936 46868 4731 12094 > sRGB D65-adapted 14573 46991 3971 > 12094 > WideGamut D50 14573 46991 3971 > 12094 > ProPhoto D50 14573 46991 3971 > 12094 > Identity D50 14573 46991 > 3971 12094 > > The 18% gray value is the same for all five conversions because the > ONLY thing that determines the value of a neutral patch in an RGB > image is the profile's TRC. And a technically correct Luminance > conversion must always be done in a linear gamma color space. > > For neutral R=G=B starting "color image" values like 18% gray, correct > OR incorrect Y values gives the same black and white R=G=B value after > the Luminance conversion (it's always the same RGB values as before > the conversion). But the more saturated the color is, the more wrong > the results are using the UNadapted Y values. > > A technically correct Luminance conversion to black and white should > give the same resulting black and white image ***regardless*** of the > RGB working space the image happens to be in (this is not true of most > methods of converting an image to black and white). > > Your incorrect use of the unadapted "Y" values makes the Luminance > conversion dependent on the image profile's white point. But Luminance > is determined by the value of Y in the color's XYZ coordinates. And > those coordinates are determined using the adapted Y values from the > actual image ICC profile. Those values don't change just because you > converted the image from one color space to another, as long as you > use relative colorimetric intent to do the conversion. > > The **un**adapted sRGB Luminance values for red, blue and green are > different from the adapted values because the unadapted values use the > wrong "Y" values. The Luminance values calculated using the unadapted > "Y" values don't correspond to the equivalent "Y" value in XYZ space > for the color before the conversion to black and white. > > You can download the test image and the four ICC profiles from this web page: > http://ninedegreesbelow.com/photography/luminance-black-white.html > > Elle _______________________________________________ gimp-developer-list mailing list List address: gimp-developer-list@xxxxxxxxx List membership: https://mail.gnome.org/mailman/listinfo/gimp-developer-list
