Channel blending when converting to black and white can fail completely in the unbounded sRGB color space
Every once in a while a software developer will propose that unbounded sRGB can be used as a universal color space for image editing. This article provides an example showing how channel blending during a conversion to black and white worked perfectly in the source color space but failed completely after a conversion to the unbounded sRGB color space.
When working with interpolated camera raw files, normally I use a custom RGB working space that's the same size and shape as my camera input profile. As an experiment, I tried to recreate in the unbounded sRGB color space a conversion to black and white that I had previously done in my custom RGB working space.
I initially envisioned the conversion to black and white after inspecting the image channel data during raw processing. Part B below shows the steps I followed in the conversion to black and white in my custom color space. Part C shows what happened when I tried to replicate the steps in the unbounded sRGB color space.
Converting an image from a wider gamut color space to the unbounded sRGB color space necessarily rearranges channel data, and the more saturated the image, the more radically the channel data is rearranged. My envisioned conversion to black and white cannot be done in the unbounded sRGB color space.
Software used in this demonstration:
High bit depth GIMP 2.9 from git is somewhat unusual among RGB image editors in that it actually can be used to edit images in the unbounded sRGB color space. So I used GIMP 2.9 to prepare the illustrations given below. The default GIMP 2.9 from git uses linear gamma RGB values for some editing operations and uses "gamma corrected" RGB values for other editing operations. To avoid any "apples to oranges" comparisions, I compiled and used a version of GIMP from git that I modified to ensure that all editing operations were done using linear gamma processing.
Converting to black and white in a custom "camera sized" RGB working space
Converting to black and white in the unbounded sRGB color space
All colors can be encoded and displayed in the very small sRGB color space by using an unbounded ICC profile conversion, thus allowing otherwise out of gamut colors to be expressed using RGB values that are greater than one and/or less than zero. The yellow-orange portions of the flower shown in Figure 1 above are in fact out of gamut with respect to sRGB, having red channel values that are greater than one, and blue channel values that are less than zero.
After the image has been converted to the unbounded sRGB color space, reproducing the black and white rendition shown in Figure 3 above is impossible.
Certainly the saturated image colors are not clipped by converting from the camera input profile to an unbounded sRGB working space. And certainly the luminance-based conversion to black and white produced the same result when done in either color space.
But as shown in Figure 4 below, the flower's red, green, and blue channel information is radically altered, destroying the original blue channel detail and rendering the blue channel unuseable as a blending layer.
My envisioned conversion to black and white was simple to achieve in my custom RGB working space: make a luminance-based conversion to black and white, pull over the blue channel and set it to multiply blend mode, season to taste.
Converting the image to unbounded sRGB made my envisioned conversion to black and white completely impossible.
Worse, if I hadn't examined the blue channel data before the conversion to the unbounded sRGB color space, I never would have seen the original blue channel data. As the original blue channel contained all the interesting information in this particular image, I would not have been inspired to convert the image to black and white. No doubt no loss to art! but that is not the point. The point is that a conversion to unbounded sRGB radically rearranges channel data, which in turn radically alters the information the artist has to work with.
If you would like to check for yourself and you don't have any reasonably saturated interpolated camera raw files to play with, here are the image file and ICC profiles used in this demonstration: