Using GIMP 2.9 updated yesterday, two different ways of adding layers
produce different results. But it seems to me that the two ways should
produce the same results. Here is a screenshot:
Are the different results a bug, a feature, or am I making an obvious
mistake or just not understanding something?
Looking at the screenshot, the "channel" layers were produced by making
a solid white layer and dragging the Red, Blue, and Green channels over
to the layer stack. So of course each channel layer also has R=G=B=1.0.
The channel layers are added using layer percent opacities of 22.2 for
the Red channel layer, 71.7% opacity for the Green channel layer, and
6.1% opacity for the Blue channel layer. The percentages are the correct
percentages for producing a Luminance conversion to black and white by
adding the Red, Green, and Blue channels together as layers.
The result of adding the three layers should be white, R=G=B=1.0, which
is what happens with the second way of adding the layers. But the first
way, using the more obvious "add each layer to the layers below",
Here's a download link for the actual XCF file:
You'll need to reset *all* of the layer opacities to the values given
above, because for some reason saving to disk and reopening causes layer
opacities to shift slightly (for example, the Blue layer opacities shift
to 5.9% instead of staying at 6.1%).
The image is an sRGB image and the precision is 32-bit floating point
(linear) in order to get the layers to properly add up to R=G=B=1.0,
which they should anyway for solid white, but results would be wrong for
colors other than solid white or solid black.
Color management and free/libre photography
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