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", produces R=G=B=0.793257.

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.

Elle, puzzled
Color management and free/libre photography
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