Re: [Gimp-user] Layer addition - bug, feature, or user misunderstanding?

[Elle Stone <ellestone ninedegreesbelow com>]

On 10/13/2015 12:25 AM, Richard wrote:
> The problem is that a layer's opacity doesn't add -- it multiplies, like
> this:
>
> Result = (opacity) * (this layer) + (100%-opacity) * (result of layers
> below)
>
> This formula holds true regardless of the layer's assigned blending mode
> (and it's recursive, with the "result of layers below" defined by
> inserting the next layer down into the same formula).

Hmm, I tried setting the layer blend mode for the three layers to 
Addition, Normal, Lighten only, and Dodge, and regardless of the chosen 
blend mode, the result was always 79%. But these identical results are 
because each layer is white, R=G=B=1.0f or 255 at 8-bit integer. Change 
to some lower value, such as R=G=B=0.5f or 128 at 8-bit integer, and the 
result of blending the three layers over black does depend on the chosen 
blend mode.

> = (22.2% + 55.8% + 1.3%) * (white)
> = 79.3% white
>
> Doesn't that 79% look rather familiar? :)

Many thanks! for explaining this math. The math behind Normal blend mode 
isn't exactly intuitively obvious, at least not to me.

But as explained below, I think that's not the right math for Addition 
blend mode. I think something might be wrong in the Addition layer blend 
mode code.

> Aside - the left half of your image is totally reproducible on GIMP 2.8
> .  (I can't seem to reproduce the right half in 2.8, but I haven't
> examined the actual XCF either, so I don't have all the details.)

I uploaded a GIMP 2.8 version of the xcf file:
http://ninedegreesbelow.com/bug-reports/gimp29/layer-addition/gimp28-addition.xcf

> Now to fix the values ... first, Red is on top so it can keep the 22.2%;
> this leaves a translucency of 77.8% for everything below it.
> For Green, below Red, divide its opacity by Red's translucency (above):
> Green's opacity should be (71.7% / 77.8%) = 92.1%.  This, in turn,
> leaves 7.9% of translucency for Blue below it.
> For Blue (which is below both Green and Red), divide its opacity by the
> overall translucencies of both Red and Green.  You can do the math if
> you want (6.1% / 77.8% / 7.9%), but it conveniently works out to exactly
> 100% opacity -- i.e. Blue doesn't need any translucency for itself
> because with both Red and Green on top of it (at the above opacities)
> only 6.1% of Blue will be visible anyway.
>
> To prove it, just plug the new opacities back into the above formula:
>
> Image = 22.2% * (red) + 77.8% * (92.1% * (green) + (100% - 92.1%) *
> (100% * blue) )
> = 22.2% * (red) + 77.8% * (92.1% * (green) + 7.9% * (blue) )
> = 22.2% * red + 71.7% * green + 6.1% * blue

Many, many thanks! I tried your percentages with a real image rather 
than a solid white layer, using Normal blend mode, and your percentages 
produced *exactly* the right black and white image.

Here's why I think there might be a bug in the code for Addition blend 
mode. But maybe you can explain why it's not a bug:

Working in GIMP 2.9, change the color of the three layers that are being 
added together to 0.50f (the same as you'd get if you started with a 50% 
gray layer and dragged the three channels over to the layer stack).

The result of setting the percentages to 22.2% of the Red channel layer, 
71.7% of the Green channel layer, and 6.1% of the Blue channel layer, 
and then setting the layer blend mode for each layer to Addition, is 
exactly 0.50f, which is intuitively what I would expect, and coheres 
with the equations:

(0.222 * Red) + (0.717 * Green) + (0.061 * Blue)

If Red=Green=Blue=0.5, the above equation simplifies to

(0.222 + 0.717 + 0.061) * 0.5 = 1.0 * 0.5 = 0.5

The corresponding Normal blend mode math that you provided produces 0.3965.

I tried the same percentages using GIMP 2.8, using R=G=B=128. Well, 
actually I set the percentages to the "easier to type" 22%, 72%, 6%.

In GIMP 2.8, Addition blend mode for the three layers produces R=G=B=127 
or 50% (rounded by the color picker), pretty close to what I expected 
Addition blend mode to produce. And Normal blend mode produces 
R=G=B=101, or 40%, exactly as you describe the Normal blend mode math above.

In GIMP 2.9, using Addition blend mode as described produces the 
intuitively expected results for all shades of gray less than or equal 
to 55% gray (R=G=B=0.55f, or 140 for 8-bit integer).

But at 56% gray and higher, results are progressively less than I would 
expect, assuming my equations for Addition blend mode are correct. 
Instead results progressively converge on the Normal blend mode results 
as the color of the blended layers approaches solid white.

So does this seem like a bug in the Addition blend mode? Or is there 
something I'm not still not understanding about how Addition layer blend 
mode is supposed to work?

Best,
Elle

As an aside (in case anyone is interested in using stacked channel 
layers to produce a black and white image), although the math is the 
same, the required percentages for converting to Luminance will change 
for other RGB color spaces. And the image needs to be in a linear gamma 
color space, or else the addition needs to be done on linearized RGB (as 
GIMP 2.9 does if you choose linear precision), otherwise you get Luma 
instead of Luminance.