set a ONE pixel image's alpha value using gimp

Question

i have gimp 2.10.22 on Kubuntu.

I want a one pixel png image with r=102 g=127 b=128 a=128

I know about "add alpha channel". I have played with "add layer mask", setting opacity on various tools, tried the range tool and other tools with add layer mask. I know about edit layer mask / show layer mask / apply layer mask

I just can NOT get a pixel with alpha of 128.

Everything I try gives me a weird alpha value. Sometimes too low, sometimes too high, or 0 or 255 but can't get 128 to save my life.

It looks ok, but when I apply the layer, or look at the tool result with the color picker tool, the alpha is not what I tried to get.

Is gimp just this hard to use (so many settings with bad defaults) or am i dumb (a veryyyy real possibility)

thanks for any help you can throw my way.

EDIT - given that the gradient tool DOES have alpha values you can pick in the from and to point, I figured it should work.

I put in my exact r,g,b,a values into both the from and the to point and picked linear gradient. Then had to click on one of the points and THEN hit ENTER to apply it (I'm def blaming the devs on unusability here)

Nope - alpha turned into 64. Then I noticed opacity was at 50% for the gradient tool.

Set it to 100%, set r,g,b,a in both from and to points, reclicked a point, hit ENTER to apply

Nope - alpha turned into 160.

Anybody know how to file a bug. That's gotta be a bug??

At least I can keep adjusting until I figure out the magic opacity to not change my dang alpha.

Is there something I don't get here. Cuz there sure seems like it.

Answer 1

First of all, alpha usually ranges from 0 to 100 in GIMP. So when you say 128 opacity, I'm assuming you mean half opacity.

I created a 8-bit precision linear light 72 DPI 1-pixel image. Then I changed the foreground color to 102,127,128. Then, I selected the Pencil tool and set its opacity to 50%. After setting "Aspect Ratio" and "Angle" to zero, and "Force" to 100%, and unchecking all the checkboxes, I clicked the image. And the "Pointer Information" tool told me that the pixel was 102,127,128,128. Isn't that exactly what you want?

I'm using the exact same version of GIMP as you - 2.10.22. My OS, though, is Windows 10. Maybe try doing exactly what I did?

Answer 2

Quick answer: best solution is to create a 1-pixel layer, set its opacity to 50% using the slider at the top of the Layers list, and export.

Now, about your problem: since Gimp 2.10, the layer mask is gamma-corrected(*). In other words, the true alpha is mask^2.2. So if you want a half-transparent pixel the mask value is (^ is the power/exponent operation):

0.5 ^ (1/2.2) = 0.5 ^ 0.454545 = 0 .73

(or 186 in the 0..255 notation). And you can check that despite this value in the mask, if you export to PNG, the PNG will indeed have Alpha=.5.

If you don't want to compute this, you can obtain it this way:

• create a plain layer
• use the opacity slider to set it to the require opacity
• Edit > Copy visible, File > Create > From clipboard
• Layer > Mask > Add layer mask and Transfer alpha channel
• Use the Pointer dialog (with deselected Sample merged) to check the mask value.

(*) this stems from the big change in Gimp 2.10 where computations are done on "linear light" values. In previous versions, Gimp worked directly on the gamma-corrected values (because JPG/PNG are natively gamma-corrected) but this creates nasty problems (for instances, color gradients will get dark in the middle). Some of the old behavior can be restored using the "Standard/Legacy" layer mode selector (small selector at the end of the Layer mode selector) but not how the mask is handled.

Answer 3

1. Create a new image, and make sure the background is set to transparent. Here's an example, I made a new 3px x 3px, transparent document.

If you forgot to do this and started with a solid white background layer, no worries, you can just delete the background layer in the layers panel, and add a new transparent layer instead. You can add as many transparent layers as you like. GIMP is layer based. Everything happens on layers.

If you can't see the Layers panel, hit Ctrl+L to bring it up, here's what you want to see, a layer thumbnail with the checkered pattern, this indicates transparency.

2. Now to get to your specific problem. Set the foreground colour to RGB 102,127,128 = #667F80

3. Select the Pencil Tool in the toolbox, select the 1px Brush

4. In the Tool Options, set the opacity to what you want. For example 50% alpha = 128, or 49.8% = 127

5. Now, back in the image paint a pixel

6. Select the colour picker tool, and Shift+Click on the pixel, and check it.