Alpha channel for color behaves inconsistently between web and desktop

[RybekU]

Description
Using Color::with_alpha on web and locally produces different results as illustrated on image below:

Left rectangle - web
Right rectangle - desktop application
To Reproduce
Background color is Color::CYAN.
Rectangle is created using:

  gfx.fill_rect(&area, Color::BLUE.with_alpha(0.2));
  gfx.stroke_rect(&area, Color::BLUE);

Environment and versions:

• Environment: Windows 10 and Firefox
• Rust compiler version: stable
• Quicksilver version: commit rev = "8fdff10"

[ryanisaacg]

That's... interesting! I'll read the specs on blending for WebGL vs Desktop GL to see if there's anything that jumps out at me. I hope this is an easy fix but I can't promise that it will be.

[lenscas]

So... here is an interesting one. I decided to check the difference for myself mostly because I was curious and although the build for native looks quite similar to what is on the right. When I use cargo web to build for the web the square seems to become green

Also, if I change the background color to white, the square becomes yellow instead.

Browser:76.0.1 (64-bit)
OS: Linux
Quicskilver version: d105757

In order to produce this, I simply cloned quicksilver and changed the 01_example to contain the above mentioned code.

[AnthonyYoManz]

Pretty sure this is caused by the alpha-blending happening against the web-page. If you change the body element's background color to white and run some code that draws a semi-transparent black square over a sprite then the sprite will be blended with white. If you change the body element's background colour to black and repeat the process, the sprite will be blended with black.

[ryanisaacg]

Both issues here should be resolved by ryanisaacg/blinds#28: the alpha should show up the same on desktop and web, and the canvas should not be blended with the page background.

[ryanisaacg]

Let me know if either of these issues crop up after you upgrade dependencies, but the new blinds version should take care of them.