Shaders and image_alpha

[dull sand]

Working on an enhancement of one of my shaders, and I found out, that, when image_alpha is set, the shader renders the original sprite texture almost transparent, like with a 0.5 image_alpha, it seems, 0.5 * 0.5 * 0.5 gets applied or so...

Anything I should know about image alpha and shaders?
I send the image_alpha down to the shader through a uniform, together with the sprite texture but I can't get it to work...

I get the next pixel from the sprite's texture through

vec4  spriteSample  = texture2D(u_sSpriteSurface, v_vSurfaceUV);

and when I simply set

gl_FragColor = spriteSample;

the shader already renders the thing almost fully transparent, like I said, as if a 0.5 alpha gets applied multiple times.

What do I miss?

[rare hatch]

image_alpha is the alpha component of vertex color, which by default gets multiplied by the pixel color in shaders. so, not in your shader here, but normally, you get image_alpha * texture alpha and then that does gm's default alpha blending

so if image alpha is 0.5 and the texture alpha is 0.5, in a normal shader you get 0.5 * 0.5 which is a final pixel alpha of .25. that is the source, which is then multiplied (not in the shader, later on) using the blend equation by the destination alpha

your shader example is not using vertex color so you would get texture alpha only

and then blending

[dull sand]

ooookay ... wait, that explains LOTS