Alpha ignored on shader?

[Azaratur]

I am writing my custom shader and I did notice something strange.
The shader example i wrote is very simple:

vec4 c = texture(u_tex0, v_texCoord); FragColor.r = 0.0; FragColor.g = 0.0; FragColor.b = 1.0; FragColor.a = c.a;
This code should paint the sprite to whole blue and it's working more or less.
The problem is that it's painting with blue where alpha should be 0:

If I add the a discard with alpha is 0, it works correctly, but as far as I know it should work even without.
`
vec4 c = texture(u_tex0, v_texCoord);

if ( c.a == 0.0 )
discard;

FragColor.r = 0.0;
FragColor.g = 0.0;
FragColor.b = 1.0;
FragColor.a = c.a;
`

Can you explain me why?

[aismann]

Have you tried this one:

if ( c.a != 0.0 )
{
  FragColor.r = 0.0;
  FragColor.g = 0.0;
  FragColor.b = 1.0;
  FragColor.a = c.a;
}

[Azaratur]

This won't answer my question. I don't need a solution to fix a bug, the code I wrote worked.
My question is why if alpha is 0.0 it still paint something?

[rh101]

Could you please test the shader on shadertoy to see how it behaves on there?

Also, show the complete code used to create that sprite that the shader is applied to.

[aismann]

@Azaratur
Its a spriteframe right?
Look which frame be shown wrong and check its alpha.
Sorry we have too few input for full help what we write is more a guess .

[Azaratur]

I can undestand it's hard to follow, unfortunately this project it's quite big and old, so it's hard to me to find a way to share piece of code.
Anyway if I don't change the color value the result is correct:

So it's something that happens when I change the color values.

[Azaratur]

Anyway will keep investigating and if I understand why it's happening will post it here.

[Azaratur]

The bug was on the blend mode.
BlendFunc blend; blend.src = backend::BlendFactor::SRC_ALPHA; blend.dst = backend::BlendFactor::ONE_MINUS_SRC_ALPHA; _sprite->setBlendFunc ( blend );

This fixed the issue.

[aismann]

@Azaratur
I think it can closed too. Right?