Can't set lower alpha value

[Meelusyn]

Hello

I'm trying to erase parts of a render_target with a shader by setting the alpha to 0 but it's stay to 1 (255).

here is the fragment shader :

void main() {
     
   gl_FragColor = vec4(0.0, 0.0, 0.0,       0.0       );
}

and here the sfml code :

_renderTarget.clear(sf::Color(0,0,0,      255       ));
_window.clear(sf::Color(255,255,255,255);

_renderTarget.draw(_sprite,&erasingShader);// a 32x32 sprite at mouse location

_renderTarget.display();

sf::Sprite tempSprite(_renderTarget.getTexture());

_window.draw(tempSprite, sf::BlendAlpha);

_window.display();
 

Am I missing something ? it should displays a white square but its all black;

[Laurent]

The default alpha-blending mode blends using the source alpha. In your case it is 0, so you're doing nothing to the render-texture. What you want is to disable alpha-blending (sf::Blending::None), so that your 0 alpha is actually written instead of interpreted.

And using a shader to generate transparent pixels is a little overkill, a simple shape with a transparent color will do the job.

[Meelusyn]

Thanks, i didn't know shader wasn't replacing blendmodes;

I put sf::BlendNone instead of the shader and it works, but i am using a shader because i want not to erase all the square of the sprite(i have simplified the code for looking what i was doing wrong);

but when i do

draw(sprite, sf::RenderStates(sf::BlendNone, sprite.getTransform(), sprite.getTexture(), &erasingShader));

it still doesn't work
(when it will works i will put the renderStates as member instead of recreating it each draw call i suppose it would be better ? but here it just for the test)
also i have seen we could use vertex array instead of sprite now, thats great.

Edit : with a renderState outside the calldraw its work