Alpha blending

[yanner]

Hello,

I am trying to pass color values as attributes to fragment shader in order to implement a custom geometry gradient with alpha blending using QSGSimpleMaterial and QSGSimpleMaterialShader. The problem is that where there is low opacity the resulting color goes to white. I suspect this is because the alpha blending is not happening correctly but the color at a fragment is getting added to the destination color and gets saturated to white. How can I prevent this? Is there a way to access the destination color or the background color (not gl_Fragcolor) in the fragment shader somehow to be able to change the alpha blending? My code looks like this...

vertice shader:

attribute highp vec4 pos;
attribute highp vec4 color;

uniform highp mat4 qt_Matrix;
varying highp vec4 vC;

void main(void)
{
gl_Position = qt_Matrix * pos;
vC = color;
}

fragment shader:

uniform lowp float qt_Opacity;

varying highp vec4 vC;

void main(void)
{
gl_FragColor = vC * qt_Opacity;
}

Any help appreciated, thanks!