GLSL particle's rendering (premult)

Question

I have been trying to get a particle rendering library to work with my shaders. It works perfect with fixed function and I know the result I am aiming for. I am just unsure how to do the final blending bit.

It packs the particle life like this into the DWORD and sets the dword as one of the vertex attributes. A quad has 4 vertices like that. 3 float pos, 3 float normal, 4 byte diffuse, 2 float uv. What should I do with the 4byte attribute, do I use it as a vec4 or float attribute in my glsl shader? Also since this library uses premultiplied alpha, what kind of blending equation should I use? The library is pyro particle library.

int ir = (int) (r * 255.0f + 0.5f);
int ig = (int) (g * 255.0f + 0.5f);
int ib = (int) (b * 255.0f + 0.5f);
int ia = (int) (a * Particle.Visibility * 255.0f + 0.5f);
DWORD diffuse;

#ifdef __BIG_ENDIAN__
if (GetLibrary()->GetGraphicsDevice()->IsRGBA())
diffuse = (ir << 24) | (ig << 16) | (ib << 8) | ia;
else diffuse = (ib << 24) | (ig << 16) | (ir << 8) | ia;
#else
if (GetLibrary()->GetGraphicsDevice()->IsRGBA())
diffuse = ir | (ig << 8) | (ib << 16) | (ia << 24);
else diffuse = ib | (ig << 8) | (ir << 16) | (ia << 24);
#endif /* __BIG_ENDIAN__ */

Answer 1

Your 4x unsigned bytes will be converted to vec4 (floating point type). glVertexAttribPointer always converts any input type to float in the shader.

Their value will range from 0-255 if you select normalize == GL_FALSE, otherwise they will range from 0-1 (byte/255.f) if you select normalize == GL_TRUE .

As to your question about premultiplied alpha, if you don't want to multiply the source data by alpha in the blending operation, then just use:

glBlendFunc(GL_ONE,                 //multiply source value by 1
            GL_ONE_MINUS_SRC_ALPHA  //multiply destination value by (1-src_alpha)
            )