MSAA performance in deferred shading

[somboon]

I have implemented MSAA in deferred shading by calculate lighting at every sample of the pixel and then average it,something like this

vec4 pixelIntenisty = vec4(0.0,0.0,0.0,0.0);
for(int i=0;i<MSAACount;i++){
   float depth = texelFetch(depthTextureMSAA,texCoord,i);
   vec4 diffuse = texelFetch(diffuseTextureMSAA,texCoord,i);
   vec4 specular = texelFetch(specularTextureMSAA,texCoord,i);
   vec4 normal = textelFetch(normalTextureMSAA,texCoord,i);
   //calculate lighting
   pixelIntenisty += calculateLighting(......);
}
//average the pixel intensity
pixelIntensity = pixelIntensity/MSAACount;

while this give a good result ,the performane is totally killed.

so , I am thinking about doing antialiasing only at the edge pixel but that will also invole some kind of edge detection and stencil operation to mark the pixel(which might make thing even more slower).

Is there any other approch (in OpenGL 3.2-3.3 compatible way) to handle real antialiasing in deferred shader ?

Cheer
somboon

[kowal]

Texel fetches are slow, but they can be executed in pararel with other operations (math), try mixing texture access with computation instructions.

[somboon]

Can you give some short example ?

I dont understand the "mixing texture access with computation instruction".

Is it something like this ?

in stead of

float depth = texelFetch(depthTextureMSAA,intTexCoord.st,i).r;
pos.z = - planes.y / (planes.x + depth);
pos.xy = intViewVector.xy / intViewVector.z*pos.z;

do this

pos.z = - planes.y / (planes.x + texelFetch(depthTextureMSAA,intTexCoord.st,i).r);
pos.xy = intViewVector.xy / intViewVector.z*pos.z;

Thanks

[kowal]

No, I meant,

instead of:

diffuse = texelFetch(...);
specular = textelFetch(...);
somethingelse = texelFetch(...);

do:

diffuse = texelFetch(...);
(do some math here, but do not use 'diffuse' variable)
specular = texelFetch(...);
(math again, assume that 'diffuse' is ready, but do not use 'specular')
...

For me this trick boosted performance about 1.5x (I had 14 texelFetches in shader)

[somboon]

Thank a lot , will definately try this.

Is this trick also work with standard texture2D ? or the function is already fast enought ?

[kowal]

AFAIK all texturing functions are slow, but I'm not sure.
It is definitely worth testing .