Gaussian Blur on texture

[devdept]

Hi All,

I have a texture generated on the fly (via FBO) with the model shadow. This is a black and transparent texture with - of course - many stair steps between (on/off) pixels.

I was thiking to add Gaussian Blur on the whole bitmap. I found the algorithm you see below but it works pixel by pixel.

Now the questions are:

1) Does OpenGL provide some built in algorithm to do this?
2) What is the fastest method to change pixel color on a texture?
3) Does some other different approach exist to obtain the same result?

Thanks so much as usually!

Alberto

#define gauss_width 7
 
sumr=0;
sumg=0;
sumb=0;
 
int gauss_fact[gauss_width]={1,6,15,20,15,6,1};
int gauss_sum=64;
 
for(i=1;i<temp->w-1;i++){
  for(j=1;j<temp->h-1;j++){
    sumr=0;
    sumg=0;
    sumb=0;
    for(k=0;k<gauss_width;k++){
      color=getpixel(temp,i-((gauss_width-1)>>1)+k,j);
      r=getr32(color);
      g=getg32(color);
      b=getb32(color);
      sumr+=r*gauss_fact[k];
      sumg+=g*gauss_fact[k];
      sumb+=b*gauss_fact[k];
 
    }
    putpixel(temp1,i,j,makecol(sumr/gauss_sum,sumg/gauss_sum,
      sumb/gauss_sum));
  } 
}
 
for(i=1;i<temp->w-1;i++){
  for(j=1;j<temp->h-1;j++){
    sumr=0;
    sumg=0;
    sumb=0;
    for(k=0;k<gauss_width;k++){
      color=getpixel(temp1,i,j-((gauss_width-1)>>1)+k);
      r=getr32(color);
      g=getg32(color);
      b=getb32(color);
 
      sumr+=r*gauss_fact[k];
      sumg+=g*gauss_fact[k];
      sumb+=b*gauss_fact[k];
    }
    sumr/=gauss_sum;
    sumg/=gauss_sum;
    sumb/=gauss_sum;
 
    putpixel(temp2,i,j,makecol(sumr,sumg,sumb));
  } 
}

[yooyo]

As you can see in that algorithm.. do the same.. but you cant write (setpixel) into the same texture. So.. do the ping-pong... create another texture (same size as original) and do horizontal blur. Then use that new texture as source for vertical blur.

fbo1 is original fbo
fbo2 is temporary fbo
 
fbo2 = horizontal_blur(fbo1) // step 1.. read from fbo1, write to fbo2
fbo1 = vertical_blur(fbo2)   // step 2.. read from fbo2, write to fbo1
 
now, fbo1 contains blurred image.

Even more, you can repeat step1 and step2 several times to blur more.

[dletozeun]

I think this would be useful too:
http://www.gamasutra.com/features/20010209/evans_02.htm

[devdept]

Thanks guy,

I now have the doubt that using this extension "EXT_convolution" I can do it via hardware. Do you know if it is true?

Thanks.

Alberto

[Seth Hoffert]

In my experience some/most of the imaging extensions (including convolution) are not HW accelerated.

[devdept]

No I am certain, the best way to do what we need are Shaders. Does anybody know a good 'basic' tutorial on the Shaders based convulution filter?

Thanks.

Alberto