# Thread: Gaussian Blur on texture

1. ## Gaussian Blur on texture

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

Code :
#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));
}
}

2. ## Re: Gaussian Blur on texture

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.
Code :
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.

4. ## Re: Gaussian Blur on texture

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

5. ## Re: Gaussian Blur on texture

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

6. ## Re: Gaussian Blur on texture

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

7. ## Re: Gaussian Blur on texture

Code :
/////////////////////////////////////////////////
// 7x1 gaussian blur fragment shader
/////////////////////////////////////////////////

varying vec2 v_Coordinates;

uniform vec2 u_Scale;
uniform sampler2D u_Texture0;

const vec2 gaussFilter[7] =
{
-3.0,	0.015625,
-2.0,	0.09375,
-1.0,	0.234375,
0.0,	0.3125,
1.0,	0.234375,
2.0,	0.09375,
3.0,	0.015625
};

void main()
{
vec4 color = 0.0;
for( int i = 0; i < 7; i++ )
{
color += texture2D( u_Texture0, vec2( v_Coordinates.x+gaussFilter[i].x*u_Scale.x, v_Coordinates.y+gaussFilter[i].x*u_Scale.y ) )*gaussFilter[i].y;
}

gl_FragColor = color;
}

u_Scale is vec2(0, 1.0/height ) for vertical blur and vec2(1.0/width, 0 ) for horizontal blur.

8. ## Re: Gaussian Blur on texture

Thanks a lot vexator,

You all Shaders expert use shader program only code samples but for us old OpenGL programmers the rest is still not so easy to add.

Can you please add also the gl calls to make this shader to run?

Thanks so much,

Alberto

9. ## Re: Gaussian Blur on texture

You will need a good glsl tutorial:
http://www.lighthouse3d.com/opengl/glsl/

10. ## Re: Gaussian Blur on texture

Hi Vexator,

Please help me to make this shader to work.

What are variables we need to initialize?

How can we perform the blur in both directions?

Thanks,

Alberto

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