Block filters using fragment shaders
- by Nils
I was following this tutorial using Apple's OpenGL Shader Builder (tool similar to Nvidia's fx composer, but simpler).
I could easily apply the filters, but I don't understand if they worked correct (and if so how can I improve the output). For example the blur filter: OpenGL itself does some image processing on the textures, so if they are displayed in a higher resolution than the original image, they are blurred already by OpenGL. Second the blurred part is brighter then the part not processed, I think this does not make sense, since it just takes pixels from the direct neighborhood. This is defined by
float step_w = (1.0/width);
Which I don't quite understand: The pixels are indexed using floating point values??
Edit: I forgot to attach the exact code I used:
Fragment Shader
// Originally taken from: http://www.ozone3d.net/tutorials/image_filtering_p2.php#part_2
#define KERNEL_SIZE 9
float kernel[KERNEL_SIZE];
uniform sampler2D colorMap;
uniform float width;
uniform float height;
float step_w = (1.0/width);
float step_h = (1.0/height);
// float step_w = 20.0;
// float step_h = 20.0;
vec2 offset[KERNEL_SIZE];
void main(void)
{
int i = 0;
vec4 sum = vec4(0.0);
offset[0] = vec2(-step_w, -step_h); // south west
offset[1] = vec2(0.0, -step_h); // south
offset[2] = vec2(step_w, -step_h); // south east
offset[3] = vec2(-step_w, 0.0); // west
offset[4] = vec2(0.0, 0.0); // center
offset[5] = vec2(step_w, 0.0); // east
offset[6] = vec2(-step_w, step_h); // north west
offset[7] = vec2(0.0, step_h); // north
offset[8] = vec2(step_w, step_h); // north east
// Gaussian kernel
// 1 2 1
// 2 4 2
// 1 2 1
kernel[0] = 1.0; kernel[1] = 2.0; kernel[2] = 1.0;
kernel[3] = 2.0; kernel[4] = 4.0; kernel[5] = 2.0;
kernel[6] = 1.0; kernel[7] = 2.0; kernel[8] = 1.0;
// TODO make grayscale first
// Laplacian Filter
// 0 1 0
// 1 -4 1
// 0 1 0
/*
kernel[0] = 0.0; kernel[1] = 1.0; kernel[2] = 0.0;
kernel[3] = 1.0; kernel[4] = -4.0; kernel[5] = 1.0;
kernel[6] = 0.0; kernel[7] = 2.0; kernel[8] = 0.0;
*/
// Mean Filter
// 1 1 1
// 1 1 1
// 1 1 1
/*
kernel[0] = 1.0; kernel[1] = 1.0; kernel[2] = 1.0;
kernel[3] = 1.0; kernel[4] = 1.0; kernel[5] = 1.0;
kernel[6] = 1.0; kernel[7] = 1.0; kernel[8] = 1.0;
*/
if(gl_TexCoord[0].s<0.5)
{
// For every pixel sample the neighbor pixels and sum up
for( i=0; i<KERNEL_SIZE; i++ )
{
// select the pixel with the concerning offset
vec4 tmp = texture2D(colorMap, gl_TexCoord[0].st + offset[i]);
sum += tmp * kernel[i];
}
sum /= 16.0;
}
else if( gl_TexCoord[0].s>0.51 )
{
sum = texture2D(colorMap, gl_TexCoord[0].xy);
}
else // Draw a red line
{
sum = vec4(1.0, 0.0, 0.0, 1.0);
}
gl_FragColor = sum;
}
Vertex Shader
void main(void)
{
gl_TexCoord[0] = gl_MultiTexCoord0;
gl_Position = ftransform();
}
