rohan west - Page 17 - Developer IT

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(); }

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