How to get pixel information inside a fragment shader?

Question

In my fragment shader I can load a texture, then do this:

uniform sampler2D tex;

void main(void) {
   vec4 color = texture2D(tex, gl_TexCoord[0].st);
   gl_Frag         


        

Answer 1

Easy peasy.

Just compute the size of a pixel based on resolution. Then look up +1 and -1.

vec2 onePixel = vec2(1.0, 1.0) / u_textureSize;
gl_FragColor = (
   texture2D(u_image, v_texCoord) +
   texture2D(u_image, v_texCoord + vec2(onePixel.x, 0.0)) +
   texture2D(u_image, v_texCoord + vec2(-onePixel.x, 0.0))) / 3.0;

There's a good example here