I've asked the same question on SO here and they suggested I should ask here instead.

I'm currently trying to create a gaussian blur shader, and while I've successfully created the blur effect my shader only renders the lower right quarter of my screen like the image shows:Screenshot of effect And just to make it clear; it has NOT re-scaled the image, it just doesn't render the rest of it.

Here's my shader code with comments of my thoughts:

Vertex (horizontal, vertical almost identical. See below)

#version 400

in vec2 a_texCoord0; //My tex coords

out vec2 blurTextureCoords[11]; //Sample 11px (i.e, 5 right 5 left of center)

uniform float u_width; //Width of the screen. Used to calculate pixel size

void main(void){
    gl_Position = vec4(a_texCoord0, 0.0, 1.0);
    vec2 centerTexCoords = a_texCoord0 * 0.5 + 0.5; //Maybe this is somehow wrong?
    float pixelSize = 1.0 / u_width; //This is kind of interesting, because my shader sometimes tells me that "no uniform value was found with name 'u_width', but it still seems to work as if I change the values manually (ex. set it to 1920) it still looks normal.

    for(int i=-5; i<=5; i++) {
        blurTextureCoords[i+5] = centerTexCoords + vec2(pixelSize * i, 0.0); //I also thought that it might be because I multiply a float with an integer, but if I do float(i) instead of just i it still looks the same.



#version 400
#ifdef GL_ES
precision mediump float;

in vec2 blurTextureCoords[11];

out vec4 out_colour;

uniform sampler2D u_texture;

void main(void){
    //The actual bluring
    out_colour = vec4(0.0);
    out_colour += texture(u_texture, blurTextureCoords[0]) * 0.0093;
    out_colour += texture(u_texture, blurTextureCoords[1]) * 0.028002;
    out_colour += texture(u_texture, blurTextureCoords[2]) * 0.065984;
    out_colour += texture(u_texture, blurTextureCoords[3]) * 0.121703;
    out_colour += texture(u_texture, blurTextureCoords[4]) * 0.175713;
    out_colour += texture(u_texture, blurTextureCoords[5]) * 0.198596;
    out_colour += texture(u_texture, blurTextureCoords[6]) * 0.175713;
    out_colour += texture(u_texture, blurTextureCoords[7]) * 0.121703;
    out_colour += texture(u_texture, blurTextureCoords[8]) * 0.065984;
    out_colour += texture(u_texture, blurTextureCoords[9]) * 0.028002;
    out_colour += texture(u_texture, blurTextureCoords[10]) * 0.0093;

Additional code

Creating a shader:

//I also have one for the vertical shader, it's almost exactly the same.
horizontalShader = new ShaderProgram(
horizontalShader.pedantic = false;
horizontalShader.setUniformf("u_width", Gdx.graphics.getWidth());

if (horizontalShader.getLog().length() != 0) {
    System.out.println("Horizontal shader! \n" + horizontalShader.getLog());

Rendering to FBO then to screen:

// Horozontal blur

// Vertical blur
spriteBatch.draw(horizontalFBO.getColorBufferTexture(), 0, 0);

// Normal FBO (screen)
spriteBatch.draw(verticalFBO.getColorBufferTexture(), 0, 0);

Additional info

I use two FBOs, but it seems that those are not the root of the problem, as the problem still persists if I just render directly to the screen using these shaders.

I have two vertex shaders, one for the horizontal and one for the vertical blur. The only difference is the uniform name u_width becomes u_height and the blurTextureCoords[i+5] = centerTexCoords + vec2(pixelSize * i, 0.0); becomes blurTextureCoords[i+5] = centerTexCoords + vec2(0.0, pixelSize * i);


The screen coords go from {-1,-1} to {1,1} but you are most likely feeding it {0,0} to {1,1} - hence the fact that only one-quarter of the screen gets rendered.

So just to be clear - point {0,0} is not the top-left (or bottom-left, depending on your co-ordinate system) point; it's the center.

This also explains the texture coord tranformation that you are querying:

vec2 centerTexCoords = a_texCoord0 * 0.5 + 0.5; //Maybe this is somehow wrong?

This transformation maps position coords of {-1,-1 ... 1,1} to texture coords of {0,0 ... 1,1}, and is therefore correct.

  • \$\begingroup\$ This was what some other people told me as well, but I'm unsure on how to change this (or compensate for it) in LibGDX. Any solution or possibly a link? \$\endgroup\$ – Charanor May 3 '16 at 20:14
  • \$\begingroup\$ @Charanor - I would imagine that one of the SpriteBatch.Draw overloads would do it: libgdx.badlogicgames.com/nightlies/docs/api/com/badlogic/gdx/… - alternatively change it in your shader, e.g using a_texCoord0 * 2 - 1. \$\endgroup\$ – Maximus Minimus May 3 '16 at 20:23
  • \$\begingroup\$ So I changed the "centerTexCoords" to what you suggested, and that kind of "stretched" the edges of the image instead. I also tried just using "a_texCoord0" instead of "centerTexCoords" in my calculations, which resuted in the entire image being drawn, but at 1/8th of the size (still in the lower right corner). \$\endgroup\$ – Charanor May 3 '16 at 20:52

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