GLSL Motion Blur Jittering

Question

I tried to implement a motion-blur post processing effect as described in GPU Gems 3 Chapter 27, but I am encountering issues because the blur jitters when i move the camera and does not work as expected. This is my fragment shader:

varying vec3 pos;
varying vec3 normal;
varying vec2 tex;
varying vec3 tangent;
uniform mat4 matrix;
uniform mat4 VPmatrix;
uniform mat4 matrixPrev;
uniform mat4 VPmatrixPrev;
uniform sampler2D diffuseTexture;
uniform sampler2D zTexture;
void main() {
    vec2 texCoord = tex;
    float zOverW = texture2D(zTexture, texCoord).r;
    vec4 H = vec4(texCoord.x * 2.0 - 1.0, (1 - texCoord.y) * 2.0 - 1.0, zOverW, 1.0);
    mat4 inv1 = inverse(matrix);
    mat4 inv2 = inverse(VPmatrix);
    vec4 D = H*(inv2*inv1);
    vec4 worldPos = D/D.w;
    mat4 prev = matrixPrev*VPmatrixPrev;
    vec4 previousPos = worldPos*prev;
    previousPos /= previousPos.w;
    vec2 velocity = vec2((H.x-previousPos.x)/2.0, (H.y-previousPos.y)/2.0);
    vec3 color = vec3(texture2D(diffuseTexture, texCoord));
    for(int i = 0; i < 16; i++) {
        texCoord += velocity;
        vec3 color2 = vec3(texture2D(diffuseTexture, texCoord));
        color += color2;
    }
    color /= 16;
    gl_FragColor = vec4(color, 1.0);
}

The uniforms matrix and VPmatrix are the ModelView and Projection matrixes that were got as following:

float matrix[16];
float VPmatrix[16];
glGetFloatv(GL_MODELVIEW_MATRIX, matrix);
glGetFloatv(GL_PROJECTION_MATRIX, VPmatrix);

The uniforms matrixPrev and VPmatrixPrev are the previous ModelView and Projection matrixes got as following after rendering: (in the code below matrixPrev and VPmatrixPrev are global variables)

for(int i = 0; i < 16; i++) {
    matrixPrev[i] = matrix[i];
    VPmatrixPrev[i] = VPmatrix[i];
}

All four matrixes are passed to the shader as following:

glUniformMatrix4fvARB(glGetUniformLocationARB(postShader, "matrix"), 16, GL_FALSE, matrix);
glUniformMatrix4fvARB(glGetUniformLocationARB(postShader, "VPmatrix"), 16, GL_FALSE, VPmatrix);
glUniformMatrix4fvARB(glGetUniformLocationARB(postShader, "matrixPrev"), 16, GL_FALSE, matrixPrev);
glUniformMatrix4fvARB(glGetUniformLocationARB(postShader, "VPmatrixPrev"), 16, GL_FALSE, VPmatrixPrev);

In the shader, the uniform zTexture is a texture containing the depth values of the frame buffer. (Not sure if they are divided by W)

I hoped the shader would work but what I get instead is that when i rotate the camera around the blur jitters really fast with subtle rotations. I tried rendering the zTexture and the result I get is a grayscale image so It seems alright. I also tried setting the fragment color to H.xyz and previousPos.xyz and while rendering H.xyz produces a colored screen, previousPos.xyz produces the same colored screen, except that when the camera rotates the colors seem to invert, so I suspect there is something wrong with extracting the world position from depth.

Am I missing something here? Any help would be greatly appreciated.

In your code, you do the blur sampling 16 times. Notice that this means you need to sample 16x per pixel. That's a lot. Did you try reducing this to e.g. 4 times? Is the effect OK? Do you still see the jitter? It's hard to say anything based on the code alone, maybe some experiments will show what can be wrong. — Krystian

piecemontee piecemontee · Accepted Answer · 2015-01-21T23:31:09

forget my previous answer, is a matrix error:

matrix multiplications are in the wrong order or otherwise you must send transpose matrix (explaining why only the rotation was taken in acount, translation/scale components where messed up):

glUniformMatrix4fvARB(glGetUniformLocationARB(postShader, "matrix"), 16, GL_FALSE, matrix);

becomes

glUniformMatrix4fvARB(glGetUniformLocationARB(postShader, "matrix"), 1, GL_TRUE, matrix);

(note 16 was not the matrix size but the matrix count, so it should be only only one here)

Another note: you should compute inverse matrix and project*view result matrix in your main application, not the pixel shader: it's done once per pixels but should be done only once per frame!)

Past post note: Google "RuinIsland_GLSL_Demo.zip": it contains many good glsl sample, helped me solve this issue

GLSL Motion Blur Jittering

2 Answers