I suspect I'm not correctly rendering particle positions to my FBO, or correctly sampling those positions when rendering, though that may not be the actual problem with my code, admittedly.
I have a complete jsfiddle here: http://jsfiddle.net/p5mdv/53/
A brief overview of the code:
Initialization:
Create an array of random particle positions in x,y,z
Create an array of texture sampling locations (e.g. for 2 particles, first particle at 0,0, next at 0.5,0)
Create a Frame Buffer Object and two particle position textures (one for input, one for output)
Create a full-screen quad (-1,-1 to 1,1)
Particle simulation:
Render a full-screen quad using the particle program (bind frame buffer, set viewport to the dimensions of my particle positions texture, bind input texture, and draw a quad from -1,-1 to 1,1). Input and output textures are swapped each frame.
Particle fragment shader samples the particle texture at the current fragment position (gl_FragCoord.xy), makes some modifications, and writes out the modified position
Particle rendering:
Draw using the vertex buffer of texture sampling locations
Vertex shader uses the sampling location to sample the particle position texture, then transforms them using view projection matrix
Draw the particle using a sprite texture (gl.POINTS)
Questions:
Am I correctly setting the viewport for the FBO in the particle simulation step? I.e. am I correctly rendering a full-screen quad?
// 6 2D corners = 12 vertices var vertexBuffer = new Float32Array(12); // -1,-1 to 1,1 screen quad vertexBuffer[0] = -1; vertexBuffer[1] = -1; vertexBuffer[2] = -1; vertexBuffer[3] = 1; vertexBuffer[4] = 1; vertexBuffer[5] = 1; vertexBuffer[6] = -1; vertexBuffer[7] = -1; vertexBuffer[8] = 1; vertexBuffer[9] = 1; vertexBuffer[10] = 1; vertexBuffer[11] = -1; // Create GL buffers with this data g.particleSystem.vertexObject = gl.createBuffer(); gl.bindBuffer(gl.ARRAY_BUFFER, g.particleSystem.vertexObject); gl.bufferData(gl.ARRAY_BUFFER, vertexBuffer, gl.STATIC_DRAW); ... gl.viewport(0, 0, g.particleSystem.particleFBO.width, g.particleSystem.particleFBO.height); ... // Set the quad as vertex buffer gl.bindBuffer(gl.ARRAY_BUFFER, g.screenQuad.vertexObject); gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0); // Draw! gl.drawArrays(gl.TRIANGLES, 0, 6);Am I correctly setting the texture coordinates to sample the particle positions?
for(var i=0; i<numParticles; i++) { // Coordinates of particle within texture (normalized) var texCoordX = Math.floor(i % texSize.width) / texSize.width; var texCoordY = Math.floor(i / texSize.width) / texSize.height; particleIndices[ pclIdx ] = texCoordX; particleIndices[ pclIdx + 1 ] = texCoordY; particleIndices[ pclIdx + 2 ] = 1; // not used in shader }
The relevant shaders:
Particle simulation fragment shader:
precision mediump float; uniform sampler2D mParticleTex; void main() { // Current pixel is the particle's position on the texture vec2 particleSampleCoords = gl_FragCoord.xy; vec4 particlePos = texture2D(mParticleTex, particleSampleCoords); // Move the particle up particlePos.y += 0.1; if(particlePos.y > 2.0) { // Reset particlePos.y = -2.0; } // Write particle out to texture gl_FragColor = particlePos; }Particle rendering vertex shader:
attribute vec4 vPosition; uniform mat4 u_modelViewProjMatrix; uniform sampler2D mParticleTex; void main() { vec2 particleSampleCoords = vPosition.xy; vec4 particlePos = texture2D(mParticleTex, particleSampleCoords); gl_Position = u_modelViewProjMatrix * particlePos; gl_PointSize = 10.0; }Let me know if there's a better way to go about debugging this, if nothing else. I'm using webgl-debug to find gl errors and logging what I can to the console.