[SOLVED] Using pc.TransformFeedback

Help & Support
1

There is an example on how to use pc.TransformFeedback here

I tried to follow it, but without using mesh API and manually creating a vertex buffer. In my example below, the vertex shader should reset the vertex position values to zero. However, the output buffer still shows them as 0.5 after processing. What am I missing?

// pseudo code

// shader
attribute vec4 vertex_position;
varying vec4 out_vertex_position;
void main(void)
{
    out_vertex_position = vec4(0.);
}

// initialize
const vertexFormat = new pc.VertexFormat(device, [
    { semantic: pc.SEMANTIC_POSITION, components: 4, type: pc.TYPE_FLOAT32 }
]);

const pos = new Float32Array([0.5, 0.5, 0.5, 0.5]);
const vb = new pc.VertexBuffer(device, vertexFormat, 1, pc.BUFFER_DYNAMIC, pos.buffer);
tf = new pc.TransformFeedback(vb);

const shader = pc.TransformFeedback.createShader(
    device, shader, "test"
);

// update
tf.process(shader);

const output = tf.outputBuffer.lock();
new Float32Array(output); // [ 0.5, 0.5, 0.5, 0.5 ]
2

I dont think you can read from the buffer on the CPU … only the GPU copy is written to by GPU … it’s expected to use the result on the GPU again for some follow up draw call - for example render the results as points sprites.

3

Ah, that makes sense. Thank you, @mvaligursky !