TransformFeedback

This object allows you to configure and use the transform feedback feature (WebGL2 only). How to use:

1. First, check that you're on WebGL2, by looking at the `app.graphicsDevice.isWebGL2`` value.
2. Define the outputs in your vertex shader. The syntax is out vec3 out_vertex_position, note that there must be out_ in the name. You can then simply assign values to these outputs in VS. The order and size of shader outputs must match the output buffer layout.
3. Create the shader using TransformFeedback.createShader(device, vsCode, yourShaderName).
4. Create/acquire the input vertex buffer. Can be any VertexBuffer, either manually created, or from a Mesh.
5. Create the TransformFeedback object: const tf = new TransformFeedback(inputBuffer). This object will internally create an output buffer.
6. Run the shader: tf.process(shader). Shader will take the input buffer, process it and write to the output buffer, then the input/output buffers will be automatically swapped, so you'll immediately see the result.

// *** shader asset ***
attribute vec3 vertex_position;
attribute vec3 vertex_normal;
attribute vec2 vertex_texCoord0;
out vec3 out_vertex_position;
out vec3 out_vertex_normal;
out vec2 out_vertex_texCoord0;
void main(void) {
    // read position and normal, write new position (push away)
    out_vertex_position = vertex_position + vertex_normal * 0.01;
    // pass other attributes unchanged
    out_vertex_normal = vertex_normal;
    out_vertex_texCoord0 = vertex_texCoord0;
}

// *** script asset ***
var TransformExample = pc.createScript('transformExample');

// attribute that references shader asset and material
TransformExample.attributes.add('shaderCode', { type: 'asset', assetType: 'shader' });
TransformExample.attributes.add('material', { type: 'asset', assetType: 'material' });

TransformExample.prototype.initialize = function() {
    const device = this.app.graphicsDevice;
    const mesh = pc.createTorus(device, { tubeRadius: 0.01, ringRadius: 3 });
    const meshInstance = new pc.MeshInstance(mesh, this.material.resource);
    const entity = new pc.Entity();
    entity.addComponent('render', {
        type: 'asset',
        meshInstances: [meshInstance]
    });
    app.root.addChild(entity);

    // if webgl2 is not supported, transform-feedback is not available
    if (!device.isWebGL2) return;
    const inputBuffer = mesh.vertexBuffer;
    this.tf = new pc.TransformFeedback(inputBuffer);
    this.shader = pc.TransformFeedback.createShader(device, this.shaderCode.resource, "tfMoveUp");
};

TransformExample.prototype.update = function(dt) {
    if (!this.app.graphicsDevice.isWebGL2) return;
    this.tf.process(this.shader);
};

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