Transform (Experimental, WebGL2)

The Transform class provides easy interface to perform Transform Feedback operations on given data. Applications can use this class to move data processing from CPU to GPU, where multiple parallel execution units will be used for processing. Data is handled in form of Buffer objects, i.e. data resides in the GPU memory. Output of this class can directly set as attributes on Model or VertexArray for regular rendering operations, CPU access is not required hence avoids expensive CPU and GPU sync.

Transform class creates and holds Model and TransformFeedback instances.

This class is only supported when using WebGL2RenderingContext.

Use case : Specify source and destination buffers.

Create a Transform object by passing, vs (vertex shader), source buffer(s), varyings (output variable names in vertex shader) and destination buffers. Then call run to perform one transform feedback iteration.

const VS = `\
#version 300 es
attribute float inValue;
varying float outValue;

void main()
{
  outValue = 2.0 * inValue;
}
`;

const sourceData = new Float32Array([10, 20, 31, 0, -57]);
const sourceBuffer = new Buffer(gl, {data: sourceData});

// Default values applied for size (1) and type (gl.FLOAT)
const feedbackBuffer = new Buffer(gl, {bytes: sourceData.length * 4});

const transform = new Transform(gl2, {
  sourceBuffers: {
    inValue: sourceBuffer
  },
  feedbackBuffers: {
    outValue: feedbackBuffer
  },
  vs: VS,
  varyings: ['outValue'],
  elementCount: 5
});

// Perform one transform feedback iteration
transform.run();

Use case : Create destination buffers automatically.

Transform can internally create destination buffers (i.e. feedback buffers), when feedbackMap is provided. Each destination buffer is created with same settings and layout as corresponding source buffer as per feedbackMap.

const transform = new Transform(gl2, {
  sourceBuffers: {
    inValue: sourceBuffer
  },
  feedbackMap: {
    inValue: 'outValue'
  },
  vs: VS,
  varyings: ['outValue'],
  elementCount: 5
});

Use case : Multiple iterations using swapBuffers().

When feedbackMap is specified buffers can be swapped using a single call to swapBuffers(), this is useful for cases like particle simulation, where output of one transform feedback iteration is piped as input to the next iteration.


// Setup Transform with `feedbackMap` as above

transform.run();

let bufferWithNewValues = transform.getBuffer('outValue');
...
// Render using 'bufferWithNewValues'
...

//swap buffers
transform.swapBuffers();
transform.run();
bufferWithNewValues = transform.getBuffer('outValue');
...
// Render using 'bufferWithNewValues'
...

Use case : Update one or more buffers using update() method..

Once Transform object is constructed and used, one or more source or destination buffers can be updated using update.

// transform is set up as above
...

// update buffer binding for 'inValue' attribute
const newSourceBuffer = new Buffer(gl, {data: newSourceData});
transform.update({
  sourceBuffers: {
    inValue: newSourceBuffer
  }
});

// now data is provided from newly bound buffer.
transform.run();

Constructor

Transform(gl : WebGL2RenderingContext, props: Object)

Constructs a Transform object. It then creates destination buffers if needed and binds the buffers to Model and TransformFeedback objects.

gl (WebGL2RenderingContext) gl - context
opts (Object={}) - options
- sourceBuffers (Object) - key and value pairs, where key is the name of vertex shader attribute and value is the corresponding Attribute, Buffer or attribute descriptor object.
- feedbackBuffers (Object, Optional) - key and value pairs, where key is the name of vertex shader attribute and value is the corresponding Buffer object.
- vs (String) - vertex shader string.
- varyings (Array) - Array of vertex shader varyings names.
- feedbackMap (Object, Optional) - key and value pairs, where key is a vertex shader attribute name and value is a vertex shader varying name.
- drawMode (GLEnum = gl.POINTS, Optional) - Draw mode to be set on Model and TransformFeedback objects during draw/render time.
- elementCount (Integer) - Number set to vertex count when rendering the model.

Notes:

Internally, creates Model and TransformFeedback instances

delete() : Transform

Deletes all owned resources, Model, TransformFeedback and any Buffer objects that are crated internally.

Methods

getBuffer(varyingName : String) : Buffer

Returns current destination buffer corresponding to given varying name.

varyingName (String) - varying name.

update(props) : Transform

Updates buffer bindings with provided buffer objects for one or more source or destination buffers.

props.sourceBuffers (Object) - key and value pairs, where key is the name of vertex shader attribute and value is the corresponding Attribute, Buffer or attribute descriptor object.
props.feedbackBuffers (Object, Optional) - key and value pairs, where key is the name of vertex shader varying and value is the corresponding Buffer object.
props.elementCount (Integer, Optional) - Number set to vertex count when rendering the model. If not supplied, the previously set element count is used.

run(uniforms : Object) : Transform

Performs one transform feedback iteration.

uniforms (Object = {}, Optional) - Sets uniforms before rendering.

swapBuffers() : Transform

Swaps source and destination buffers. If buffer swapping is used, sourceBuffers supplied to the constructor and/or the update method must be Buffer objects.

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