luma.gl/docs/api-reference/webgl/transform-feedback.md

TransformFeedback (WebGL2)

TransformFeedback objects holds state needed to perform transform feedback operations. They store the buffer bindings that are being recorded to. This makes it easy to switch between different sets of feedback buffer bindings (somewhat similar to how VertexArrayObjects hold input vertex buffers.

The state managed by TransformFeedback objects includes the buffers the GPU will use to record the requested varyings.

When TransformFeedback objects must be "activated" (TransformFeedback.begin) before it can be used. There a number of caveats to be aware of when manually managing TransformFeedback object activation, see the remarks. For this reason, luma.gl Program.draw call takes an optional TransformFeedback object as a parameter and activates and deactivates it before and after the draw call.

Finally, note that when using transform feedback it is frequently desirable to turn off rasterization: gl.enable(GL.RASTERIZER_DISCARD) to prevent the fragment shader from running.

For more information, see OpenGL Wiki.

Usage

Setting up a model object for transform feedback.

const model = new Model(gl, {
  vs,
  fs,
  varyings: ['gl_Position', 'outputColor'],
  ...
});

Setting up a transform feedback object and binding buffers

const transformFeedback = new TransformFeedback(gl)
  .bindBuffer({index: 0, bufferPosition, })
  .bindBuffer({index: 1, bufferColor, });

When binding the buffers, index should be equal to the corresponding varying entry in varyings array passed to Program constructor.

Buffers can also be bound using varying name and varyingMap that can be retrieved from Model object.

const transformFeedback = new TransformFeedback(gl, {
  buffers: {
    outputColor: bufferColor,
    gl_Position: bufferPosition
  },
  varyingMap: model.varyingMap
});

Running program (drawing) with implicit activation of transform feedback (will call begin and end on supplied transformFeedback)

model.draw({
  drawMode,
  vertexCount,
  ...,
  transformFeedback
});

Running program (drawing) with explicit activation of transform feedback

transformFeedback.begin();
model.draw({...});
transformFeedback.end();

Turning off rasterization

const parameters = {[GL.RASTERIZER_DISCARD]: true}
model.draw({..., transformFeedback, parameters});

Methods

constructor

• gl - (WebGL2RenderingContext) gl - context
• buffers - buffers that gets bound to TRANSFORM_FEEDBACK_BUFFER target for recording vertex shader outputs.
• varyingMap - Object mapping varying name to buffer index it needs to be bound.

WebGL APIs gl.createTransformFeedback

delete

WebGL APIS gl.deleteTransformFeedback

bindBuffer

WebGL APIs gl.bindBufferBase, gl.bindBufferRange

begin(primitiveMode)

Activates transform feedback using the buffer bindings in this TransformFeedback object. Buffers can not be accessed until TransformFeedback.end or TransformFeedback.pause have been called.

Buffers can not be changed until TransformFeedback.end or has been called.

Doing anything which reads from or writes to any part of these buffers (outside of feedback writes, of course).

Reallocating storage for any of these buffers. This includes invalidation.

begin(primitiveMode)

• primitiveMode (GLenum) -

returns (TransformFeedback) - returns self to enable chaining

Transform Feedback primitiveMode	Compatible Draw Modes
GL.POINTS	GL.POINTS
GL.LINES	GL.LINES, GL.LINE_LOOP, GL.LINE_STRIP
GL.TRIANGLES	GL.TRIANGLES, GL.TRIANGLE_STRIP, GL.TRIANGLE_FAN

WebGL APIs gl.beginTransformFeedback

pause()

Pauses transform feedback operations. When paused, transform feedback is still considered active and changing most transform feedback state related to the object results in an error. However, a new transform feedback object may be bound while transform feedback is paused.

The current program can be changed and so forth. Feedback operations can be paused indefinitely, and it is legal to read from buffers that are in a paused feedback operation (though you need to unbind the feedback object first).

Returns (TransformFeedback) - returns self to enable chaining

WebGL APIs gl.pauseTransformFeedback

resume()

Resumes TranformFeedback

Bind the exact program that was used when TransformFeedback.begin was called.

returns (TransformFeedback) - returns self to enable chaining

WebGL APIs gl.resumeTransformFeedback

returns (TransformFeedback) - returns self to enable chaining

end()

returns (TransformFeedback) - returns self to enable chaining

WebGL APIs gl.endTransformFeedback

See also

• Program constructor - varyings argument to specify which
• Program.varyings - contains a map of the indices of

Parameters

None

Limits

Limit	Value	Description
GL.MAX_TRANSFORM_FEEDBACK_SEPARATE_ATTRIBS	>=4	total number of variables that can be captured }
GL.MAX_TRANSFORM_FEEDBACK_SEPARATE_COMPONENTS	>=4	number of components that any particular variable can contain
GL.MAX_TRANSFORM_FEEDBACK_INTERLEAVED_COMPONENTS	>= 64	total number of components in interleaved capture
GL.MAX_TRANSFORM_FEEDBACK_BUFFERS	TBD	Advanced interleaving total number of buffers

Remarks

• All of the indexed GL.TRANSFORM_FEEDBACK_BUFFER bindings. So all calls to Buffer.bindBase or Buffer.bindRange with {target: GL.TRANSFORM_FEEDBACK_BUFFER, ...} will attach the given region of the buffer to the currently bound feedback object.
• Whether the transform feedback is active and/or paused.

About TransformFeedback activation caveats

• When activated, TransformFeedback are coupled to the "current" Program
• Note that a started and unpaused TransformFeedback prevents the app from changing or re-linking the current program. So for instance, Program.use (gl.useProgram) cannot be called.