KhronosGroup/WebGL

Clarify OffscreenCanvas.transferToImageBitmap and preserveDrawingBuffer

Opened this issue · 5 comments

kkinnunen-apple commented

Clarify OffscreenCanvas.transferToImageBitmap and preserveDrawingBuffer

It mandates that color buffer is cleared.
Are depth and stencil buffers preserved or cleared?

The transferToImageBitmap() method, when invoked, must run the following steps:
....

Set this OffscreenCanvas object's bitmap to reference a newly created bitmap of the same dimensions and color space as the previous bitmap, and with its pixels initialized to [transparent black>(https://drafts.csswg.org/css-color/#transparent-black), or opaque black if the rendering context's alpha flag is set to false.

This means that if the rendering context of this OffscreenCanvas is a WebGLRenderingContext, the value of preserveDrawingBuffer will have no effect. [WEBGL]

kenrussell commented

I think the depth and stencil buffers, if they exist, should be cleared to the default values defined in https://registry.khronos.org/webgl/specs/latest/1.0/#2.2 . What do you and others think?

kkinnunen-apple commented

The depth, stencil and color buffers should be cleared if preserveDrawingBuffer == false.
The depth, stencil and color buffers should be preserved if preserveDrawingBuffer == true.

Otherwise transferToImageBitmap + Placeholder Canvas + preserveDrawingBuffer == true seems hard to implement:

  • A early transferToImageBitmap would clear
  • A later commit to placeholder canvas would preserve
  • --> code needs to support both
kdashg commented

I agree, clearing implies clearing depth-stencil as well.

kdashg commented

WebGL meeting 2024-03-21:

  • KR: Kimmo, could you write a test for this?
  • KK: If someone was relying on the previous spec - we're going to say that you should change your code
  • KG: thought this was just a clarification. Thought depth/stencil was cleared if and only if the color buffer was cleared.
  • KK: clarity is around depth/stencil buffer. Not mentioned at all. Other thing to clarify - how does preserveDrawingBuffer affect all the buffers?
  • Today, most implementations clear upon transferToImageBitmap regardless of preserveDrawingBuffer state
  • KG: if you want to change the preserveDrawingBuffer semantics, that's not a clarification. But if you want to state that depth/stencil are cleared, that's a clarification.
  • Agreed:
    • We'll clear depth/stencil if and only if color is cleared
  • AIs:
    • KG to write the spec change
    • KK to write the test