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Last Modifed Date: May 21, 2018
OpenGL ES 3.0 is required.
This extension provides a query mechanism that allows for non-blocking
device-to-host readback of GL buffer object contents.
In Chromium's multiprocess GL architecture, buffer readback
(MapBufferRange(MAP_READ_BIT)) may be accelerated by a shared-memory shadow
copy of a GL buffer object's memory. This shadow copy is allocated based on
the buffer allocation usage hint (*_READ). The contents of the shadow copy
are updated based on the usage of the
InvalidateReadbackBufferShadowDataCHROMIUM command. (The management of the
shadow copies is purely internal to the implementation, and not actually
exposed by this extension.)
When a buffer is read back, if such a shadow copy is up-to-date with the
contents of the real buffer, then the MapBufferRange operation can return
that shadow copy.
(1) a signal from client to service (EndQuery) that any previously written
*_READ buffers might be read back later, and that their shadow copies should
be updated.
(2) a signal from service to client (QUERY_RESULT_AVAILABLE) that it will no
longer use any of the shadow copies it received from the command buffer
client prior to the point in the command stream when the query was issued,
and that any previously written *_READ buffers' shadow copies have been
updated to mirror the contents of the buffers (at a point in the command
stream no earlier than the query was issued).
(The QUERY_RESULT is undefined.)
Except in the presence of transform feedback operations (see below), this
query, and the existence of shadow copies, affect only the performance of
the GL, and not its observable semantic behavior. The semantic behavior of
READBACK_SHADOW_COPIES_UPDATED_CHROMIUM is otherwise equivalent to that of
the COMMANDS_COMPLETED_CHROMIUM query from CHROMIUM_sync_query, except that
it is a distinct query target, and the QUERY_RESULT may return a different
When a buffer is written by any OpenGL ES 3.0 mechanism aside from transform
feedback - ReadPixels with a PIXEL_PACK_BUFFER bound, MapBufferRange with
MAP_WRITE_BIT, BufferData, BufferSubData, and CopyBufferSubData - the
buffer's shadow copy will be automatically invalidated.
If the buffer's shadow copy is invalidated by a transform feedback operation
writing to the buffer, the application must manually invalidate it via
New Procedures and Functions
The command
void InvalidateReadbackBufferShadowDataCHROMIUM(GLuint buffer_id)
manually invalidates the shadow copy associated with a given buffer,
preventing it from being returned for readbacks.
This is necessary only when a buffer is written by transform feedback; all
other OpenGL ES 3.0 mechanisms which write to the buffer will automatically
invalidate the shadow copy.
New Tokens
Accepted by the <target> parameter of BeginQuery, EndQuery, and GetQueryiv:
New State
Revision History
5/21/2018 Documented the extension