blob: 01bd695c09f3f891e83777565772ccf8e2a164cb [file] [log] [blame]
Name Strings
Colin Sharp
Jeff Leger
Chuck Smith, Qualcomm (chucks 'at'
Maurice Ribble, Qualcomm (mribble 'at'
Copyright Qualcomm 2009.
IP Status
Qualcomm Proprietary.
Last Modified Date: August 20, 2009
Revision: #1.6
OpenGL ES Extension #70
OpenGL ES 1.0 or higher is required.
This extension interacts with QCOM_write_only_rendering.
This extension is written based on the wording of the OpenGL ES 2.0
In the handheld graphics space, a typical challenge is achieving efficient
rendering performance given the different characteristics of the various
types of graphics memory. Some types of memory ("slow" memory) are less
expensive but have low bandwidth, higher latency, and/or higher power
consumption, while other types ("fast" memory) are more expensive but have
higher bandwidth, lower latency, and/or lower power consumption. In many
cases, it is more efficient for a graphics processing unit (GPU) to render
directly to fast memory, but at most common display resolutions it is not
practical for a device to contain enough fast memory to accommodate both the
full color and depth/stencil buffers (the frame buffer). In some devices,
this problem can be addressed by providing both types of memory; a large
amount of slow memory that is sufficient to store the entire frame buffer,
and a small, dedicated amount of fast memory that allows the GPU to render
with optimal performance. The challenge lies in finding a way for the GPU
to render to fast memory when it is not large enough to contain the actual
frame buffer.
One approach to solving this problem is to design the GPU and/or driver
using a tiled rendering architecture. With this approach the render target
is subdivided into a number of individual tiles, which are sized to fit
within the available amount of fast memory. Under normal operation, the
entire scene will be rendered to each individual tile using a multi-pass
technique, in which primitives that lie entirely outside of the tile being
rendered are trivially discarded. After each tile has been rendered, its
contents are saved out to the actual frame buffer in slow memory (a process
referred to as the "resolve"). The resolve introduces significant overhead,
both for the CPU and the GPU. However, even with this additional overhead,
rendering using this method is usually more efficient than rendering
directly to slow memory.
This extension allows the application to specify a rectangular tile
rendering area and have full control over the resolves for that area. The
information given to the driver through this API can be used to perform
various optimizations in the driver and hardware. One example optimization
is being able to reduce the size or number of the resolves. Another
optimization might be to reduce the number of passes needed in the tiling
approach mentioned above. Even traditional rendering GPUs that don't use
tiles may benefit from this extension depending on their implemention of
certain common GPU operations.
One typical use case could involve an application only rendering to select
portions of the render target using this technique (which shall be referred
to as "application tiling"), leaving all other portions of the render target
untouched. Therefore, in order to preserve the contents of the untouched
portions of the render target, the application must request an EGL (or other
context management API) configuration with a non-destructive swap. A
destructive swap may only be used safely if the application renders to the
entire area of the render target during each frame (otherwise the contents
of the untouched portions of the frame buffer will be undefined).
Additionally, care must be taken to avoid the cost of mixing rendering with
and without application tiling within a single frame. Rendering without
application tiling ("normal" rendering) is most efficient when all of the
rendering for the entire scene can be encompassed within a single resolve.
If any portions of the scene are rendered prior to that resolve (such as via
a prior resolve, or via application tiling), then that resolve becomes much
more heavyweight. When this occurs, prior to rendering each tile the fast
memory must be populated with the existing contents of the frame buffer
region corresponding to that tile. This operation can double the cost of
resolves, so it is recommended that applications avoid mixing application
tiling and normal rendering within a single frame. If both rendering
methods must be used in the same frame, then the most efficient approach is
to perform all normal rendering first, followed by rendering done with
application tiling. An implicit resolve will occur (if needed) at the start
of application tiling, so any pending normal rendering operations will be
flushed at the time application tiling is initiated. This extension
provides interfaces for the application to communicate to the driver whether
or not rendering done with application tiling depends on the existing
contents of the specified tile, and whether or not the rendered contents of
the specified tile need to be preserved upon completion. This mechanism can
be used to obtain optimal performance, e.g. when the application knows that
every pixel in a tile will be completely rendered or when the resulting
contents of the depth/stencil buffers do not need to be preserved.
(1) How do Viewport and Scissor interact with this extension?
RESOLVED: They don't. When application tiling is used, the viewport and
scissor retain their existing values, relative to the render target, not the
specified tile. Therefore, all rendering commands issued between
StartTilingQCOM and EndTilingQCOM will be subject to the same scissor, and
will undergo the same viewport transformation, as normal rendering commands.
(2) How do Flush and Finish interact with this extension?
RESOLVED: When Flush or Finish is called while application tiling is
active, the behavior will be as if EndTilingQCOM was called, except that the
application tiling state will remain unchanged (meaning the active tile will
not be reset). This means that any pending rendering commands will be
performed to the active tile, and application tiling will continue to be
active for any following rendering commands.
(3) How does SwapBuffers interact with this extension?
RESOLVED: It doesn't. If SwapBuffers is called while application tiling is
active, the contents of the entire back buffer will be copied to the visible
window, ignoring the active tile. SwapBuffers will have no effect on the
application tiling state.
(4) What happens if the render target is changed while application tiling
is active?
RESOLVED: If the current render target is changed, either by binding a new
framebuffer object or changing the write surface of the active framebuffer
(either explicitly or by deleting the currently bound framebuffer or write
surface), an implicit EndTilingQCOM will occur. The active tile will be
reset and application tiling will be deactivated. This is necessary because
the active tile may not be valid for the new render target.
(5) Should this extension provide a query mechanism for determining things
such as tile offset, alignment, and size requirements so a developer
can intelligently choose tile regions?
RESOLVED: No. This information is very device-dependent and difficult to
present in an easily understood manner. Instead, this extension will let
developers specify an arbitrary rectangular tile region and all these
requirements, including subdividing the given tile into multiple tiles if
necessary, will be handled by the driver and hardware.
(6) Should this extension allow multiple tiles?
RESOLVED: No. While earlier versions of this extension allowed for this,
after support for arbitrary tile sizes was added the benefit of multiple
tiles became negligible. Allowing multiple tiles complicated the API and
made it much more difficult for traditional rendering and some tile-based
rendering GPUs to support this extension.
(7) Should multiple render targets be supported? They are not supported
by either the OpenGL ES core specification or any existing OpenGL ES
extensions. Support could be added with some new bitmasks for the
<preserveMask> parameter. Should this be added now, or deferred for
inclusion in any possible future MRT extension?
RESOLVED: Yes. It is not difficult to add now and doing it now makes
supporting MRTs in the future easier.
New Procedures and Functions
void StartTilingQCOM(uint x, uint y, uint width, uint height,
bitfield preserveMask);
void EndTilingQCOM(bitfield preserveMask);
New Tokens
Accepted by the <preserveMask> parameter of StartTilingQCOM and
Additions to Chapter 2 of the OpenGL ES 2.0 Specification (OpenGL Operation)
Add a new section "Rendering with Application Tiling" after section 2.13:
"2.14 Rendering with Application Tiling
The application may specify an arbitrary rectangular region (a 'tile') to
which rendering commands should be restricted.
The command
void StartTilingQCOM(uint x, uint y, uint width, uint height,
bitfield preserveMask);
specifies the tile described by <x>, <y>, <width>, <height>. Until the next
call to EndTilingQCOM, all rendering commands (including clears) will only
update the contents of the render target defined by the extents of this
tile. The parameters <x> and <y> specify the screen-space origin of the
tile, and <width> and <height> specify the screen-space width and height of
the tile. The tile origin is located at the lower left corner of the tile.
If the size of the tile is too large for the fast memory on the device then
it will be internally subdivided into multiple tiles. The parameter
<preserveMask> is the bitwise OR of a number of values indicating which
buffers need to be initialized with the existing contents of the frame
buffer region corresponding to the specified tile prior to rendering, or the
single value NONE. The values allowed are COLOR_BUFFER_BIT*_QCOM,
MULTISAMPLE_BUFFER_BIT*_QCOM. These indicate the color buffer, the depth
buffer, the stencil buffer, and a multisample buffer modifier, respectively.
The multisample bits are different since they modify the meaning of the
color, depth, and stencil bits if the active surface is a multisample
surface. If a multisample bit is set then the corresponding color, depth,
and/or stencil bit will cause all the samples to be copied across the memory
bus in devices that are using fast tiled memory, but if the multisample bit
is not set then only a single resolved sample is copied across the bus. In
practice, not setting the multisample bit when rendering to a multisample
buffer can greatly improve performance, but could cause small rendering
artifacts in some multiple-pass rendering algorithms. The 0-7 number is to
specify which render target is being used. If multiple render targets are
not being used then 0 should be specified. Any buffers specifed in
<preserveMask> that do not exist in the current rendering state will be
silently ignored (simlilar to the behavior of Clear). If NONE is specified,
then no buffers will be initialized. For any buffers not initialized in
this manner, the initial contents will be undefined.
The values of <x>, <y>, <width> and <height> are silently clamped to the
extents of the render target.
The command
void EndTilingQCOM(bitfield preserveMask);
notifies the driver that the application has completed all desired rendering
to the tile specified by StartTilingQCOM. This allows the driver to flush
the contents of the specified tile to the corresponding region of the render
target, and disables application tiling (resuming normal rendering). The
parameter <preserveMask> is specified using the same values as the
equivalent argument of StartTilingQCOM, but indicates which buffers need to
be preserved upon completion of all rendering commands issued with
application tiling. For any buffers not preserved in this manner, the
resulting contents of the buffer regions corresponding to the active tile
will be undefined.
GLX Protocol
INVALID_OPERATION error is generated if StartTilingQCOM is called while
WRITEONLY_RENDERING_QCOM is enabled or the current framebuffer is not
framebuffer complete
INVALID_OPERATION error is generated if EndTilingQCOM is called without a
corresponding call to StartTilingQCOM
INVALID_OPERATION error is generated if StartTilingQCOM is called after
calling StartTilingQCOM without a corresponding call to EndTilingQCOM
is called between StartTilingQCOM and EndTilingQCOM
New State
Sample Usage
GLboolean renderTiledTriangle(GLuint x, GLuint y, GLuint width, GLuint height)
// set the active tile and initialize the color and depth buffers with
// the existing contents
glStartTilingQCOM(x, y, width, height,
// draw the triangle
glDrawArrays(GL_TRIANGLES, 0, 3);
// finished with this tile -- preserve the color buffer
// return success
return GL_TRUE;
Revision History
#09 08/20/2009 Chuck Smith Cosmetic changes
#08 08/19/2009 Maurice Ribble Add support for multiple render targets
#07 07/28/2009 Maurice Ribble Clean up spec
Remove multiple tile support
#06 07/23/2009 Maurice Ribble Updated overview to match latest spec
#05 07/15/2009 Maurice Ribble Changed from spec to subdivide tiles
instead of returning out of memory
#04 07/06/2009 Maurice Ribble Update due to the AMD->Qualcomm move;
general extension cleanup.
#03 11/17/2008 Chuck Smith Clarified the results of EndTilingQCOM
for unpreserved buffers.
#02 11/10/2008 Chuck Smith Updates to clarify behavior; additions
to the Issues section.
#01 11/04/2008 Chuck Smith First draft.