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Name
ANGLE_texture_usage
Name Strings
GL_ANGLE_texture_usage
Contributors
Nicolas Capens, TransGaming
Daniel Koch, TransGaming
Contact
Daniel Koch, TransGaming (daniel 'at' transgaming.com)
Status
Complete
Version
Last Modified Date: November 10, 2011
Version: 2
Number
OpenGL ES Extension #112
Dependencies
This extension is written against the OpenGL ES 2.0 Specification.
Overview
In some implementations it is advantageous to know the expected
usage of a texture before the backing storage for it is allocated.
This can help to inform the implementation's choice of format
and type of memory used for the allocation. If the usage is not
known in advance, the implementation essentially has to make a
guess as to how it will be used. If it is later proven wrong,
it may need to perform costly re-allocations and/or reformatting
of the texture data, resulting in reduced performance.
This extension adds a texture usage flag that is specified via
the TEXTURE_USAGE_ANGLE TexParameter. This can be used to
indicate that the application knows that this texture will be
used for rendering.
IP Status
No known IP claims.
New Procedures and Functions
None
New Tokens
Accepted as a value for <pname> for the TexParameter{if} and
TexParameter{if}v commands and for the <value> parameter of
GetTexParameter{if}v:
TEXTURE_USAGE_ANGLE 0x93A2
Accepted as a value to <param> for the TexParameter{if} and
to <params> for the TexParameter{if}v commands with a <pname> of
TEXTURE_USAGE_ANGLE; returned as possible values for <data> when
GetTexParameter{if}v is queried with a <value> of TEXTURE_USAGE_ANGLE:
NONE 0x0000
FRAMEBUFFER_ATTACHMENT_ANGLE 0x93A3
Additions to Chapter 2 of the OpenGL ES 2.0 Specification (OpenGL ES Operation)
None
Additions to Chapter 3 of the OpenGL ES 2.0 Specification (Rasterization)
Add a new row to Table 3.10 (Texture parameters and their values):
Name | Type | Legal Values
------------------------------------------------------------
TEXTURE_USAGE_ANGLE | enum | NONE, FRAMEBUFFER_ATTACHMENT_ANGLE
Add a new section 3.7.x (Texture Usage) before section 3.7.12 and
renumber the subsequent sections:
"3.7.x Texture Usage
Texture usage can be specified via the TEXTURE_USAGE_ANGLE value
for the <pname> argument to TexParameter{if}[v]. In order to take effect,
the texture usage must be specified before the texture contents are
defined either via TexImage2D or TexStorage2DEXT.
The usage values can impact the layout and type of memory used for the
texture data. Specifying incorrect usage values may result in reduced
functionality and/or significantly degraded performance.
Possible values for <params> when <pname> is TEXTURE_USAGE_ANGLE are:
NONE - the default. No particular usage has been specified and it is
up to the implementation to determine the usage of the texture.
Leaving the usage unspecified means that the implementation may
have to reallocate the texture data as the texture is used in
various ways.
FRAMEBUFFER_ATTACHMENT_ANGLE - this texture will be attached to a
framebuffer object and used as a desination for rendering or blits."
Modify section 3.7.12 (Texture State) and place the last 3 sentences
with the following:
"Next, there are the three sets of texture properties; each consists of
the selected minification and magnification filters, the wrap modes for
<s> and <t>, and the usage flags. In the initial state, the value assigned
to TEXTURE_MIN_FILTER is NEAREST_MIPMAP_LINEAR, and the value for
TEXTURE_MAG_FILTER is LINEAR. <s> and <t> wrap modes are both set to
REPEAT. The initial value for TEXTURE_USAGE_ANGLE is NONE."
Additions to Chapter 4 of the OpenGL ES 2.0 Specification (Per-Fragment
Operations and the Framebuffer)
None
Additions to Chapter 5 of the OpenGL ES 2.0 Specification (Special
Functions):
None
Additions to Chapter 6 of the OpenGL ES 2.0 Specification (State and
State Requests)
None
Dependencies on EXT_texture_storage
If EXT_texture_storage is not supported, omit any references to
TexStorage2DEXT.
Errors
If TexParameter{if} or TexParamter{if}v is called with a <pname>
of TEXTURE_USAGE_ANGLE and the value of <param> or <params> is not
NONE or FRAMEBUFFER_ATTACHMENT_ANGLE the error INVALID_VALUE is
generated.
Usage Example
/* create and bind texture */
glGenTextures(1, &texture);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture);
/* specify texture parameters */
glTexParameteri(GL_TEXTURE_2D, GL_*, ...); /* as before */
/* specify that we'll be rendering to the texture */
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_USAGE_ANGLE, GL_FRAMEBUFFER_ATTACHMENT_ANGLE);
glTexStorage2DEXT(GL_TEXTURE_2D, levels, ...); // Allocation
for(int level = 0; level < levels; ++level)
glTexSubImage2D(GL_TEXTURE_2D, level, ...); // Initialisation
Issues
1. Should there be a dynamic usage value?
DISCUSSION: We could accept a dynamic flag to indicate that a texture will
be updated frequently. We could map this to D3D9 dynamic textures. This would
allow us to avoid creating temporary surfaces when updating the texture.
However renderable textures cannot be dynamic in D3D9, which eliminates the
primary use case for this. Furthermore, the memory usage of dynamic textures
typically increases threefold when you lock it.
2. Should the texture usage be an enum or a bitfield?
UNRESOLVED. Using a bitfield would allow combination of values to be specified.
On the other hand, if combinations are really required, additional <pnames>
could be added as necessary. Querying a bitfield via the GetTexParameter command
feels a bit odd.
3. What should happen if TEXTURE_USAGE_ANGLE is set/changed after the texture
contents have been specified?
RESOLVED: It will have no effect. However, if the texture is redefined (for
example by TexImage2D) the new allocation will use the updated usage.
GetTexParameter is used to query the value of the TEXTURE_USAGE_ANGLE
state that was last set by TexParameter for the currently bound texture, or
the default value if it has never been set. There is no way to determine the
usage that was in effect at the time the texture was defined.
Revision History
Rev. Date Author Changes
---- ----------- --------- ----------------------------------------
1 10 Nov 2011 dgkoch Initial revision
2 10 Nov 2011 dgkoch Add overview
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