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