EXT_texture_rg

GL_EXT_texture_rg

Contributors to ARB_texture_rg, on which this extension is based
Kyle Haughey
Richard Schreyer

Benj Lipchak, Apple (lipchak 'at' apple.com)

Complete

Date: July 22, 2011
Revision: 3

OpenGL ES Extension #103

Requires OpenGL ES 2.0.

Written based on the wording of the OpenGL ES 2.0.25 Full Specification
(November 2, 2010).

OES_texture_float affects the definition of this extension.

OES_texture_half_float affects the definition of this extension.

APPLE_framebuffer_multisample affects the definition of this extension.

Historically one- and two-component textures have been specified in OpenGL
ES using the luminance or luminance-alpha (L/LA) formats. With the advent
of programmable shaders and render-to-texture capabilities these legacy
formats carry some historical artifacts which are no longer useful.

For example, when sampling from such textures, the luminance values are
replicated across the color components. This is no longer necessary with
programmable shaders.

It is also desirable to be able to render to one- and two-component format
textures using capabilities such as framebuffer objects (FBO), but
rendering to L/LA formats is under-specified (specifically how to map
R/G/B/A values to L/A texture channels).

This extension adds new base internal formats for one-component RED and
two-component RG (red green) textures as well as sized RED and RG internal
formats for renderbuffers. The RED and RG texture formats can be used for
both texturing and rendering into with framebuffer objects.

None

Accepted by the <internalformat> parameter of TexImage2D and CopyTexImage2D,
and the <format> parameter of TexImage2D, TexSubImage2D, and ReadPixels:

    RED_EXT                 0x1903
    RG_EXT                  0x8227

Accepted by the <internalformat> parameter of RenderbufferStorage and
RenderbufferStorageMultisampleAPPLE:

    R8_EXT                  0x8229
    RG8_EXT                 0x822B

None

(Add the following to Table 3.3: "TexImage2D and ReadPixels formats")

Format Name     Element Meaning and Order      Target Buffer
-----------     -------------------------      -------------
RED_EXT         R                              Color
RG_EXT          R, G                           Color

(Add the following to Table 3.4: "Valid pixel format and type combinations")
(as modified by OES_texture_float and OES_texture_half_float)

Format          Type                           Bytes per Pixel
-----------     -------------------------      ---------------
RED_EXT         FLOAT                          4
RED_EXT         HALF_FLOAT_OES                 2
RED_EXT         UNSIGNED_BYTE                  1
RG_EXT          FLOAT                          8
RG_EXT          HALF_FLOAT_OES                 4
RG_EXT          UNSIGNED_BYTE                  2

(Add the following to Table 3.8: "Conversion from RGBA and depth pixel
components to internal texture")

Base Internal Format     RGBA       Internal Components
--------------------     ------     -------------------
RED_EXT                  R          R
RG_EXT                   R,G        R,G

(Modify Table 3.9: "CopyTexImage internal format/color buffer combinations")

                        Texture Format
Color Buffer      A  L  LA  R  RG  RGB  RGBA
------------      -  -  --  -  --  ---  ----
A                 X
R                    X      X
RG                   X      X  X
RGB                  X      X  X   X
RGBA              X  X  X   X  X   X    X

(Add the following to Table 3.12: "Correspondence of filtered texture
components to texture source color components")

Texture Base        Texture source color
Internal Format     C_s             A_s
---------------     -------------   ------
RED_EXT             (R_t, 0, 0)     1
RG_EXT              (R_t, G_t, 0)   1

In section 4.3.1 "Reading Pixels", subsection "Obtaining Pixels from the
Framebuffer", modify the last sentence to read:

"If the framebuffer does not support G, B, or A values then the G, B, and A
values that are obtained are 0.0, 0.0, and 1.0 respectively."

In section 4.4.5 "Framebuffer Completeness", modify the last sentence of
the second paragraph to read:

"Color-renderable formats contain red, and possibly green, blue, and alpha
components; depth-renderable formats contain depth components; and
stencil-renderable formats contain stencil components."

(Add the following to Table 4.5: "Renderbuffer image formats, showing their
renderable type (color-, depth-, or stencil-renderable) and the number of
bits each format contains for color (R, G, B, A), depth (D), and stencil
(S) components")

Sized Internal  Renderable Type   R bits G bits B bits A bits D bits S bits
Format
--------------  ----------------  ------ ------ ------ ------ ------ ------
R8_EXT        color-renderable  8
RG8_EXT       color-renderable  8      8

None

None

If OES_texture_float is not supported, then omit the rows of
Table 3.4 that have Type FLOAT.

If OES_texture_half_float is not supported, then omit the rows of 
Table 3.4 that have Type HALF_FLOAT_OES.

If APPLE_framebuffer_multisample is not supported, then all references to
RenderbufferStorageMultisampleAPPLE should be ignored.

#1 February 22, 2011, khaughey
    - initial version adapted from ARB_texture_rg.
#2 June 16, 2011, benj
    - add interaction with APPLE_framebuffer_multisample
#3 July 22, 2011, benj
    - rename from APPLE to EXT