extensions/SGIS/SGIS_texture_color_mask.txt

Name

    SGIS_texture_color_mask

Name Strings

    GL_SGIS_texture_color_mask

Contact

    Jon Leech, SGI (ljp 'at' sgi.com)

Status

    Complete. Not shipping yet.

Version

    Last Modified Date: December 13, 2004
    Author Revision: $Header: //depot/main/doc/registry/extensions/SGI/texture_color_mask.spec#7 $

Number

    214

Dependencies

    ARB_texture_compression affects the definition of this extension.

Overview

    This extension implements the same functionality for texture
    updates that glColorMask implements for color buffer updates.
    Masks for updating textures with indexed internal formats
    (the analog for glIndexMask) should be supported by a separate extension.

    The extension allows an application to update a subset of
    components in an existing texture.	The masks are applied after
    all pixel transfer operations have been performed, immediately
    prior to writing the texel value into texture memory.  They
    apply to all texture updates.

IP Status

    No issues.

Issues

    * The functionality provided by this extension can already be
      performed by using the color mask when rendering to the frame
      buffer and then using CopyTexture to load the texture.

    * This functionality may be hard to achieve with compressed textures.
      It will require a read->decompress->modify->compress->write
      operation.  This may be slower than the DrawPixels/CopyTexture
      approach.

New Procedures and Functions

    void TextureColorMaskSGIS(boolean r, boolean g, boolean, b, boolean a );

New Tokens

    Accepted by the <pname> parameter of GetBooleanv, GetDoublev,
    GetIntegerv, and GetFloatv:

	TEXTURE_COLOR_WRITEMASK_SGIS		= 0x81EF

Additions to Chapter 2 of the OpenGL 1.1 Specification (OpenGL Operation)

    None

Additions to Chapter 3 of the OpenGL 1.1 Specification (Rasterization)

    Before the section of Texture Parameters, add a section on
    texture download masking.

    Fine Control of Texture Image Updates

    A set of masks can be used to mask the writing of bits into
    textures. These masks are applied after all pixel transfer
    operations have been performed, immediately before the bits are
    written into the texture.

    If the texture image being written is compressed, either in one of
    the generic compressed formats defined by the base
    ARB_texture_compression extension or in a compressed format defined
    by an extension layering on ARB_texture_compression, then the
    texture color mask is not used when writing that texture.

    The command

	void TextureColorMaskSGIS( boolean r, boolean g, boolean, b,
	     boolean a );

    is used to control updates to texture images with non-indexed
    internal formats.

    For an RGBA texture image, TextureColorMaskSGIS is used
    to mask the writing of R, G, B, and A values to the texture image.
    The r, g, b, and a parameters indicate whether R, G, B,
    and A values, respectively, are written.  A value of TRUE
    means the component is written; a value of FALSE means it is not.

    For Luminance or Intensity texture images, the r parameter
    indicates whether the Luminance or Intensity component
    is written.  In this case, the g, b, and a parameters are
    ignored.

    In the initial state, all color masks are enabled for writing.

    If a color mask is in effect when a texture image is being
    defined for the first time, the values of the components whose
    updates have been disabled are undefined.


Additions to Chapter 4 of the OpenGL 1.1 Specification (Per-Fragment Operations
and the Frame Buffer)

    None

Additions to Chapter 5 of the OpenGL 1.1 Specification (Special Functions)

    None

Additions to Chapter 6 of the OpenGL OpenGL 1.1 Specification (State and State Requests)

    The mask values are queried using GetIntegerv with <pname> set to
    TEXTURE_COLOR_WRITEMASK_SGIS.

    The masks are saved and restored when PushAttrib and PopAttrib
    are called with TEXTURE_BIT set. They are included in the part of
    texture state that is associated with the currently bound texture
    objects.

Additions to the GLX Specification

    None

GLX Protocol

    A new GL rendering command is added to support
    TextureColorMaskSGIS(). The following command is sent to the server
    as part of a glXRender request:

	TextureColorMaskSGIS
	    2		8		rendering command length
	    2		2082		rendering command opcode
	    1		BOOL		r
	    1		BOOL		g
	    1		BOOL		b
	    1		BOOL		a

Errors

    INVALID_OPERATION is generated if TextureColorMaskSGIS is executed
    between the execution of Begin and the corresponding execution of
    End.

New State

(table 6.?, p. ???)

    Get Value			    Type    Get Command     Initial Value   Description     Sec Attribute
    ---------			    ----    -----------     -------------   -----------     --- ---------
    TEXTURE_COLOR_WRITEMASK_SGIS    4xB     GetIntegerv     (T,T,T,T)	    Texture color   3.? texture
									    writemask

Revision History

  * Version 6, 2004/12/13 - there was an error listed in the GLX
    protocol (length 20, using 4 byte BOOL fields). In actuality the
    generated protocol in use on IRIX, the development platform for this
    extension, is 8 bytes long using single byte BOOLs. Thanks to Ian
    Romanick for catching this.
  * Version 5, 2001/03/14 - defined interaction with compressed textures
    (that the write mask is ignored).