extensions/SGIX/SGIX_interlace.txt

Name

    SGIX_interlace

Name Strings

    GL_SGIX_interlace

Version

    $Date: 1995/02/22 20:58:22 $ $Revision: 1.2 $

Number

    45

Dependencies

    None.

Overview

    This extension provides a way to interlace rows of pixels when
    rasterizing pixel rectangles, and loading texture images.  In this
    context, interlacing means skiping over rows of pixels or texels
    in the destination.  This is useful for dealing with video data
    since a single frame of video is typically composed from two images
    or fields: one image specifying the data for even rows of the frame
    and the other image specifying the data for odd rows of the frame.

New Procedures and Functions

    None.

New Tokens

    Accepted by the <cap> parameter of of Enable, Disable, IsEnabled,
    and by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv,
    GetDoublev:

	INTERLACE_SGIX				0x8094

Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)

    None.

Additions to Chapter 3 of the 1.0 Specification (Rasterization)

    The specification of interlace operation is added the GL Specification
    in section 3.6.3 "Rasterization of Pixel Rectangles", immediately
    following the operations described by EXT_histogram, and immediately
    prior to the subsection "Final Conversion".

    The interlacing of rows of pixels is enabled or disabled with Enable or
    Disable using the symbolic constant INTERLACE_SGIX.  If INTERLACE_SGIX
    is enabled, then all of the groups which belong to a row m are treated
    as if they belonged to the row 2 * m.  If the source image has a height
    of h rows, this effectively expands the height of the image to 2 * h - 1
    rows.  After interlacing, only every other row of the image is defined.
    If the interlaced pixel rectangle is rasterized to the framebuffer, then
    only these rows are converted to fragments.  If the interlaced pixel
    rectangle is a texture image, then only these rows are written to texure
    memory.

    In cases where errors can result from the specification of invalid
    image dimensions, it is the resulting dimensions that are tested, not
    the dimensions of the source image.  (A specific example is TexImage2D,
    which specifies constraints for image dimensions.  Even if TexImage2D
    is called with a null pixel pointer, the dimensions of the resulting
    texture image are those that would result from the effective expansion
    of the specified image due to interlacing.)

Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Frame Buffer)

    In section 4.3 "Drawing, Reading, and Copying Pixels"

    INTERLACE_SGIX has no effect on the operation of ReadPixels.

Additions to Chapter 5 of the 1.0 Specification (Special Functions)

    None.

Additions to Chapter 6 of the 1.0 Specification (State and State Requests)

    None.

Additions to the GLX Specification

    None.

Errors

    None.

New State

    Get Value				Get Command			Type		Initial Value		Attribute
    ---------				-----------			----		-------------		---------
    INTERLACE_SGIX			IsEnabled			B		False			pixel/enable

New Implementation Dependent State

    None.