MESA_tile_raster_order

GL_MESA_tile_raster_order

Eric Anholt, Broadcom (eric@anholt.net)

Proposal

Last modified date: 24 September 2017

OpenGL Extension #515
OpenGL ES Extension #292

GL_ARB_texture_barrier or GL_NV_texture_barrier is required.

This extension is written against the OpenGL 4.4 (Compatibility
Profile) specification, as modified by the GL_ARB_texture_barrier
extension.

This extension extends the sampling-from-the-framebuffer behavior provided
by GL_ARB_texture_barrier to allow setting the rasterization order of the
scene, so that overlapping blits can be implemented.  This can be used for
scrolling or window movement within in 2D scenes, without first copying to
a temporary.

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1.  Should this extension also affect BlitFramebuffer?

    NOT RESOLVED: BlitFramebuffer could use the same underlying
    functionality to provide defined results for 1:1 overlapping blits,
    but one could use the coordinates being copied to just produce the
    right result automatically, rather than requiring the state flags to
    be adjusted.

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None

Modify Section 9.3.1 Rendering Feedback Loops, p. 289

Replace the bullet point "If a texel has been written..." with:

  - A texel has been written, but it has been separated from this
    Draw call by the command:

      void TextureBarrier(void);

    TextureBarrier() will guarantee that writes have completed and
    caches have been invalidated before subsequent Draws are
    executed."

  - TILE_RASTER_ORDER_FIXED_MESA is enabled, and there is only a
    single write of each texel, and primitives are emitted in the
    order of TILE_RASTER_ORDER_INCREASING_X/Y_MESA (where those
    being disabled mean negative texel offsets), and reads are
    only performed from texels offset from the current fragment
    shader invocation in the direction specified by
    TILE_RASTER_ORDER_INCREASING_X/Y_MESA, e.g. using
    "texelFetch2D(sampler, ivec2(gl_FragCoord.xy + vec2(dx, dy)),
    0);".

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Get Value                            Type    Get Command    Initial Value Description                  Section   Attribute
-----------------------------------  ------  -------------  ------------- ---------------------------  --------  ------------
TILE_RASTER_ORDER_FIXED_MESA         B       IsEnabled      True          Tile rasterization order is  9.3.1     enable
                                                                          defined by
                                                                          TILE_RASTER_ORDER_INCREASING_*_MESA.
                                                                          in increasing X direction
TILE_RASTER_ORDER_INCREASING_X_MESA  B       IsEnabled      True          Tiles are rasterized         9.3.1     enable
                                                                          in increasing X direction
TILE_RASTER_ORDER_INCREASING_Y_MESA  B       IsEnabled      True          Tiles are rasterized         9.3.1     enable
                                                                          in increasing Y direction

26 July 2017 - Initial draft

24 September 2017 - Improved wording of the new specification
                    paragraph.  Adjust the ARB_texture_barrier
                    paragraph, to make it clear that it's not
                    required when in tile raster order mode.

2 October 2017 - Give it an ES extension number, mention
                 NV_texture_barrier for ES.

5 October 2017 - Mention what spec it's written against.