mesa: add new signed rgba texture format
This is a (partial) backport of the signed texture format support in OGL 3.1.
Since it wasn't promoted from an existing extension roll our own.
diff --git a/docs/MESA_texture_signed_rgba.spec b/docs/MESA_texture_signed_rgba.spec
new file mode 100644
index 0000000..49c8e9e
--- /dev/null
+++ b/docs/MESA_texture_signed_rgba.spec
@@ -0,0 +1,214 @@
+Name
+
+ MESA_texture_signed_rgba
+
+Name Strings
+
+ GL_MESA_texture_signed_rgba
+
+Contact
+
+
+
+Notice
+
+
+
+IP Status
+
+ No known IP issues
+
+Status
+
+
+
+Version
+
+ 0.3, 2009-03-24
+
+Number
+
+ Not assigned ?
+
+Dependencies
+
+ Written based on the wording of the OpenGL 2.0 specification.
+
+ This extension trivially interacts with ARB_texture_float.
+ This extension shares some language with ARB_texture_compression_rgtc
+ but does not depend on it.
+
+Overview
+
+ OpenGL prior to 3.1 does not support any signed texture formats.
+ ARB_texture_compression_rgtc introduces some compressed red and
+ red_green signed formats but no uncompressed ones, which might
+ still be useful. NV_texture_shader adds signed texture formats,
+ but also a lot of functionality which has been superceded by fragment
+ shaders.
+ It is usually possible to get the same functionality
+ using a unsigned format by doing scale and bias in a shader, but this
+ is undesirable since modern hardware has direct support for this.
+ This extension adds a signed 4-channel texture format by backporting
+ the relevant features from OpenGL 3.1, as a means to support this in
+ OpenGL implementations only supporting older versions.
+
+Issues
+
+ 1) What should this extension be called?
+
+ RESOLVED: MESA_texture_signed_rgba seems reasonable.
+ The rgba part is there because only 4 channel format is supported.
+
+
+ 2) Should the full set of signed formats (alpha, luminance, rgb, etc.)
+ be supported?
+
+ RESOLVED: NO. To keep this extension simple, only add the most
+ universal format, rgba. alpha/luminance can't be trivially supported
+ since OpenGL 3.1 does not support them any longer, and there is some
+ implied dependency on ARB_texture_rg for red/red_green formats so
+ avoid all this. Likewise, only 8 bits per channel is supported.
+
+
+ 3) Should this extension use new enums for the texture formats?
+
+ RESOLVED: NO. Same enums as those used in OpenGL 3.1.
+
+
+ 4) How are signed integer values mapped to floating-point values?
+
+ RESOLVED: Same as described in issue 5) of
+ ARB_texture_compression_rgtc (quote):
+ A signed 8-bit two's complement value X is computed to
+ a floating-point value Xf with the formula:
+
+ { X / 127.0, X > -128
+ Xf = {
+ { -1.0, X == -128
+
+ This conversion means -1, 0, and +1 are all exactly representable,
+ however -128 and -127 both map to -1.0. Mapping -128 to -1.0
+ avoids the numerical awkwardness of have a representable value
+ slightly more negative than -1.0.
+
+ This conversion is intentionally NOT the "byte" conversion listed
+ in Table 2.9 for component conversions. That conversion says:
+
+ Xf = (2*X + 1) / 255.0
+
+ The Table 2.9 conversion is incapable of exactly representing
+ zero.
+
+ (Difference to ARB_texture_compression_rgtc):
+ This is the same mapping as OpenGL 3.1 uses.
+ This is also different to what NV_texture_shader used.
+ The above mapping should be considered the reference, but there
+ is some leeway so other mappings are allowed for implementations which
+ cannot do this. Particulary the mapping given in NV_texture_shader or
+ the standard OpenGL byte/float mapping is considered acceptable too, as
+ might be a mapping which represents -1.0 by -128, 0.0 by 0 and 1.0 by
+ 127 (that is, uses different scale factors for negative and positive
+ numbers).
+ Also, it is ok to store incoming GL_BYTE user data as-is, without
+ converting to GL_FLOAT (using the standard OpenGL float/byte mapping)
+ and converting back (using the mapping described here).
+ Other than those subtle issues there are no other non-standard
+ conversions used, so when using for instance CopyTexImage2D with
+ a framebuffer clamped to [0,1] all converted numbers will be in the range
+ [0, 127] (and not scaled and biased).
+
+
+ 5) How will signed components resulting from RGBA8_SNORM texture
+ fetches interact with fragment coloring?
+
+ RESOLVED: Same as described in issue 6) of
+ ARB_texture_compression_rgtc (quote):
+ The specification language for this extension is silent
+ about clamping behavior leaving this to the core specification
+ and other extensions. The clamping or lack of clamping is left
+ to the core specification and other extensions.
+
+ For assembly program extensions supporting texture fetches
+ (ARB_fragment_program, NV_fragment_program, NV_vertex_program3,
+ etc.) or the OpenGL Shading Language, these signed formats will
+ appear as expected with unclamped signed components as a result
+ of a texture fetch instruction.
+
+ If ARB_color_buffer_float is supported, its clamping controls
+ will apply.
+
+ NV_texture_shader extension, if supported, adds support for
+ fixed-point textures with signed components and relaxed the
+ fixed-function texture environment clamping appropriately. If the
+ NV_texture_shader extension is supported, its specified behavior
+ for the texture environment applies where intermediate values
+ are clamped to [-1,1] unless stated otherwise as in the case
+ of explicitly clamped to [0,1] for GL_COMBINE. or clamping the
+ linear interpolation weight to [0,1] for GL_DECAL and GL_BLEND.
+
+ Otherwise, the conventional core texture environment clamps
+ incoming, intermediate, and output color components to [0,1].
+
+ This implies that the conventional texture environment
+ functionality of unextended OpenGL 1.5 or OpenGL 2.0 without
+ using GLSL (and with none of the extensions referred to above)
+ is unable to make proper use of the signed texture formats added
+ by this extension because the conventional texture environment
+ requires texture source colors to be clamped to [0,1]. Texture
+ filtering of these signed formats would be still signed, but
+ negative values generated post-filtering would be clamped to
+ zero by the core texture environment functionality. The
+ expectation is clearly that this extension would be co-implemented
+ with one of the previously referred to extensions or used with
+ GLSL for the new signed formats to be useful.
+
+
+ 6) Should the RGBA_SNORM tokens also be accepted by CopyTexImage
+ functions?
+
+ RESOLVED: YES.
+
+
+ 7) What to do with GetTexParameter if ARB_texture_float is supported,
+ in particular what datatype should this return for TEXTURE_RED_TYPE_ARB,
+ TEXTURE_GREEN_TYPE_ARB, TEXTURE_BLUE_TYPE_ARB, TEXTURE_ALPHA_TYPE_ARB?
+
+ RESOLVED: ARB_texture_float states type is either NONE,
+ UNSIGNED_NORMALIZED_ARB, or FLOAT. This extension adds a new enum,
+ SIGNED_NORMALIZED, which will be returned accordingly. This is the
+ same behaviour as in OpenGL 3.1.
+
+
+New Tokens
+
+
+ Accepted by the <internalformat> parameter of
+ TexImage1D, TexImage2D, TexImage3D, CopyTexImage1D, and CopyTexImage2D:
+
+ RGBA_SNORM 0x8F93
+ RGBA8_SNORM 0x8F97
+
+ Returned by the <params> parameter of GetTexLevelParameter:
+
+ SIGNED_NORMALIZED 0x8F9C
+
+
+Additions to Chapter 3 of the OpenGL 2.0 Specification (Rasterization):
+
+ -- Section 3.8.1, Texture Image Specification
+
+ Add to Table 3.16 (page 154): Sized internal formats
+
+ Sized Base R G B A L I D
+ Internal Format Internal Format bits bits bits bits bits bits bits
+ --------------- --------------- ---- ---- ---- ---- ---- ---- ----
+ RGBA8_SNORM RGBA 8 8 8 8 0 0 0
+
+
+Dependencies on ARB_texture_float extension:
+
+ If ARB_texture_float is supported, GetTexParameter queries with <value>
+ of TEXTURE_RED_TYPE_ARB, TEXTURE_GREEN_TYPE_ARB, TEXTURE_BLUE_TYPE_ARB or
+ TEXTURE_ALPHA_TYPE_ARB return SIGNED_NORMALIZED if
+ the base internal format is RGBA_SNORM.
diff --git a/docs/extensions.html b/docs/extensions.html
index dbb8eba..91ed20e 100644
--- a/docs/extensions.html
+++ b/docs/extensions.html
@@ -24,6 +24,7 @@
<LI><A HREF="MESA_resize_buffers.spec">MESA_resize_buffers.spec</A>
<LI><A HREF="MESA_set_3dfx_mode.spec">MESA_set_3dfx_mode.spec</A>
<LI><A HREF="MESA_sprite_point.spec">MESA_sprite_point.spec</A> (obsolete)
+<LI><A HREF="MESA_texture_signed_rgba.spec">MESA_texture_signed_rgba.spec</A>
<LI><A HREF="MESA_trace.spec">MESA_trace.spec</A> (obsolete)
<LI><A HREF="MESA_window_pos.spec">MESA_window_pos.spec</A>
<LI><A HREF="MESA_ycbcr_texture.spec">MESA_ycbcr_texture.spec</A>