docs/MESA_texture_signed_rgba.spec

   1 Name
   2
   3     MESA_texture_signed_rgba
   4
   5 Name Strings
   6
   7     GL_MESA_texture_signed_rgba
   8
   9 Contact
  10
  11
  12
  13 Notice
  14
  15
  16
  17 IP Status
  18
  19     No known IP issues
  20
  21 Status
  22
  23
  24
  25 Version
  26
  27     0.3, 2009-03-24
  28
  29 Number
  30
  31     Not assigned ?
  32
  33 Dependencies
  34
  35     Written based on the wording of the OpenGL 2.0 specification.
  36
  37     This extension trivially interacts with ARB_texture_float.
  38     This extension shares some language with ARB_texture_compression_rgtc
  39     but does not depend on it.
  40
  41 Overview
  42
  43     OpenGL prior to 3.1 does not support any signed texture formats.
  44     ARB_texture_compression_rgtc introduces some compressed red and
  45     red_green signed formats but no uncompressed ones, which might
  46     still be useful. NV_texture_shader adds signed texture formats,
  47     but also a lot of functionality which has been superceded by fragment
  48     shaders.
  49     It is usually possible to get the same functionality
  50     using a unsigned format by doing scale and bias in a shader, but this
  51     is undesirable since modern hardware has direct support for this.
  52     This extension adds a signed 4-channel texture format by backporting
  53     the relevant features from OpenGL 3.1, as a means to support this in
  54     OpenGL implementations only supporting older versions.
  55
  56 Issues
  57
  58     1) What should this extension be called?
  59
  60        RESOLVED: MESA_texture_signed_rgba seems reasonable.
  61        The rgba part is there because only 4 channel format is supported.
  62
  63
  64     2) Should the full set of signed formats (alpha, luminance, rgb, etc.)
  65        be supported?
  66
  67        RESOLVED: NO. To keep this extension simple, only add the most
  68        universal format, rgba. alpha/luminance can't be trivially supported
  69        since OpenGL 3.1 does not support them any longer, and there is some
  70        implied dependency on ARB_texture_rg for red/red_green formats so
  71        avoid all this. Likewise, only 8 bits per channel is supported.
  72
  73
  74     3) Should this extension use new enums for the texture formats?
  75
  76        RESOLVED: NO. Same enums as those used in OpenGL 3.1.
  77
  78
  79     4) How are signed integer values mapped to floating-point values?
  80
  81        RESOLVED: Same as described in issue 5) of
  82        ARB_texture_compression_rgtc (quote):
  83        A signed 8-bit two's complement value X is computed to
  84        a floating-point value Xf with the formula:
  85
  86                 { X / 127.0, X > -128
  87            Xf = {
  88                 { -1.0,      X == -128
  89
  90        This conversion means -1, 0, and +1 are all exactly representable,
  91        however -128 and -127 both map to -1.0.  Mapping -128 to -1.0
  92        avoids the numerical awkwardness of have a representable value
  93        slightly more negative than -1.0.
  94
  95        This conversion is intentionally NOT the "byte" conversion listed
  96        in Table 2.9 for component conversions.  That conversion says:
  97
  98            Xf = (2*X + 1) / 255.0
  99
 100        The Table 2.9 conversion is incapable of exactly representing
 101        zero.
 102
 103        (Difference to ARB_texture_compression_rgtc):
 104        This is the same mapping as OpenGL 3.1 uses.
 105        This is also different to what NV_texture_shader used.
 106        The above mapping should be considered the reference, but there
 107        is some leeway so other mappings are allowed for implementations which
 108        cannot do this. Particulary the mapping given in NV_texture_shader or
 109        the standard OpenGL byte/float mapping is considered acceptable too, as
 110        might be a mapping which represents -1.0 by -128, 0.0 by 0 and 1.0 by
 111        127 (that is, uses different scale factors for negative and positive
 112        numbers).
 113        Also, it is ok to store incoming GL_BYTE user data as-is, without
 114        converting to GL_FLOAT (using the standard OpenGL float/byte mapping)
 115        and converting back (using the mapping described here).
 116        Other than those subtle issues there are no other non-standard
 117        conversions used, so when using for instance CopyTexImage2D with
 118        a framebuffer clamped to [0,1] all converted numbers will be in the range
 119        [0, 127] (and not scaled and biased).
 120
 121
 122     5) How will signed components resulting from RGBA8_SNORM texture
 123        fetches interact with fragment coloring?
 124
 125        RESOLVED: Same as described in issue 6) of
 126        ARB_texture_compression_rgtc (quote):
 127        The specification language for this extension is silent
 128        about clamping behavior leaving this to the core specification
 129        and other extensions.  The clamping or lack of clamping is left
 130        to the core specification and other extensions.
 131
 132        For assembly program extensions supporting texture fetches
 133        (ARB_fragment_program, NV_fragment_program, NV_vertex_program3,
 134        etc.) or the OpenGL Shading Language, these signed formats will
 135        appear as expected with unclamped signed components as a result
 136        of a texture fetch instruction.
 137
 138        If ARB_color_buffer_float is supported, its clamping controls
 139        will apply.
 140
 141        NV_texture_shader extension, if supported, adds support for
 142        fixed-point textures with signed components and relaxed the
 143        fixed-function texture environment clamping appropriately.  If the
 144        NV_texture_shader extension is supported, its specified behavior
 145        for the texture environment applies where intermediate values
 146        are clamped to [-1,1] unless stated otherwise as in the case
 147        of explicitly clamped to [0,1] for GL_COMBINE.  or clamping the
 148        linear interpolation weight to [0,1] for GL_DECAL and GL_BLEND.
 149
 150        Otherwise, the conventional core texture environment clamps
 151        incoming, intermediate, and output color components to [0,1].
 152
 153        This implies that the conventional texture environment
 154        functionality of unextended OpenGL 1.5 or OpenGL 2.0 without
 155        using GLSL (and with none of the extensions referred to above)
 156        is unable to make proper use of the signed texture formats added
 157        by this extension because the conventional texture environment
 158        requires texture source colors to be clamped to [0,1].  Texture
 159        filtering of these signed formats would be still signed, but
 160        negative values generated post-filtering would be clamped to
 161        zero by the core texture environment functionality.  The
 162        expectation is clearly that this extension would be co-implemented
 163        with one of the previously referred to extensions or used with
 164        GLSL for the new signed formats to be useful.
 165
 166
 167     6) Should the RGBA_SNORM tokens also be accepted by CopyTexImage
 168        functions?
 169
 170        RESOLVED: YES.
 171
 172
 173     7) What to do with GetTexParameter if ARB_texture_float is supported,
 174        in particular what datatype should this return for TEXTURE_RED_TYPE_ARB,
 175        TEXTURE_GREEN_TYPE_ARB, TEXTURE_BLUE_TYPE_ARB, TEXTURE_ALPHA_TYPE_ARB?
 176
 177        RESOLVED: ARB_texture_float states type is either NONE,
 178        UNSIGNED_NORMALIZED_ARB, or FLOAT. This extension adds a new enum,
 179        SIGNED_NORMALIZED, which will be returned accordingly. This is the
 180        same behaviour as in OpenGL 3.1.
 181
 182
 183 New Tokens
 184
 185
 186     Accepted by the <internalformat> parameter of
 187     TexImage1D, TexImage2D, TexImage3D, CopyTexImage1D, and CopyTexImage2D:
 188
 189         RGBA_SNORM                                0x8F93
 190         RGBA8_SNORM                               0x8F97
 191
 192     Returned by the <params> parameter of GetTexLevelParameter:
 193
 194         SIGNED_NORMALIZED                         0x8F9C
 195
 196
 197 Additions to Chapter 3 of the OpenGL 2.0 Specification (Rasterization):
 198
 199  -- Section 3.8.1, Texture Image Specification
 200
 201     Add to Table 3.16 (page 154): Sized internal formats
 202
 203     Sized             Base             R    G    B    A    L    I    D
 204     Internal Format   Internal Format bits bits bits bits bits bits bits
 205     ---------------   --------------- ---- ---- ---- ---- ---- ---- ----
 206     RGBA8_SNORM       RGBA             8    8    8    8    0    0    0
 207
 208
 209 Dependencies on ARB_texture_float extension:
 210
 211     If ARB_texture_float is supported, GetTexParameter queries with <value>
 212     of TEXTURE_RED_TYPE_ARB, TEXTURE_GREEN_TYPE_ARB, TEXTURE_BLUE_TYPE_ARB or
 213     TEXTURE_ALPHA_TYPE_ARB return SIGNED_NORMALIZED if
 214     the base internal format is RGBA_SNORM.