EXT_render_snorm

 GL_EXT_render_snorm

 Daniel Koch, NVIDIA
 Jan-Harald Fredriksen, ARM
 Mathias Heyer, NVIDIA

 Mathias Heyer, NVIDIA (mheyer [at] nvidia.com)

 Complete

 Last Modified Date:  2014-10-24
 Revision:            4

OpenGL ES Extension #206

 OpenGL ES 3.1 is required.

 This extension is written against the OpenGL ES 3.1 (June 4, 2014)
 specification.

 This extension interacts with EXT_texture_norm16.

 OpenGL ES 3.1 supports a variety of signed normalized texture and
 renderbuffer formats which are not color-renderable.

 This extension enables signed normalized texture and renderbuffer
 formats to be color-renderable.

 None

Accepted by the <type> parameter of ReadPixels

BYTE                            0x1400  // core OpenGL ES 3.1
SHORT                           0x1402  // core OpenGL ES 3.1

Accepted by the <internalFormat> parameter of RenderbufferStorage
and RenderbufferStorageMultisample:

R8_SNORM                        0x8F94  // core OpenGL ES 3.1
RG8_SNORM                       0x8F95  // core OpenGL ES 3.1
RGBA8_SNORM                     0x8F97  // core OpenGL ES 3.1
R16_SNORM_EXT                   0x8F98  // EXT_texture_norm16
RG16_SNORM_EXT                  0x8F99  // EXT_texture_norm16
RGBA16_SNORM_EXT                0x8F9B  // EXT_texture_norm16

Changes to Section 8.6 "Alternate Texture ImageSpecification Commands":

Modify INVALID_OPERATION errors section on CopyTexImage2D,
first bullet point from:

"if signed integer, unsigned integer, or fixed-point RGBA data is
required and the format of the current color buffer does not match
the required format."

to

"if FRAMEBUFFER_ATTACHMENT_COMPONENT_TYPE for the framebuffer
 attachment corresponding to the read buffer does not match
 the component type of the requested <internalformat>."


Change Table 8.13 "Correspondence of sized internal formats to base
internal formats,...":

    Sized            Base            Bits/component              CR   TF  Req.  Req.
    Internal         Internal        S are shared bits                    rend. tex.
    Format           Format          R     G    B    A    S
    ------------     -----------     ----  ---- ---- ---  ----   ---  --- ----  ---
    R8_SNORM         RED             s8                           X    X   X     X
    RG8_SNORM        RG              s8    s8                     X    X   X     X
    RGBA8_SNORM      RGBA            s8    s8   s8   s8           X    X   X     X
    R16_SNORM_EXT    RED             s16                          X    X   X     X
    RG16_SNORM_EXT   RG              s16   s16                    X    X   X     X
    RGBA16_SNORM_EXT RGBA            s16   s16  s16  s16          X    X   X     X

Change Table 8.15: "ReadPixels format and type used during CopyTex*."

Replace the first row with the following:

Read Buffer Format                    format    type
------------------------------------  ------    -------------
8bit Unsigned Normalized Fixed-point  RGBA      UNSIGNED_BYTE
8bit Signed Normalized Fixed-point    RGBA      BYTE
16bit Signed Normalized Fixed-point   RGBA      SHORT

Changes to Section 15.1.7 "Blending":

    Replace

    "The components of the source and destination values and blend
    factors are clamped to [0, 1] prior to evaluating the blend
    equation."

    with

    "If the color buffer is fixed-point, the components of the
    source and destination values and blend factors are each clamped
    to [0, 1] or [-1, 1] respectively for an unsigned normalized or
    signed normalized color buffer prior to evaluating the blend
    equation."

Changes to Section 15.1.7.1 "Blend Equation":

    Replace

    "Unsigned normalized fixed-point destination (framebuffer)
    components are represented as described in section 2.3.4."

    with

    "Normalized fixed-point destination (framebuffer) components are
    represented as described in section 2.3.4."

    Replace

    "Prior to blending, unsigned normalized fixed-point color
    components undergo an implied conversion to floating-point using
    equation 2.1."

    with

    "Prior to blending, unsigned and signed normalized fixed-point
    color components undergo an implied conversion to floating-point
    using equation 2.1 and 2.2, respectively."

Changes to Section 15.1.7.3 "Blend Color"

    Replace

    "If destination framebuffer components use an unsigned
    normalized fixed-point representation, the constant color
    components are clamped to the range [0, 1] when computing blend
    factors."

    with

    "If destination framebuffer components use an unsigned or
    signed normalized fixed-point representation, the constant color
    components are clamped to the range [0, 1] or [-1, 1],
    respectively, when computing blend factors."

Changes to Section 15.2.3 "Clearing the Buffers":

    Replace

    "Unsigned normalized fixed-point RGBA color buffers are cleared
    to color values derived by clamping each component of the clear
    color to the range [0, 1], then converting the (possibly sRGB
    converted and/or dithered) color to fixed-point using equations
    2.3 or 2.4, respectively."

    with

    "Unsigned normalized fixed-point or signed normalized
    fixed-point RGBA color buffers are cleared to color values
    derived by clamping each component of the clear color to the
    range [0, 1] or [-1, 1] respectively, then converting the
    (possibly sRGB converted and/or dithered) color to fixed-point
    using equations 2.3 or 2.4, respectively."

    Add to the second paragraph of Section 16.1.2 "ReadPixels":

    "For 8bit signed normalized fixed-point rendering surfaces, the
    combination format RGBA and type BYTE is accepted. For a 16bit
    signed normalized fixed point buffer, the combination RGBA and
    SHORT is accepted."

     Add to Section 16.1.4 "Conversion of RGBA values":

     For signed normalized fixed-point color buffer, each element
     is converted to floating-point using equation 2.2.

No new errors.

If EXT_texture_norm16 is not supported, remove references to R16_SNORM_EXT,
RG16_SNORM_EXT, RGB16_SNORM_EXT, RGBA16_SNORM_EXT. Remove language and
additions referring to 16bit (signed / unsigned) normalized fixed
point buffers.

1. How  does this extension differ from the functionality offered by GL4.4?

   On GL, the affected formats are generally exposed as 'color
   renderable' but not as 'required renderbuffer format', which
   might be interpreted as 'optionally renderable'. EXT_render_snorm
   tries to avoid this ambiguity by introducing formats as 'neither
   color-renderable nor req. renderbuffer format' or as
   'color-renderable plus required renderbuffer format', but not a
   mixture of both. DX 10.1 level hardware is expected to supported
   these formats as being renderable. 3-component snorm formats were
   left out as they are not expected to map well to native hardware
   formats.

2. Are format conversions between UNORM and SNORM formats allowed
   for CopyTexImage2D?

   The OpenGL ES specs describe the CopyTexImage2D operation in
   terms of ReadPixels followed by TexImage2D. The ReadPixels
   command will use the appropriate format/type combination from
   Table 8.15. For an internal format of RGBA8_SNORM, this will be
   GL_RGBA and GL_BYTE. (Special rules apply to framebuffer
   attachments that have been created using unsized internalformats.
   These rules don't apply to attachments of _SNORM formats as they
   could only have been created using sized internalformats.) The
   following 'virtual' TexImage2D call will use the same format/type
   combination together with the provided internalformat. The
   resulting combination of internalformat/format/type must be one
   of those listed in Table 8.12. For signed fixed point formats it
   is not possible to create them from unsigned fixed point data and
   vice versa. Effectively this means, CopyTexImage2D cannot perform
   any conversions that glTexImage could not do (except dropping
   color components); in particular it cannot convert between
   mismatching component types.

   Source Effective | Requested              | CopyTexImage Supported
   Internalformat   | Internalformat         |
   -----------------+------------------------+------------------------------------
   sized SNORM      | sized SNORM            | yes (provided component sizes match
                    |                        | and the same or more components
                    |                        | are present in the source buffer)
   sized SNORM      | sized or unsized UNORM | no (conversions not supported)