Difference between revisions of "NV2A/Pixel Combiner"

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(Add some BRDF links)
(Add more nvidia BRDF links)
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* [https://www.merl.com/brdf/ BRDF Database by Mitsubishi]
 
* [https://www.merl.com/brdf/ BRDF Database by Mitsubishi]
  
{{FIXME|reason=Describe Xbox specific BRDF texture shader}}
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* nvidia resources ''(The code and technique is probably not using the texture shader that is described here)'':
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** [http://www.nvidia.in/attach/6670 BRDFs.pdf] / [http://www.nvidia.in/attach/6669 BRDFs.ppt]
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** [https://www.nvidia.com/attach/6568 BRDFIntro.pdf] / [https://www.nvidia.com/attach/6569 BRDFIntro.doc]
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** [https://www.nvidia.com/attach/6567 BRDFSeparable.pdf] / [https://www.nvidia.com/attach/6566 BRDFSeparable.doc]
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** [https://www.nvidia.com/attach/6570 brdfseparate.zip]
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** [https://www.nvidia.com/attach/6571 brdfview.zip]
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{{FIXME|reason=Describe Xbox specific BRDF texture shader}}
 
{{FIXME|reason=Describe Xbox specific BRDF texture shader}}
  

Revision as of 10:29, 30 December 2019

Data types

NV_texture_shader suggests that: "The 8-bit and 16-bit signed fixed-point types are used for signed internal texture formats, while the 9-bit signed fixed-point type is used for register combiners computations." Here is a table from the GL extension:

floating-point 8-bit fixed-point 9-bit fixed-point 16-bit fixed-point
1.0 n/a 255 n/a
0.99996... n/a n/a 32767
0.99218... 127 n/a n/a
0.0 0 0 0
-1.0 -128 -255 -32768
-1.00392... n/a -256 n/a

This means:

  • 8-bit fixed-point: [-128, 127] → [-128/128, 127/128] → [-1.0, 0.99218...]
  • 9-bit fixed-point: [-256, 255] → [-256/255, 255/255] → [-1.00392..., 1.0]
  • 16-bit fixed-point: [-32768, 32767] → [-32768/32768, 32767/32768] → [-1.0, 0.99996...]

It is not known if the NV2A really implements these 3 datatypes. It is also not yet known how exactly conversion or negation of these types would work.

Texture Shaders

The NV2A implements at least parts of the following OpenGL extensions:

Texturing modes

ID Name D3D name GL Name Stage 1 Stage 2 Stage 3 Stage 4 Notes
0x00 PS_TEXTUREMODES_NONE NONE
0x01 PS_TEXTUREMODES_PROJECT2D tex TEXTURE_2D
0x02 PS_TEXTUREMODES_PROJECT3D tex TEXTURE_3D
0x03 PS_TEXTUREMODES_CUBEMAP tex TEXTURE_CUBE_MAP_ARB
0x04 PS_TEXTUREMODES_PASSTHRU texcoord PASS_THROUGH_NV
0x05 PS_TEXTUREMODES_CLIPPLANE texkill CULL_FRAGMENT_NV
0x06 PS_TEXTUREMODES_BUMPENVMAP texbem OFFSET_TEXTURE_2D_NV
0x07 PS_TEXTUREMODES_BUMPENVMAP_LUM texbeml OFFSET_TEXTURE_2D_SCALE_NV
0x08 PS_TEXTUREMODES_BRDF texbrdf
0x09 PS_TEXTUREMODES_DOT_ST texm3x2tex DOT_PRODUCT_TEXTURE_2D_NV
0x0A PS_TEXTUREMODES_DOT_ZW texm3x2depth DOT_PRODUCT_DEPTH_REPLACE_NV
0x0B PS_TEXTUREMODES_DOT_RFLCT_DIFF texm3x3diff DOT_PRODUCT_DIFFUSE_CUBE_MAP_NV[citation needed]
0x0C PS_TEXTUREMODES_DOT_RFLCT_SPEC texm3x3vspec DOT_PRODUCT_CONST_EYE_REFLECT_CUBE_MAP_NV
0x0D PS_TEXTUREMODES_DOT_STR_3D texm3x3tex DOT_PRODUCT_TEXTURE_3D_NV
0x0E PS_TEXTUREMODES_DOT_STR_CUBE texm3x3vspec DOT_PRODUCT_REFLECT_CUBE_MAP_NV
0x0F PS_TEXTUREMODES_DPNDNT_AR texreg2ar DEPENDENT_AR_TEXTURE_2D_NV
0x10 PS_TEXTUREMODES_DPNDNT_GB texreg2gb DEPENDENT_GB_TEXTURE_2D_NV
0x11 PS_TEXTUREMODES_DOTPRODUCT texm3x3pad
texm3x2pad
DOT_PRODUCT_NV
0x12 PS_TEXTUREMODES_DOT_RFLCT_SPEC_CONST texm3x3spec DOT_PRODUCT_CONST_EYE_REFLECT_CUBE_MAP_NV

0x08: PS_TEXTUREMODES_BRDF / texbrdf

The BRDF texture shader is probably only exposed on original Xbox, but not in standard OpenGL or D3D drivers.

These are some generic resources about BRDFs:

[FIXME]

Register combiners

The NV2A implements at least parts of the following OpenGL extensions:

There's some additional features and oddities.

DISCARD and ZERO are the same register

On NV2A the DISCARD and ZERO register are the same index: writes are discarded / reads return zero.

This is different from NV_register_combiners where 2 different constants are used.

Encoding of input swizzle

NV2A uses a single ALPHA flag to specify the swizzle of inputs:

  • 0 for .rgb (RGB portion only) and .b (ALPHA portion only).
  • 1 for .aaa (RGB portion only) and .a (ALPHA portion only).

This is different from NV_register_combiners where each swizzle has its own constant.

Per stage constant-colors

The combiner setup switches between using the same const0 and const1 for all stages (FACTOR#_SAME_FACTOR_ALL), or using different constant-colors per stage (FACTOR#_EACH_STAGE).

On NV2A, the final-combiner does always have unique constants (even using FACTOR#_SAME_FACTOR_ALL) from all other stages. If FACTOR#_SAME_FACTOR_ALL is used, the constant-colors for all other stages are taken from the very first stage. This setting can be controlled independently for const0 and const1.

This is different from NV_register_combiners2. If that GL extension isn't available / enabled, then the constants are shared between general combiner stages and the final combiner (which doesn't have unique colors then). Additionally, the GL extension can only control this for both constant-colors at the same time.

Encoding of constant-colors

On NV2A, the constant-colors are encoded as 8-bit unsigned int values, packed into a 32-bit ARGB value ((a<<24 | r<<16 | g<<8 | b)).

This is different from NV_register_combiners where constant-colors are specified as floats in RGBA format.

BLUETOALPHA in RGB portion

NV2A has a special flag to write the blue result (RGB portion) of the A/B and C/D computations to the alpha channel of the RGB portion output register. There's no such option for the AB/CD result. [FIXME]

This feature isn't available in GL, probably. This is different from NV_register_combiners where the RGB portion always writes to .rgb of the output.

Special "or" operation (MUX) modifier

NV2A has a special flag to switch between MSB and LSB[FIXME] for the special "or" operation (MUX). [FIXME]

This feature isn't available in GL, probably. This is different from NV_register_combiners where the special "or" operation (MUX) is always doing: spare0_alpha >= 0.5 ? C*D : A*B.

Debugging

PIX from the Microsoft XDK provides great debugging capabilities.

References and links