• Advertisement
Sign in to follow this  

DX11 Standard expansion of DXTn colors?

This topic is 1823 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

Is the way the 5.6.5. colors expanded into 8 bits or normalised floats documented/standardized anywhere. Specifically for DX9 and DX11.

So this could be done a few different ways. For Red and Blue the binary values can be divided by 31.0f to give a color in the range 0.0 to 1.0.

Alternatively the color could be expanded into 8 bits and then divided by 255. A common way of expanding bits to large word sizes is to replicate high order bits to the low order bits. So to expand 5 bits in the work ABCDE to 8 bits is ABCDEABC. Is this what occurs for DXTn?

Share this post

Link to post
Share on other sites

The way how K bits wide fixed-bit-width channels with unsigned integer values [0, 2^K-1] (UNORM types) are interpreted is standard: The unsigned number k in the range [0, 2^K-1] represents the rational number k / 2^k-1.


This means that a 5-bit channel can represent the rationals 0/31, 1/31, 2/31, 3/31, ..., 30/31, 31/31 = 1. A 8-bit color channel can represent the rationals 0/255, 1/255, ..., 254/255, 255/255=1.



When encoding a rational represented as a floating point as an unsigned integer, we (usually) pick the integer that is nearest to the floating point number in question, since that minimizes the generated rounding error.



Converting the other way, from 5-bit integer to floating point, or 8-bit integer to floating point is lossless (but not necessarily exact), since floats have more precision. 


The method of "expanding bits" you specify is an optimization that does not have mathematical basis, and it introduces an error.


Here is a small code snippet that converts colors encoded as 5-bit UNORM to floating point and to their closest 8-bit representative, as well as directly 5-bit to 8-bit using the approximation you describe:


[code]for(int c = 0; c <= 31; ++c)
double d = c / 31.0; // Convert 5-bit UNORM color to nearest double representation.
int u = d * 255.0; // Convert UNORM color as double to nearest 8-bit UNORM encoded representation.
int u2 = (c << 3) | (c >> 2); // Approximate conversion from 5-bit directly to 8-bit.
printf("5-bit:%3d as UNORM:%.05g Stored as 8-bit:%3d approx 5-bit->8-bit: %3d\n", c, d, u, u2);


The output for that is:


[code]5-bit: 0 as UNORM:0 Stored as 8-bit: 0 approx 5-bit->8-bit: 0
5-bit: 1 as UNORM:0.032258 Stored as 8-bit: 8 approx 5-bit->8-bit: 8
5-bit: 2 as UNORM:0.064516 Stored as 8-bit: 16 approx 5-bit->8-bit: 16
5-bit: 3 as UNORM:0.096774 Stored as 8-bit: 24 approx 5-bit->8-bit: 24
5-bit: 4 as UNORM:0.12903 Stored as 8-bit: 32 approx 5-bit->8-bit: 33
5-bit: 5 as UNORM:0.16129 Stored as 8-bit: 41 approx 5-bit->8-bit: 41
5-bit: 6 as UNORM:0.19355 Stored as 8-bit: 49 approx 5-bit->8-bit: 49
5-bit: 7 as UNORM:0.22581 Stored as 8-bit: 57 approx 5-bit->8-bit: 57
5-bit: 8 as UNORM:0.25806 Stored as 8-bit: 65 approx 5-bit->8-bit: 66
5-bit: 9 as UNORM:0.29032 Stored as 8-bit: 74 approx 5-bit->8-bit: 74
5-bit: 10 as UNORM:0.32258 Stored as 8-bit: 82 approx 5-bit->8-bit: 82
5-bit: 11 as UNORM:0.35484 Stored as 8-bit: 90 approx 5-bit->8-bit: 90
5-bit: 12 as UNORM:0.3871 Stored as 8-bit: 98 approx 5-bit->8-bit: 99
5-bit: 13 as UNORM:0.41935 Stored as 8-bit:106 approx 5-bit->8-bit: 107
5-bit: 14 as UNORM:0.45161 Stored as 8-bit:115 approx 5-bit->8-bit: 115
5-bit: 15 as UNORM:0.48387 Stored as 8-bit:123 approx 5-bit->8-bit: 123
5-bit: 16 as UNORM:0.51613 Stored as 8-bit:131 approx 5-bit->8-bit: 132
5-bit: 17 as UNORM:0.54839 Stored as 8-bit:139 approx 5-bit->8-bit: 140
5-bit: 18 as UNORM:0.58065 Stored as 8-bit:148 approx 5-bit->8-bit: 148
5-bit: 19 as UNORM:0.6129 Stored as 8-bit:156 approx 5-bit->8-bit: 156
5-bit: 20 as UNORM:0.64516 Stored as 8-bit:164 approx 5-bit->8-bit: 165
5-bit: 21 as UNORM:0.67742 Stored as 8-bit:172 approx 5-bit->8-bit: 173
5-bit: 22 as UNORM:0.70968 Stored as 8-bit:180 approx 5-bit->8-bit: 181
5-bit: 23 as UNORM:0.74194 Stored as 8-bit:189 approx 5-bit->8-bit: 189
5-bit: 24 as UNORM:0.77419 Stored as 8-bit:197 approx 5-bit->8-bit: 198
5-bit: 25 as UNORM:0.80645 Stored as 8-bit:205 approx 5-bit->8-bit: 206
5-bit: 26 as UNORM:0.83871 Stored as 8-bit:213 approx 5-bit->8-bit: 214
5-bit: 27 as UNORM:0.87097 Stored as 8-bit:222 approx 5-bit->8-bit: 222
5-bit: 28 as UNORM:0.90323 Stored as 8-bit:230 approx 5-bit->8-bit: 231
5-bit: 29 as UNORM:0.93548 Stored as 8-bit:238 approx 5-bit->8-bit: 239
5-bit: 30 as UNORM:0.96774 Stored as 8-bit:246 approx 5-bit->8-bit: 247
5-bit: 31 as UNORM:1 Stored as 8-bit:255 approx 5-bit->8-bit: 255[/code]


I can't outright find a page from MSDN that describes this, but you can find the same interpretation from OpenGL specification pdfs where it is explained with formal rigor.

Share this post

Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Now

  • Advertisement
  • Similar Content

    • By AxeGuywithanAxe
      I wanted to see how others are currently handling descriptor heap updates and management.
      I've read a few articles and there tends to be three major strategies :
      1 ) You split up descriptor heaps per shader stage ( i.e one for vertex shader , pixel , hull, etc)
      2) You have one descriptor heap for an entire pipeline
      3) You split up descriptor heaps for update each update frequency (i.e EResourceSet_PerInstance , EResourceSet_PerPass , EResourceSet_PerMaterial, etc)
      The benefits of the first two approaches is that it makes it easier to port current code, and descriptor / resource descriptor management and updating tends to be easier to manage, but it seems to be not as efficient.
      The benefits of the third approach seems to be that it's the most efficient because you only manage and update objects when they change.
    • By evelyn4you
      until now i use typical vertexshader approach for skinning with a Constantbuffer containing the transform matrix for the bones and an the vertexbuffer containing bone index and bone weight.
      Now i have implemented realtime environment  probe cubemaping so i have to render my scene from many point of views and the time for skinning takes too long because it is recalculated for every side of the cubemap.
      For Info i am working on Win7 an therefore use one Shadermodel 5.0 not 5.x that have more options, or is there a way to use 5.x in Win 7
      My Graphic Card is Directx 12 compatible NVidia GTX 960
      the member turanszkij has posted a good for me understandable compute shader. ( for Info: in his engine he uses an optimized version of it )
      Now my questions
       is it possible to feed the compute shader with my orignial vertexbuffer or do i have to copy it in several ByteAdressBuffers as implemented in the following code ?
        the same question is about the constant buffer of the matrixes
       my more urgent question is how do i feed my normal pipeline with the result of the compute Shader which are 2 RWByteAddressBuffers that contain position an normal
      for example i could use 2 vertexbuffer bindings
      1 containing only the uv coordinates
      2.containing position and normal
      How do i copy from the RWByteAddressBuffers to the vertexbuffer ?
      (Code from turanszkij )
      Here is my shader implementation for skinning a mesh in a compute shader:
      1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 struct Bone { float4x4 pose; }; StructuredBuffer<Bone> boneBuffer;   ByteAddressBuffer vertexBuffer_POS; // T-Pose pos ByteAddressBuffer vertexBuffer_NOR; // T-Pose normal ByteAddressBuffer vertexBuffer_WEI; // bone weights ByteAddressBuffer vertexBuffer_BON; // bone indices   RWByteAddressBuffer streamoutBuffer_POS; // skinned pos RWByteAddressBuffer streamoutBuffer_NOR; // skinned normal RWByteAddressBuffer streamoutBuffer_PRE; // previous frame skinned pos   inline void Skinning(inout float4 pos, inout float4 nor, in float4 inBon, in float4 inWei) {  float4 p = 0, pp = 0;  float3 n = 0;  float4x4 m;  float3x3 m3;  float weisum = 0;   // force loop to reduce register pressure  // though this way we can not interleave TEX - ALU operations  [loop]  for (uint i = 0; ((i &lt; 4) &amp;&amp; (weisum&lt;1.0f)); ++i)  {  m = boneBuffer[(uint)inBon].pose;  m3 = (float3x3)m;   p += mul(float4(pos.xyz, 1), m)*inWei;  n += mul(nor.xyz, m3)*inWei;   weisum += inWei;  }   bool w = any(inWei);  pos.xyz = w ? p.xyz : pos.xyz;  nor.xyz = w ? n : nor.xyz; }   [numthreads(1024, 1, 1)] void main( uint3 DTid : SV_DispatchThreadID ) {  const uint fetchAddress = DTid.x * 16; // stride is 16 bytes for each vertex buffer now...   uint4 pos_u = vertexBuffer_POS.Load4(fetchAddress);  uint4 nor_u = vertexBuffer_NOR.Load4(fetchAddress);  uint4 wei_u = vertexBuffer_WEI.Load4(fetchAddress);  uint4 bon_u = vertexBuffer_BON.Load4(fetchAddress);   float4 pos = asfloat(pos_u);  float4 nor = asfloat(nor_u);  float4 wei = asfloat(wei_u);  float4 bon = asfloat(bon_u);   Skinning(pos, nor, bon, wei);   pos_u = asuint(pos);  nor_u = asuint(nor);   // copy prev frame current pos to current frame prev pos streamoutBuffer_PRE.Store4(fetchAddress, streamoutBuffer_POS.Load4(fetchAddress)); // write out skinned props:  streamoutBuffer_POS.Store4(fetchAddress, pos_u);  streamoutBuffer_NOR.Store4(fetchAddress, nor_u); }  
    • By mister345
      Hi, can someone please explain why this is giving an assertion EyePosition!=0 exception?
      _lightBufferVS->viewMatrix = DirectX::XMMatrixLookAtLH(XMLoadFloat3(&_lightBufferVS->position), XMLoadFloat3(&_lookAt), XMLoadFloat3(&up));
      It looks like DirectX doesnt want the 2nd parameter to be a zero vector in the assertion, but I passed in a zero vector with this exact same code in another program and it ran just fine. (Here is the version of the code that worked - note XMLoadFloat3(&m_lookAt) parameter value is (0,0,0) at runtime - I debugged it - but it throws no exceptions.
          m_viewMatrix = DirectX::XMMatrixLookAtLH(XMLoadFloat3(&m_position), XMLoadFloat3(&m_lookAt), XMLoadFloat3(&up)); Here is the repo for the broken code (See LightClass) https://github.com/mister51213/DirectX11Engine/blob/master/DirectX11Engine/LightClass.cpp
      and here is the repo with the alternative version of the code that is working with a value of (0,0,0) for the second parameter.
    • By mister345
      Hi, can somebody please tell me in clear simple steps how to debug and step through an hlsl shader file?
      I already did Debug > Start Graphics Debugging > then captured some frames from Visual Studio and
      double clicked on the frame to open it, but no idea where to go from there.
      I've been searching for hours and there's no information on this, not even on the Microsoft Website!
      They say "open the  Graphics Pixel History window" but there is no such window!
      Then they say, in the "Pipeline Stages choose Start Debugging"  but the Start Debugging option is nowhere to be found in the whole interface.
      Also, how do I even open the hlsl file that I want to set a break point in from inside the Graphics Debugger?
      All I want to do is set a break point in a specific hlsl file, step thru it, and see the data, but this is so unbelievably complicated
      and Microsoft's instructions are horrible! Somebody please, please help.

    • By mister345
      I finally ported Rastertek's tutorial # 42 on soft shadows and blur shading. This tutorial has a ton of really useful effects and there's no working version anywhere online.
      Unfortunately it just draws a black screen. Not sure what's causing it. I'm guessing the camera or ortho matrix transforms are wrong, light directions, or maybe texture resources not being properly initialized.  I didnt change any of the variables though, only upgraded all types and functions DirectX3DVector3 to XMFLOAT3, and used DirectXTK for texture loading. If anyone is willing to take a look at what might be causing the black screen, maybe something pops out to you, let me know, thanks.
      Also, for reference, here's tutorial #40 which has normal shadows but no blur, which I also ported, and it works perfectly.
  • Advertisement