读取每个像素的PVRTC图像颜色信息

Question

如何读取PVRTC图像每个像素的图像颜色信息？

下面是我提取整数数组的代码

    NSData *data = [[NSData alloc] initWithContentsOfFile:path];

 NSMutableArray *_imageData = [[NSMutableArray alloc] initWithCapacity:10];

 BOOL success = FALSE;
 PVRTexHeader *header = NULL;
 uint32_t flags, pvrTag;
 uint32_t dataLength = 0, dataOffset = 0, dataSize = 0;
 uint32_t blockSize = 0, widthBlocks = 0, heightBlocks = 0;
 uint32_t width = 0, height = 0, bpp = 4;
 uint8_t *bytes = NULL;
 uint32_t formatFlags;

 header = (PVRTexHeader *)[data bytes];

 pvrTag = CFSwapInt32LittleToHost(header->pvrTag);

 if (gPVRTexIdentifier[0] != ((pvrTag >>  0) & 0xff) ||
  gPVRTexIdentifier[1] != ((pvrTag >>  8) & 0xff) ||
  gPVRTexIdentifier[2] != ((pvrTag >> 16) & 0xff) ||
  gPVRTexIdentifier[3] != ((pvrTag >> 24) & 0xff))
 {
  return FALSE;
 }

 flags = CFSwapInt32LittleToHost(header->flags);
 formatFlags = flags & PVR_TEXTURE_FLAG_TYPE_MASK;

 if (formatFlags == kPVRTextureFlagTypePVRTC_4 || formatFlags == kPVRTextureFlagTypePVRTC_2)
 {
  [_imageData removeAllObjects];

  if (formatFlags == kPVRTextureFlagTypePVRTC_4)
   _internalFormat = GL_COMPRESSED_RGBA_PVRTC_4BPPV1_IMG;
  else if (formatFlags == kPVRTextureFlagTypePVRTC_2)
   _internalFormat = GL_COMPRESSED_RGBA_PVRTC_2BPPV1_IMG;


  _width = width = CFSwapInt32LittleToHost(header->width);
  _height = height = CFSwapInt32LittleToHost(header->height);

  if (CFSwapInt32LittleToHost(header->bitmaskAlpha))
   _hasAlpha = TRUE;
  else
   _hasAlpha = FALSE;

  dataLength = CFSwapInt32LittleToHost(header->dataLength);

  bytes = ((uint8_t *)[data bytes]) + sizeof(PVRTexHeader);

  // Calculate the data size for each texture level and respect the minimum number of blocks
  while (dataOffset < dataLength)
  {
   if (formatFlags == kPVRTextureFlagTypePVRTC_4)
   {
    blockSize = 4 * 4; // Pixel by pixel block size for 4bpp
    widthBlocks = width / 4;
    heightBlocks = height / 4;
    bpp = 4;
   }
   else
   {
    blockSize = 8 * 4; // Pixel by pixel block size for 2bpp
    widthBlocks = width / 8;
    heightBlocks = height / 4;
    bpp = 2;
   }

   // Clamp to minimum number of blocks
   if (widthBlocks < 2)
    widthBlocks = 2;
   if (heightBlocks < 2)
    heightBlocks = 2;

   dataSize = widthBlocks * heightBlocks * ((blockSize  * bpp) / 8);

   [_imageData addObject:[NSData dataWithBytes:bytes+dataOffset length:dataSize]];

                   for (int i=0; i < mipmapCount; i++)
                    {

              NSLog(@"width:%d, height:%d",width,height);

              data = [[NSData alloc] initWithData:[_imageData objectAtIndex:i]];
              NSLog(@"data length:%d",[data length]);

//提取了20个样本数据，但您看到的都是大整数(int i= 0；i< 20；i++){ NSLog(@"data%d:%d"，i，datai)；}

Answer 1

PVRTC是一种4x4 (或8x4)纹理像素的基于块的压缩系统，它将周围的块考虑在内，以表示两个低频图像，高频调制数据与这两个低频图像相结合，以便产生实际的纹理像素输出。这里有一个更好的解释：

http://web.onetel.net.uk/~simonnihal/assorted3d/fenney03texcomp.pdf

因此，您提取的值实际上是编码块的一部分，这些值需要正确解码才能获得有意义的值。

有两种方法可以获得颜色信息:使用软件解压缩器解码/解压缩PVR纹理信息，或者使用POWERVR图形内核渲染纹理，然后读取结果。我在这里只讨论第一个选项。

仅从那里的信息组装一个解压缩器是相当棘手的，但幸运的是，POWERVR SDK中有C++解压缩源代码，您可以在此处获得-例如，下载其中一个iPhone SDK：

http://www.imgtec.com/powervr/insider/powervr-sdk.asp

它位于Tools/PVRTDecompress.cpp文件中。

希望这能有所帮助。