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Some improvements to LATC compression

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R=bsalomon@google.com

Author: krajcevski@google.com

Review URL: https://codereview.chromium.org/347673002
This commit is contained in:
krajcevski 2014-06-20 11:43:00 -07:00 committed by Commit bot
parent 909f91dc4c
commit eecc35f988
2 changed files with 257 additions and 124 deletions
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373
src/utils/SkTextureCompressor.cpp
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@ -29,26 +29,12 @@ template <typename T> inline T abs_diff(const T &a, const T &b) {
//
////////////////////////////////////////////////////////////////////////////////
// Return the squared minimum error cost of approximating 'pixel' using the
// provided palette. Return this in the middle 16 bits of the integer. Return
// the best index in the palette for this pixel in the bottom 8 bits.
static uint32_t compute_error(uint8_t pixel, uint8_t palette[8]) {
int minIndex = 0;
uint8_t error = abs_diff(palette[0], pixel);
for (int i = 1; i < 8; ++i) {
uint8_t diff = abs_diff(palette[i], pixel);
if (diff < error) {
minIndex = i;
error = diff;
}
}
uint16_t errSq = static_cast<uint16_t>(error) * static_cast<uint16_t>(error);
SkASSERT(minIndex >= 0 && minIndex < 8);
return (static_cast<uint32_t>(errSq) << 8) | static_cast<uint32_t>(minIndex);
}
// LATC compressed texels down into square 4x4 blocks
static const int kPaletteSize = 8;
static const int kLATCBlockSize = 4;
static const int kPixelsPerBlock = kLATCBlockSize * kLATCBlockSize;
// Compress LATC block. Each 4x4 block of pixels is decompressed by LATC from two
// values LUM0 and LUM1, and an index into the generated palette. LATC constructs
// Generates an LATC palette. LATC constructs
// a palette of eight colors from LUM0 and LUM1 using the algorithm:
//
// LUM0, if lum0 > lum1 and code(x,y) == 0
@ -68,142 +54,281 @@ static uint32_t compute_error(uint8_t pixel, uint8_t palette[8]) {
// ( LUM0+4*LUM1)/5, if lum0 <= lum1 and code(x,y) == 5
// 0, if lum0 <= lum1 and code(x,y) == 6
// 255, if lum0 <= lum1 and code(x,y) == 7
//
// We compute the LATC palette using the following simple algorithm:
// 1. Choose the minimum and maximum values in the block as LUM0 and LUM1
// 2. Figure out which of the two possible palettes is better.
static uint64_t compress_latc_block(uint8_t block[16]) {
// Just do a simple min/max but choose which of the
// two palettes is better
uint8_t maxVal = 0;
uint8_t minVal = 255;
for (int i = 0; i < 16; ++i) {
maxVal = SkMax32(maxVal, block[i]);
minVal = SkMin32(minVal, block[i]);
}
// Generate palettes
uint8_t palettes[2][8];
// Straight linear ramp
palettes[0][0] = maxVal;
palettes[0][1] = minVal;
for (int i = 1; i < 7; ++i) {
palettes[0][i+1] = ((7-i)*maxVal + i*minVal) / 7;
}
// Smaller linear ramp with min and max byte values at the end.
palettes[1][0] = minVal;
palettes[1][1] = maxVal;
for (int i = 1; i < 5; ++i) {
palettes[1][i+1] = ((5-i)*maxVal + i*minVal) / 5;
}
palettes[1][6] = 0;
palettes[1][7] = 255;
// Figure out which of the two is better:
// - accumError holds the accumulated error for each pixel from
// the associated palette
// - indices holds the best indices for each palette in the
// bottom 48 (16*3) bits.
uint32_t accumError[2] = { 0, 0 };
uint64_t indices[2] = { 0, 0 };
for (int i = 15; i >= 0; --i) {
// For each palette:
// 1. Retreive the result of this pixel
// 2. Store the error in accumError
// 3. Store the minimum palette index in indices.
for (int p = 0; p < 2; ++p) {
uint32_t result = compute_error(block[i], palettes[p]);
accumError[p] += (result >> 8);
indices[p] <<= 3;
indices[p] |= result & 7;
static void generate_palette(uint8_t palette[], uint8_t lum0, uint8_t lum1) {
palette[0] = lum0;
palette[1] = lum1;
if (lum0 > lum1) {
for (int i = 1; i < 7; i++) {
palette[i+1] = ((7-i)*lum0 + i*lum1) / 7;
}
} else {
for (int i = 1; i < 5; i++) {
palette[i+1] = ((5-i)*lum0 + i*lum1) / 5;
}
palette[6] = 0;
palette[7] = 255;
}
SkASSERT(indices[0] < (static_cast<uint64_t>(1) << 48));
SkASSERT(indices[1] < (static_cast<uint64_t>(1) << 48));
uint8_t paletteIdx = (accumError[0] > accumError[1]) ? 0 : 1;
// Assemble the compressed block.
uint64_t result = 0;
// Jam the first two palette entries into the bottom 16 bits of
// a 64 bit integer. Based on the palette that we chose, one will
// be larger than the other and it will select the proper palette.
result |= static_cast<uint64_t>(palettes[paletteIdx][0]);
result |= static_cast<uint64_t>(palettes[paletteIdx][1]) << 8;
// Jam the indices into the top 48 bits.
result |= indices[paletteIdx] << 16;
// We assume everything is little endian, if it's not then make it so.
return SkEndian_SwapLE64(result);
}
static SkData *compress_a8_to_latc(const SkBitmap &bm) {
// LATC compressed texels down into square 4x4 blocks
static const int kLATCBlockSize = 4;
static bool is_extremal(uint8_t pixel) {
return 0 == pixel || 255 == pixel;
}
// Compress a block by using the bounding box of the pixels. It is assumed that
// there are no extremal pixels in this block otherwise we would have used
// compressBlockBBIgnoreExtremal.
static uint64_t compress_block_bb(const uint8_t pixels[]) {
uint8_t minVal = 255;
uint8_t maxVal = 0;
for (int i = 0; i < kPixelsPerBlock; ++i) {
minVal = SkTMin(pixels[i], minVal);
maxVal = SkTMax(pixels[i], maxVal);
}
SkASSERT(!is_extremal(minVal));
SkASSERT(!is_extremal(maxVal));
uint8_t palette[kPaletteSize];
generate_palette(palette, maxVal, minVal);
uint64_t indices = 0;
for (int i = kPixelsPerBlock - 1; i >= 0; --i) {
// Find the best palette index
uint8_t bestError = abs_diff(pixels[i], palette[0]);
uint8_t idx = 0;
for (int j = 1; j < kPaletteSize; ++j) {
uint8_t error = abs_diff(pixels[i], palette[j]);
if (error < bestError) {
bestError = error;
idx = j;
}
}
indices <<= 3;
indices |= idx;
}
return
SkEndian_SwapLE64(
static_cast<uint64_t>(maxVal) |
(static_cast<uint64_t>(minVal) << 8) |
(indices << 16));
}
// Compress a block by using the bounding box of the pixels without taking into
// account the extremal values. The generated palette will contain extremal values
// and fewer points along the line segment to interpolate.
static uint64_t compress_block_bb_ignore_extremal(const uint8_t pixels[]) {
uint8_t minVal = 255;
uint8_t maxVal = 0;
for (int i = 0; i < kPixelsPerBlock; ++i) {
if (is_extremal(pixels[i])) {
continue;
}
minVal = SkTMin(pixels[i], minVal);
maxVal = SkTMax(pixels[i], maxVal);
}
SkASSERT(!is_extremal(minVal));
SkASSERT(!is_extremal(maxVal));
uint8_t palette[kPaletteSize];
generate_palette(palette, minVal, maxVal);
uint64_t indices = 0;
for (int i = kPixelsPerBlock - 1; i >= 0; --i) {
// Find the best palette index
uint8_t idx = 0;
if (is_extremal(pixels[i])) {
if (0xFF == pixels[i]) {
idx = 7;
} else if (0 == pixels[i]) {
idx = 6;
} else {
SkFAIL("Pixel is extremal but not really?!");
}
} else {
uint8_t bestError = abs_diff(pixels[i], palette[0]);
for (int j = 1; j < kPaletteSize - 2; ++j) {
uint8_t error = abs_diff(pixels[i], palette[j]);
if (error < bestError) {
bestError = error;
idx = j;
}
}
}
indices <<= 3;
indices |= idx;
}
return
SkEndian_SwapLE64(
static_cast<uint64_t>(minVal) |
(static_cast<uint64_t>(maxVal) << 8) |
(indices << 16));
}
// Compress LATC block. Each 4x4 block of pixels is decompressed by LATC from two
// values LUM0 and LUM1, and an index into the generated palette. Details of how
// the palette is generated can be found in the comments of generatePalette above.
//
// We choose which palette type to use based on whether or not 'pixels' contains
// any extremal values (0 or 255). If there are extremal values, then we use the
// palette that has the extremal values built in. Otherwise, we use the full bounding
// box.
static uint64_t compress_block(const uint8_t pixels[]) {
// Collect unique pixels
int nUniquePixels = 0;
uint8_t uniquePixels[kPixelsPerBlock];
for (int i = 0; i < kPixelsPerBlock; ++i) {
bool foundPixel = false;
for (int j = 0; j < nUniquePixels; ++j) {
foundPixel = foundPixel || uniquePixels[j] == pixels[i];
}
if (!foundPixel) {
uniquePixels[nUniquePixels] = pixels[i];
++nUniquePixels;
}
}
// If there's only one unique pixel, then our compression is easy.
if (1 == nUniquePixels) {
return SkEndian_SwapLE64(pixels[0] | (pixels[0] << 8));
// Similarly, if there are only two unique pixels, then our compression is
// easy again: place the pixels in the block header, and assign the indices
// with one or zero depending on which pixel they belong to.
} else if (2 == nUniquePixels) {
uint64_t outBlock = 0;
for (int i = kPixelsPerBlock - 1; i >= 0; --i) {
int idx = 0;
if (pixels[i] == uniquePixels[1]) {
idx = 1;
}
outBlock <<= 3;
outBlock |= idx;
}
outBlock <<= 16;
outBlock |= (uniquePixels[0] | (uniquePixels[1] << 8));
return SkEndian_SwapLE64(outBlock);
}
// Count non-maximal pixel values
int nonExtremalPixels = 0;
for (int i = 0; i < nUniquePixels; ++i) {
if (!is_extremal(uniquePixels[i])) {
++nonExtremalPixels;
}
}
// If all the pixels are nonmaximal then compute the palette using
// the bounding box of all the pixels.
if (nonExtremalPixels == nUniquePixels) {
// This is really just for correctness, in all of my tests we
// never take this step. We don't lose too much perf here because
// most of the processing in this function is worth it for the
// 1 == nUniquePixels optimization.
return compress_block_bb(pixels);
} else {
return compress_block_bb_ignore_extremal(pixels);
}
}
static bool compress_a8_to_latc(uint8_t* dst, const uint8_t* src,
int width, int height, int rowBytes) {
// Make sure that our data is well-formed enough to be
// considered for LATC compression
if (bm.width() == 0 || bm.height() == 0 ||
(bm.width() % kLATCBlockSize) != 0 ||
(bm.height() % kLATCBlockSize) != 0 ||
(bm.colorType() != kAlpha_8_SkColorType)) {
return NULL;
if (0 == width || 0 == height ||
(width % kLATCBlockSize) != 0 || (height % kLATCBlockSize) != 0) {
return false;
}
// The LATC format is 64 bits per 4x4 block.
static const int kLATCEncodedBlockSize = 8;
int blocksX = bm.width() / kLATCBlockSize;
int blocksY = bm.height() / kLATCBlockSize;
int compressedDataSize = blocksX * blocksY * kLATCEncodedBlockSize;
uint64_t* dst = reinterpret_cast<uint64_t*>(sk_malloc_throw(compressedDataSize));
int blocksX = width / kLATCBlockSize;
int blocksY = height / kLATCBlockSize;
uint8_t block[16];
const uint8_t* row = reinterpret_cast<const uint8_t*>(bm.getPixels());
uint64_t* encPtr = dst;
uint64_t* encPtr = reinterpret_cast<uint64_t*>(dst);
for (int y = 0; y < blocksY; ++y) {
for (int x = 0; x < blocksX; ++x) {
memcpy(block, row + (kLATCBlockSize * x), 4);
memcpy(block + 4, row + bm.rowBytes() + (kLATCBlockSize * x), 4);
memcpy(block + 8, row + 2*bm.rowBytes() + (kLATCBlockSize * x), 4);
memcpy(block + 12, row + 3*bm.rowBytes() + (kLATCBlockSize * x), 4);
// Load block
static const int kBS = kLATCBlockSize;
for (int k = 0; k < kBS; ++k) {
memcpy(block + k*kBS, src + k*rowBytes + (kBS * x), kBS);
}
*encPtr = compress_latc_block(block);
// Compress it
*encPtr = compress_block(block);
++encPtr;
}
row += kLATCBlockSize * bm.rowBytes();
src += kLATCBlockSize * rowBytes;
}
return SkData::NewFromMalloc(dst, compressedDataSize);
return true;
}
////////////////////////////////////////////////////////////////////////////////
namespace SkTextureCompressor {
typedef SkData *(*CompressBitmapProc)(const SkBitmap &bitmap);
static size_t get_compressed_data_size(Format fmt, int width, int height) {
switch (fmt) {
case kLATC_Format:
{
// The LATC format is 64 bits per 4x4 block.
static const int kLATCEncodedBlockSize = 8;
int blocksX = width / kLATCBlockSize;
int blocksY = height / kLATCBlockSize;
return blocksX * blocksY * kLATCEncodedBlockSize;
}
default:
SkFAIL("Unknown compressed format!");
return 0;
}
}
typedef bool (*CompressBitmapProc)(uint8_t* dst, const uint8_t* src,
int width, int height, int rowBytes);
bool CompressBufferToFormat(uint8_t* dst, const uint8_t* src, SkColorType srcColorType,
int width, int height, int rowBytes, Format format) {
CompressBitmapProc kProcMap[kFormatCnt][kLastEnum_SkColorType + 1];
memset(kProcMap, 0, sizeof(kProcMap));
kProcMap[kLATC_Format][kAlpha_8_SkColorType] = compress_a8_to_latc;
CompressBitmapProc proc = kProcMap[format][srcColorType];
if (NULL != proc) {
return proc(dst, src, width, height, rowBytes);
}
return false;
}
SkData *CompressBitmapToFormat(const SkBitmap &bitmap, Format format) {
SkAutoLockPixels alp(bitmap);
CompressBitmapProc kProcMap[kLastEnum_SkColorType + 1][kFormatCnt];
memset(kProcMap, 0, sizeof(kProcMap));
// Map available bitmap configs to compression functions
kProcMap[kAlpha_8_SkColorType][kLATC_Format] = compress_a8_to_latc;
CompressBitmapProc proc = kProcMap[bitmap.colorType()][format];
if (NULL != proc) {
return proc(bitmap);
int compressedDataSize = get_compressed_data_size(format, bitmap.width(), bitmap.height());
const uint8_t* src = reinterpret_cast<const uint8_t*>(bitmap.getPixels());
uint8_t* dst = reinterpret_cast<uint8_t*>(sk_malloc_throw(compressedDataSize));
if (CompressBufferToFormat(dst, src, bitmap.colorType(), bitmap.width(), bitmap.height(),
bitmap.rowBytes(), format)) {
return SkData::NewFromMalloc(dst, compressedDataSize);
}
sk_free(dst);
return NULL;
}

8
src/utils/SkTextureCompressor.h
View File

@ -8,6 +8,8 @@
#ifndef SkTextureCompressor_DEFINED
#define SkTextureCompressor_DEFINED
#include "SkImageInfo.h"
class SkBitmap;
class SkData;
@ -26,6 +28,12 @@ namespace SkTextureCompressor {
// associated format, then we return NULL. The caller is responsible for
// calling unref() on the returned data.
SkData* CompressBitmapToFormat(const SkBitmap& bitmap, Format format);
// Compresses the given src data into dst. The src data is assumed to be
// large enough to hold width*height pixels. The dst data is expected to
// be large enough to hold the compressed data according to the format.
bool CompressBufferToFormat(uint8_t* dst, const uint8_t* src, SkColorType srcColorType,
int width, int height, int rowBytes, Format format);
}
#endif