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/*
* Copyright 2014 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#include "SkTextureCompressor.h"
#include "SkBitmap.h"
#include "SkData.h"
#include "SkEndian.h"
////////////////////////////////////////////////////////////////////////////////
//
// Utility Functions
//
////////////////////////////////////////////////////////////////////////////////
// Absolute difference between two values. More correct than SkTAbs(a - b)
// because it works on unsigned values.
template <typename T> inline T abs_diff(const T &a, const T &b) {
return (a > b) ? (a - b) : (b - a);
}
////////////////////////////////////////////////////////////////////////////////
//
// LATC compressor
//
////////////////////////////////////////////////////////////////////////////////
// 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);
}
// 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
// a palette of eight colors from LUM0 and LUM1 using the algorithm:
//
// LUM0, if lum0 > lum1 and code(x,y) == 0
// LUM1, if lum0 > lum1 and code(x,y) == 1
// (6*LUM0+ LUM1)/7, if lum0 > lum1 and code(x,y) == 2
// (5*LUM0+2*LUM1)/7, if lum0 > lum1 and code(x,y) == 3
// (4*LUM0+3*LUM1)/7, if lum0 > lum1 and code(x,y) == 4
// (3*LUM0+4*LUM1)/7, if lum0 > lum1 and code(x,y) == 5
// (2*LUM0+5*LUM1)/7, if lum0 > lum1 and code(x,y) == 6
// ( LUM0+6*LUM1)/7, if lum0 > lum1 and code(x,y) == 7
//
// LUM0, if lum0 <= lum1 and code(x,y) == 0
// LUM1, if lum0 <= lum1 and code(x,y) == 1
// (4*LUM0+ LUM1)/5, if lum0 <= lum1 and code(x,y) == 2
// (3*LUM0+2*LUM1)/5, if lum0 <= lum1 and code(x,y) == 3
// (2*LUM0+3*LUM1)/5, if lum0 <= lum1 and code(x,y) == 4
// ( 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;
}
}
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;
// 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;
}
// 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));
uint8_t block[16];
const uint8_t* row = reinterpret_cast<const uint8_t*>(bm.getPixels());
uint64_t* encPtr = 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);
*encPtr = compress_latc_block(block);
++encPtr;
}
row += kLATCBlockSize * bm.rowBytes();
}
return SkData::NewFromMalloc(dst, compressedDataSize);
}
////////////////////////////////////////////////////////////////////////////////
namespace SkTextureCompressor {
typedef SkData *(*CompressBitmapProc)(const SkBitmap &bitmap);
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);
}
return NULL;
}
} // namespace SkTextureCompressor
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