/// <summary>
/// Collect all the references into a histogram (without reset).
/// </summary>
/// <param name="refs">The backward references.</param>
public void StoreRefs(Vp8LBackwardRefs refs)
{
using List <PixOrCopy> .Enumerator c = refs.Refs.GetEnumerator();
while (c.MoveNext())
{
this.AddSinglePixOrCopy(c.Current, false);
}
}
public static void GetHistoImageSymbols(int xSize, int ySize, Vp8LBackwardRefs refs, int quality, int histoBits, int cacheBits, List <Vp8LHistogram> imageHisto, Vp8LHistogram tmpHisto, ushort[] histogramSymbols)
{
int histoXSize = histoBits > 0 ? LosslessUtils.SubSampleSize(xSize, histoBits) : 1;
int histoYSize = histoBits > 0 ? LosslessUtils.SubSampleSize(ySize, histoBits) : 1;
int imageHistoRawSize = histoXSize * histoYSize;
int entropyCombineNumBins = BinSize;
ushort[] mapTmp = new ushort[imageHistoRawSize];
ushort[] clusterMappings = new ushort[imageHistoRawSize];
var origHisto = new List <Vp8LHistogram>(imageHistoRawSize);
for (int i = 0; i < imageHistoRawSize; i++)
{
origHisto.Add(new Vp8LHistogram(cacheBits));
}
// Construct the histograms from the backward references.
HistogramBuild(xSize, histoBits, refs, origHisto);
// Copies the histograms and computes its bitCost. histogramSymbols is optimized.
int numUsed = HistogramCopyAndAnalyze(origHisto, imageHisto, histogramSymbols);
bool entropyCombine = numUsed > entropyCombineNumBins * 2 && quality < 100;
if (entropyCombine)
{
ushort[] binMap = mapTmp;
int numClusters = numUsed;
double combineCostFactor = GetCombineCostFactor(imageHistoRawSize, quality);
HistogramAnalyzeEntropyBin(imageHisto, binMap);
// Collapse histograms with similar entropy.
HistogramCombineEntropyBin(imageHisto, histogramSymbols, clusterMappings, tmpHisto, binMap, entropyCombineNumBins, combineCostFactor);
OptimizeHistogramSymbols(clusterMappings, numClusters, mapTmp, histogramSymbols);
}
float x = quality / 100.0f;
// Cubic ramp between 1 and MaxHistoGreedy:
int thresholdSize = (int)(1 + (x * x * x * (MaxHistoGreedy - 1)));
bool doGreedy = HistogramCombineStochastic(imageHisto, thresholdSize);
if (doGreedy)
{
RemoveEmptyHistograms(imageHisto);
HistogramCombineGreedy(imageHisto);
}
// Find the optimal map from original histograms to the final ones.
RemoveEmptyHistograms(imageHisto);
HistogramRemap(origHisto, imageHisto, histogramSymbols);
}
private static void BackwardReferences2DLocality(int xSize, Vp8LBackwardRefs refs)
{
using List <PixOrCopy> .Enumerator c = refs.Refs.GetEnumerator();
while (c.MoveNext())
{
if (c.Current.IsCopy())
{
int dist = (int)c.Current.BgraOrDistance;
int transformedDist = DistanceToPlaneCode(xSize, dist);
c.Current.BgraOrDistance = (uint)transformedDist;
}
}
}
public void Build(int xSize, int cacheBits, Vp8LBackwardRefs backwardRefs)
{
var histogram = new Vp8LHistogram(cacheBits);
using System.Collections.Generic.List <PixOrCopy> .Enumerator refsEnumerator = backwardRefs.Refs.GetEnumerator();
// The following code is similar to HistogramCreate but converts the distance to plane code.
while (refsEnumerator.MoveNext())
{
histogram.AddSinglePixOrCopy(refsEnumerator.Current, true, xSize);
}
ConvertPopulationCountTableToBitEstimates(histogram.NumCodes(), histogram.Literal, this.Literal);
ConvertPopulationCountTableToBitEstimates(ValuesInBytes, histogram.Red, this.Red);
ConvertPopulationCountTableToBitEstimates(ValuesInBytes, histogram.Blue, this.Blue);
ConvertPopulationCountTableToBitEstimates(ValuesInBytes, histogram.Alpha, this.Alpha);
ConvertPopulationCountTableToBitEstimates(WebpConstants.NumDistanceCodes, histogram.Distance, this.Distance);
}
/// <summary>
/// Construct the histograms from the backward references.
/// </summary>
private static void HistogramBuild(int xSize, int histoBits, Vp8LBackwardRefs backwardRefs, List <Vp8LHistogram> histograms)
{
int x = 0, y = 0;
int histoXSize = LosslessUtils.SubSampleSize(xSize, histoBits);
using List <PixOrCopy> .Enumerator backwardRefsEnumerator = backwardRefs.Refs.GetEnumerator();
while (backwardRefsEnumerator.MoveNext())
{
PixOrCopy v = backwardRefsEnumerator.Current;
int ix = ((y >> histoBits) * histoXSize) + (x >> histoBits);
histograms[ix].AddSinglePixOrCopy(v, false);
x += v.Len;
while (x >= xSize)
{
x -= xSize;
y++;
}
}
}
private static void BackwardReferencesTraceBackwards(
int xSize,
int ySize,
MemoryAllocator memoryAllocator,
ReadOnlySpan <uint> bgra,
int cacheBits,
Vp8LHashChain hashChain,
Vp8LBackwardRefs refsSrc,
Vp8LBackwardRefs refsDst)
{
int distArraySize = xSize * ySize;
using IMemoryOwner <ushort> distArrayBuffer = memoryAllocator.Allocate <ushort>(distArraySize);
Span <ushort> distArray = distArrayBuffer.GetSpan();
BackwardReferencesHashChainDistanceOnly(xSize, ySize, memoryAllocator, bgra, cacheBits, hashChain, refsSrc, distArrayBuffer);
int chosenPathSize = TraceBackwards(distArray, distArraySize);
Span <ushort> chosenPath = distArray.Slice(distArraySize - chosenPathSize);
BackwardReferencesHashChainFollowChosenPath(bgra, cacheBits, chosenPath, chosenPathSize, hashChain, refsDst);
}
private static void AddSingleLiteral(uint pixel, bool useColorCache, ColorCache colorCache, Vp8LBackwardRefs refs)
{
PixOrCopy v;
if (useColorCache)
{
int key = colorCache.GetIndex(pixel);
if (colorCache.Lookup(key) == pixel)
{
v = PixOrCopy.CreateCacheIdx(key);
}
else
{
v = PixOrCopy.CreateLiteral(pixel);
colorCache.Set((uint)key, pixel);
}
}
else
{
v = PixOrCopy.CreateLiteral(pixel);
}
refs.Add(v);
}
/// <summary>
/// Update (in-place) backward references for the specified cacheBits.
/// </summary>
private static void BackwardRefsWithLocalCache(ReadOnlySpan <uint> bgra, int cacheBits, Vp8LBackwardRefs refs)
{
int pixelIndex = 0;
var colorCache = new ColorCache();
colorCache.Init(cacheBits);
for (int idx = 0; idx < refs.Refs.Count; idx++)
{
PixOrCopy v = refs.Refs[idx];
if (v.IsLiteral())
{
uint bgraLiteral = v.BgraOrDistance;
int ix = colorCache.Contains(bgraLiteral);
if (ix >= 0)
{
// Color cache contains bgraLiteral
v.Mode = PixOrCopyMode.CacheIdx;
v.BgraOrDistance = (uint)ix;
v.Len = 1;
}
else
{
colorCache.Insert(bgraLiteral);
}
pixelIndex++;
}
else
{
// refs was created without local cache, so it can not have cache indexes.
for (int k = 0; k < v.Len; ++k)
{
colorCache.Insert(bgra[pixelIndex++]);
}
}
}
}
private static void BackwardReferencesRle(int xSize, int ySize, ReadOnlySpan <uint> bgra, int cacheBits, Vp8LBackwardRefs refs)
{
int pixelCount = xSize * ySize;
bool useColorCache = cacheBits > 0;
var colorCache = new ColorCache();
if (useColorCache)
{
colorCache.Init(cacheBits);
}
refs.Refs.Clear();
// Add first pixel as literal.
AddSingleLiteral(bgra[0], useColorCache, colorCache, refs);
int i = 1;
while (i < pixelCount)
{
int maxLen = LosslessUtils.MaxFindCopyLength(pixelCount - i);
int rleLen = LosslessUtils.FindMatchLength(bgra.Slice(i), bgra.Slice(i - 1), 0, maxLen);
int prevRowLen = i < xSize ? 0 : LosslessUtils.FindMatchLength(bgra.Slice(i), bgra.Slice(i - xSize), 0, maxLen);
if (rleLen >= prevRowLen && rleLen >= MinLength)
{
refs.Add(PixOrCopy.CreateCopy(1, (ushort)rleLen));
// We don't need to update the color cache here since it is always the
// same pixel being copied, and that does not change the color cache state.
i += rleLen;
}
else if (prevRowLen >= MinLength)
{
refs.Add(PixOrCopy.CreateCopy((uint)xSize, (ushort)prevRowLen));
if (useColorCache)
{
for (int k = 0; k < prevRowLen; ++k)
{
colorCache.Insert(bgra[i + k]);
}
}
i += prevRowLen;
}
else
{
AddSingleLiteral(bgra[i], useColorCache, colorCache, refs);
i++;
}
}
}
/// <summary>
/// Compute an LZ77 by forcing matches to happen within a given distance cost.
/// We therefore limit the algorithm to the lowest 32 values in the PlaneCode definition.
/// </summary>
private static void BackwardReferencesLz77Box(int xSize, int ySize, ReadOnlySpan <uint> bgra, int cacheBits, Vp8LHashChain hashChainBest, Vp8LHashChain hashChain, Vp8LBackwardRefs refs)
{
int pixelCount = xSize * ySize;
int[] windowOffsets = new int[WindowOffsetsSizeMax];
int[] windowOffsetsNew = new int[WindowOffsetsSizeMax];
int windowOffsetsSize = 0;
int windowOffsetsNewSize = 0;
short[] counts = new short[xSize * ySize];
int bestOffsetPrev = -1;
int bestLengthPrev = -1;
// counts[i] counts how many times a pixel is repeated starting at position i.
int i = pixelCount - 2;
int countsPos = i;
counts[countsPos + 1] = 1;
for (; i >= 0; --i, --countsPos)
{
if (bgra[i] == bgra[i + 1])
{
// Max out the counts to MaxLength.
counts[countsPos] = counts[countsPos + 1];
if (counts[countsPos + 1] != MaxLength)
{
counts[countsPos]++;
}
}
else
{
counts[countsPos] = 1;
}
}
// Figure out the window offsets around a pixel. They are stored in a
// spiraling order around the pixel as defined by DistanceToPlaneCode.
for (int y = 0; y <= 6; y++)
{
for (int x = -6; x <= 6; x++)
{
int offset = (y * xSize) + x;
// Ignore offsets that bring us after the pixel.
if (offset <= 0)
{
continue;
}
int planeCode = DistanceToPlaneCode(xSize, offset) - 1;
if (planeCode >= WindowOffsetsSizeMax)
{
continue;
}
windowOffsets[planeCode] = offset;
}
}
// For narrow images, not all plane codes are reached, so remove those.
for (i = 0; i < WindowOffsetsSizeMax; i++)
{
if (windowOffsets[i] == 0)
{
continue;
}
windowOffsets[windowOffsetsSize++] = windowOffsets[i];
}
// Given a pixel P, find the offsets that reach pixels unreachable from P-1
// with any of the offsets in windowOffsets[].
for (i = 0; i < windowOffsetsSize; i++)
{
bool isReachable = false;
for (int j = 0; j < windowOffsetsSize && !isReachable; j++)
{
isReachable |= windowOffsets[i] == windowOffsets[j] + 1;
}
if (!isReachable)
{
windowOffsetsNew[windowOffsetsNewSize] = windowOffsets[i];
++windowOffsetsNewSize;
}
}
Span <uint> hashChainOffsetLength = hashChain.OffsetLength.GetSpan();
hashChainOffsetLength[0] = 0;
for (i = 1; i < pixelCount; i++)
{
int ind;
int bestLength = hashChainBest.FindLength(i);
int bestOffset = 0;
bool doCompute = true;
if (bestLength >= MaxLength)
{
// Do not recompute the best match if we already have a maximal one in the window.
bestOffset = hashChainBest.FindOffset(i);
for (ind = 0; ind < windowOffsetsSize; ind++)
{
if (bestOffset == windowOffsets[ind])
{
doCompute = false;
break;
}
}
}
if (doCompute)
{
// Figure out if we should use the offset/length from the previous pixel
// as an initial guess and therefore only inspect the offsets in windowOffsetsNew[].
bool usePrev = bestLengthPrev is > 1 and < MaxLength;
int numInd = usePrev ? windowOffsetsNewSize : windowOffsetsSize;
bestLength = usePrev ? bestLengthPrev - 1 : 0;
bestOffset = usePrev ? bestOffsetPrev : 0;
// Find the longest match in a window around the pixel.
for (ind = 0; ind < numInd; ind++)
{
int currLength = 0;
int j = i;
int jOffset = usePrev ? i - windowOffsetsNew[ind] : i - windowOffsets[ind];
if (jOffset < 0 || bgra[jOffset] != bgra[i])
{
continue;
}
// The longest match is the sum of how many times each pixel is repeated.
do
{
int countsJOffset = counts[jOffset];
int countsJ = counts[j];
if (countsJOffset != countsJ)
{
currLength += countsJOffset < countsJ ? countsJOffset : countsJ;
break;
}
// The same color is repeated counts_pos times at jOffset and j.
currLength += countsJOffset;
jOffset += countsJOffset;
j += countsJOffset;
}while (currLength <= MaxLength && j < pixelCount && bgra[jOffset] == bgra[j]);
if (bestLength < currLength)
{
bestOffset = usePrev ? windowOffsetsNew[ind] : windowOffsets[ind];
if (currLength >= MaxLength)
{
bestLength = MaxLength;
break;
}
else
{
bestLength = currLength;
}
}
}
}
if (bestLength <= MinLength)
{
hashChainOffsetLength[i] = 0;
bestOffsetPrev = 0;
bestLengthPrev = 0;
}
else
{
hashChainOffsetLength[i] = (uint)((bestOffset << MaxLengthBits) | bestLength);
bestOffsetPrev = bestOffset;
bestLengthPrev = bestLength;
}
}
hashChainOffsetLength[0] = 0;
BackwardReferencesLz77(xSize, ySize, bgra, cacheBits, hashChain, refs);
}
private static void BackwardReferencesLz77(int xSize, int ySize, ReadOnlySpan <uint> bgra, int cacheBits, Vp8LHashChain hashChain, Vp8LBackwardRefs refs)
{
int iLastCheck = -1;
bool useColorCache = cacheBits > 0;
int pixCount = xSize * ySize;
var colorCache = new ColorCache();
if (useColorCache)
{
colorCache.Init(cacheBits);
}
refs.Refs.Clear();
for (int i = 0; i < pixCount;)
{
// Alternative #1: Code the pixels starting at 'i' using backward reference.
int j;
int offset = hashChain.FindOffset(i);
int len = hashChain.FindLength(i);
if (len >= MinLength)
{
int lenIni = len;
int maxReach = 0;
int jMax = i + lenIni >= pixCount ? pixCount - 1 : i + lenIni;
// Only start from what we have not checked already.
iLastCheck = i > iLastCheck ? i : iLastCheck;
// We know the best match for the current pixel but we try to find the
// best matches for the current pixel AND the next one combined.
// The naive method would use the intervals:
// [i,i+len) + [i+len, length of best match at i+len)
// while we check if we can use:
// [i,j) (where j<=i+len) + [j, length of best match at j)
for (j = iLastCheck + 1; j <= jMax; j++)
{
int lenJ = hashChain.FindLength(j);
int reach = j + (lenJ >= MinLength ? lenJ : 1); // 1 for single literal.
if (reach > maxReach)
{
len = j - i;
maxReach = reach;
if (maxReach >= pixCount)
{
break;
}
}
}
}
else
{
len = 1;
}
// Go with literal or backward reference.
if (len == 1)
{
AddSingleLiteral(bgra[i], useColorCache, colorCache, refs);
}
else
{
refs.Add(PixOrCopy.CreateCopy((uint)offset, (ushort)len));
if (useColorCache)
{
for (j = i; j < i + len; j++)
{
colorCache.Insert(bgra[j]);
}
}
}
i += len;
}
}
private static void BackwardReferencesHashChainFollowChosenPath(ReadOnlySpan <uint> bgra, int cacheBits, Span <ushort> chosenPath, int chosenPathSize, Vp8LHashChain hashChain, Vp8LBackwardRefs backwardRefs)
{
bool useColorCache = cacheBits > 0;
var colorCache = new ColorCache();
int i = 0;
if (useColorCache)
{
colorCache.Init(cacheBits);
}
backwardRefs.Refs.Clear();
for (int ix = 0; ix < chosenPathSize; ix++)
{
int len = chosenPath[ix];
if (len != 1)
{
int offset = hashChain.FindOffset(i);
backwardRefs.Add(PixOrCopy.CreateCopy((uint)offset, (ushort)len));
if (useColorCache)
{
for (int k = 0; k < len; k++)
{
colorCache.Insert(bgra[i + k]);
}
}
i += len;
}
else
{
PixOrCopy v;
int idx = useColorCache ? colorCache.Contains(bgra[i]) : -1;
if (idx >= 0)
{
// useColorCache is true and color cache contains bgra[i]
// Push pixel as a color cache index.
v = PixOrCopy.CreateCacheIdx(idx);
}
else
{
if (useColorCache)
{
colorCache.Insert(bgra[i]);
}
v = PixOrCopy.CreateLiteral(bgra[i]);
}
backwardRefs.Add(v);
i++;
}
}
}
/// <summary>
/// Evaluates best possible backward references for specified quality. The input cacheBits to 'GetBackwardReferences'
/// sets the maximum cache bits to use (passing 0 implies disabling the local color cache).
/// The optimal cache bits is evaluated and set for the cacheBits parameter.
/// The return value is the pointer to the best of the two backward refs viz, refs[0] or refs[1].
/// </summary>
public static Vp8LBackwardRefs GetBackwardReferences(
int width,
int height,
ReadOnlySpan <uint> bgra,
int quality,
int lz77TypesToTry,
ref int cacheBits,
MemoryAllocator memoryAllocator,
Vp8LHashChain hashChain,
Vp8LBackwardRefs best,
Vp8LBackwardRefs worst)
{
int lz77TypeBest = 0;
double bitCostBest = -1;
int cacheBitsInitial = cacheBits;
Vp8LHashChain hashChainBox = null;
var stats = new Vp8LStreaks();
var bitsEntropy = new Vp8LBitEntropy();
for (int lz77Type = 1; lz77TypesToTry > 0; lz77TypesToTry &= ~lz77Type, lz77Type <<= 1)
{
int cacheBitsTmp = cacheBitsInitial;
if ((lz77TypesToTry & lz77Type) == 0)
{
continue;
}
switch ((Vp8LLz77Type)lz77Type)
{
case Vp8LLz77Type.Lz77Rle:
BackwardReferencesRle(width, height, bgra, 0, worst);
break;
case Vp8LLz77Type.Lz77Standard:
// Compute LZ77 with no cache (0 bits), as the ideal LZ77 with a color cache is not that different in practice.
BackwardReferencesLz77(width, height, bgra, 0, hashChain, worst);
break;
case Vp8LLz77Type.Lz77Box:
hashChainBox = new Vp8LHashChain(memoryAllocator, width * height);
BackwardReferencesLz77Box(width, height, bgra, 0, hashChain, hashChainBox, worst);
break;
}
// Next, try with a color cache and update the references.
cacheBitsTmp = CalculateBestCacheSize(bgra, quality, worst, cacheBitsTmp);
if (cacheBitsTmp > 0)
{
BackwardRefsWithLocalCache(bgra, cacheBitsTmp, worst);
}
// Keep the best backward references.
var histo = new Vp8LHistogram(worst, cacheBitsTmp);
double bitCost = histo.EstimateBits(stats, bitsEntropy);
if (lz77TypeBest == 0 || bitCost < bitCostBest)
{
Vp8LBackwardRefs tmp = worst;
worst = best;
best = tmp;
bitCostBest = bitCost;
cacheBits = cacheBitsTmp;
lz77TypeBest = lz77Type;
}
}
// Improve on simple LZ77 but only for high quality (TraceBackwards is costly).
if ((lz77TypeBest == (int)Vp8LLz77Type.Lz77Standard || lz77TypeBest == (int)Vp8LLz77Type.Lz77Box) && quality >= 25)
{
Vp8LHashChain hashChainTmp = lz77TypeBest == (int)Vp8LLz77Type.Lz77Standard ? hashChain : hashChainBox;
BackwardReferencesTraceBackwards(width, height, memoryAllocator, bgra, cacheBits, hashChainTmp, best, worst);
var histo = new Vp8LHistogram(worst, cacheBits);
double bitCostTrace = histo.EstimateBits(stats, bitsEntropy);
if (bitCostTrace < bitCostBest)
{
best = worst;
}
}
BackwardReferences2DLocality(width, best);
hashChainBox?.Dispose();
return(best);
}
private static void BackwardReferencesHashChainDistanceOnly(
int xSize,
int ySize,
MemoryAllocator memoryAllocator,
ReadOnlySpan <uint> bgra,
int cacheBits,
Vp8LHashChain hashChain,
Vp8LBackwardRefs refs,
IMemoryOwner <ushort> distArrayBuffer)
{
int pixCount = xSize * ySize;
bool useColorCache = cacheBits > 0;
int literalArraySize = WebpConstants.NumLiteralCodes + WebpConstants.NumLengthCodes + (cacheBits > 0 ? 1 << cacheBits : 0);
var costModel = new CostModel(literalArraySize);
int offsetPrev = -1;
int lenPrev = -1;
double offsetCost = -1;
int firstOffsetIsConstant = -1; // initialized with 'impossible' value.
int reach = 0;
var colorCache = new ColorCache();
if (useColorCache)
{
colorCache.Init(cacheBits);
}
costModel.Build(xSize, cacheBits, refs);
using var costManager = new CostManager(memoryAllocator, distArrayBuffer, pixCount, costModel);
Span <float> costManagerCosts = costManager.Costs.GetSpan();
Span <ushort> distArray = distArrayBuffer.GetSpan();
// We loop one pixel at a time, but store all currently best points to non-processed locations from this point.
distArray[0] = 0;
// Add first pixel as literal.
AddSingleLiteralWithCostModel(bgra, colorCache, costModel, 0, useColorCache, 0.0f, costManagerCosts, distArray);
for (int i = 1; i < pixCount; i++)
{
float prevCost = costManagerCosts[i - 1];
int offset = hashChain.FindOffset(i);
int len = hashChain.FindLength(i);
// Try adding the pixel as a literal.
AddSingleLiteralWithCostModel(bgra, colorCache, costModel, i, useColorCache, prevCost, costManagerCosts, distArray);
// If we are dealing with a non-literal.
if (len >= 2)
{
if (offset != offsetPrev)
{
int code = DistanceToPlaneCode(xSize, offset);
offsetCost = costModel.GetDistanceCost(code);
firstOffsetIsConstant = 1;
costManager.PushInterval(prevCost + offsetCost, i, len);
}
else
{
// Instead of considering all contributions from a pixel i by calling:
// costManager.PushInterval(prevCost + offsetCost, i, len);
// we optimize these contributions in case offsetCost stays the same
// for consecutive pixels. This describes a set of pixels similar to a
// previous set (e.g. constant color regions).
if (firstOffsetIsConstant != 0)
{
reach = i - 1 + lenPrev - 1;
firstOffsetIsConstant = 0;
}
if (i + len - 1 > reach)
{
int lenJ = 0;
int j;
for (j = i; j <= reach; j++)
{
int offsetJ = hashChain.FindOffset(j + 1);
lenJ = hashChain.FindLength(j + 1);
if (offsetJ != offset)
{
lenJ = hashChain.FindLength(j);
break;
}
}
// Update the cost at j - 1 and j.
costManager.UpdateCostAtIndex(j - 1, false);
costManager.UpdateCostAtIndex(j, false);
costManager.PushInterval(costManagerCosts[j - 1] + offsetCost, j, lenJ);
reach = j + lenJ - 1;
}
}
}
costManager.UpdateCostAtIndex(i, true);
offsetPrev = offset;
lenPrev = len;
}
}
/// <summary>
/// Evaluate optimal cache bits for the local color cache.
/// The input bestCacheBits sets the maximum cache bits to use (passing 0 implies disabling the local color cache).
/// The local color cache is also disabled for the lower (smaller then 25) quality.
/// </summary>
/// <returns>Best cache size.</returns>
private static int CalculateBestCacheSize(ReadOnlySpan <uint> bgra, int quality, Vp8LBackwardRefs refs, int bestCacheBits)
{
int cacheBitsMax = quality <= 25 ? 0 : bestCacheBits;
if (cacheBitsMax == 0)
{
// Local color cache is disabled.
return(0);
}
double entropyMin = MaxEntropy;
int pos = 0;
var colorCache = new ColorCache[WebpConstants.MaxColorCacheBits + 1];
var histos = new Vp8LHistogram[WebpConstants.MaxColorCacheBits + 1];
for (int i = 0; i <= WebpConstants.MaxColorCacheBits; i++)
{
histos[i] = new Vp8LHistogram(paletteCodeBits: i);
colorCache[i] = new ColorCache();
colorCache[i].Init(i);
}
// Find the cacheBits giving the lowest entropy.
for (int idx = 0; idx < refs.Refs.Count; idx++)
{
PixOrCopy v = refs.Refs[idx];
if (v.IsLiteral())
{
uint pix = bgra[pos++];
uint a = (pix >> 24) & 0xff;
uint r = (pix >> 16) & 0xff;
uint g = (pix >> 8) & 0xff;
uint b = (pix >> 0) & 0xff;
// The keys of the caches can be derived from the longest one.
int key = ColorCache.HashPix(pix, 32 - cacheBitsMax);
// Do not use the color cache for cacheBits = 0.
++histos[0].Blue[b];
++histos[0].Literal[g];
++histos[0].Red[r];
++histos[0].Alpha[a];
// Deal with cacheBits > 0.
for (int i = cacheBitsMax; i >= 1; --i, key >>= 1)
{
if (colorCache[i].Lookup(key) == pix)
{
++histos[i].Literal[WebpConstants.NumLiteralCodes + WebpConstants.NumLengthCodes + key];
}
else
{
colorCache[i].Set((uint)key, pix);
++histos[i].Blue[b];
++histos[i].Literal[g];
++histos[i].Red[r];
++histos[i].Alpha[a];
}
}
}
else
{
// We should compute the contribution of the (distance, length)
// histograms but those are the same independently from the cache size.
// As those constant contributions are in the end added to the other
// histogram contributions, we can ignore them, except for the length
// prefix that is part of the literal_ histogram.
int len = v.Len;
uint bgraPrev = bgra[pos] ^ 0xffffffffu;
int extraBits = 0, extraBitsValue = 0;
int code = LosslessUtils.PrefixEncode(len, ref extraBits, ref extraBitsValue);
for (int i = 0; i <= cacheBitsMax; i++)
{
++histos[i].Literal[WebpConstants.NumLiteralCodes + code];
}
// Update the color caches.
do
{
if (bgra[pos] != bgraPrev)
{
// Efficiency: insert only if the color changes.
int key = ColorCache.HashPix(bgra[pos], 32 - cacheBitsMax);
for (int i = cacheBitsMax; i >= 1; --i, key >>= 1)
{
colorCache[i].Colors[key] = bgra[pos];
}
bgraPrev = bgra[pos];
}
pos++;
}while (--len != 0);
}
}
var stats = new Vp8LStreaks();
var bitsEntropy = new Vp8LBitEntropy();
for (int i = 0; i <= cacheBitsMax; i++)
{
double entropy = histos[i].EstimateBits(stats, bitsEntropy);
if (i == 0 || entropy < entropyMin)
{
entropyMin = entropy;
bestCacheBits = i;
}
}
return(bestCacheBits);
}
/// <summary>
/// Initializes a new instance of the <see cref="Vp8LHistogram"/> class.
/// </summary>
/// <param name="refs">The backward references to initialize the histogram with.</param>
/// <param name="paletteCodeBits">The palette code bits.</param>
public Vp8LHistogram(Vp8LBackwardRefs refs, int paletteCodeBits)
: this(paletteCodeBits) => this.StoreRefs(refs);