SkScaledCodec class

src/codec/SkScaledCodec.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkCodecPriv.h"
+#include "SkScaledCodec.h"
+#include "SkStream.h"
+#include "SkWebpCodec.h"
+
+
+SkCodec* SkScaledCodec::NewFromStream(SkStream* stream) {
+    bool isWebp = SkWebpCodec::IsWebp(stream);
+    if (!stream->rewind()) {
+        return NULL;
+    }
+    if (isWebp) {
+        // Webp codec supports scaling and subsetting natively
+        return SkWebpCodec::NewFromStream(stream);  
+    }
+
+    SkAutoTDelete<SkScanlineDecoder> scanlineDecoder(SkScanlineDecoder::NewFromStream(stream));
+    if (NULL == scanlineDecoder) {
+        return NULL;
+    }
+
+    // wrap in new SkScaledCodec
+    return SkNEW_ARGS(SkScaledCodec, (scanlineDecoder.detach()));
+}
+
+SkCodec* SkScaledCodec::NewFromData(SkData* data) {
+    if (!data) {
+        return NULL;
+    }
+    return NewFromStream(SkNEW_ARGS(SkMemoryStream, (data)));
+}
+
+SkScaledCodec::SkScaledCodec(SkScanlineDecoder* scanlineDecoder)
+    : INHERITED(scanlineDecoder->getInfo(), NULL)
+    , fScanlineDecoder(scanlineDecoder)
+{}
+
+SkScaledCodec::~SkScaledCodec() {}
+
+// returns a scaled dimension based on the original dimension and the sampleSize
+// NOTE: we round down here for scaled dimension to match the behavior of SkImageDecoder
+static int get_scaled_dimension(int srcDimension, int sampleSize) {
+    if (sampleSize > srcDimension) {
+        return 1;
+    }
+    return srcDimension / sampleSize;
+}
+
+static SkISize best_scaled_dimensions(const SkISize& origDims, const SkISize& nativeDims, 
+                                      const SkISize& scaledCodecDims, float desiredScale) {
+    if (nativeDims == scaledCodecDims) {
+        // does not matter which to return if equal. Return here to skip below calculations
+        return nativeDims;
+    }
+    float idealWidth = origDims.width() * desiredScale;
+    float idealHeight = origDims.height() * desiredScale;
+
+    // calculate difference between native dimensions and ideal dimensions
+    float nativeWDiff = SkTAbs(idealWidth - nativeDims.width());
+    float nativeHDiff = SkTAbs(idealHeight - nativeDims.height());
+    float nativeDiff = (nativeWDiff + nativeHDiff) / 2;
+
+    // calculate difference between scaledCodec dimensions and ideal dimensions
+    float scaledCodecWDiff = SkTAbs(idealWidth - scaledCodecDims.width());
+    float scaledCodecHDiff = SkTAbs(idealHeight - scaledCodecDims.height());
+    float scaledCodecDiff = (scaledCodecWDiff + scaledCodecHDiff) / 2;
+
+    // return dimensions closest to ideal dimensions.
+    // If the differences are equal, return nativeDims, as native scaling is more efficient.
+    return nativeDiff > scaledCodecDiff ? scaledCodecDims : nativeDims;
+
+} 
+/*
+ * Return a valid set of output dimensions for this decoder, given an input scale
+ */
+SkISize SkScaledCodec::onGetScaledDimensions(float desiredScale) const {
+    SkISize nativeDimensions = fScanlineDecoder->getScaledDimensions(desiredScale);
+    // support scaling down by integer numbers. Ex: 1/2, 1/3, 1/4 ...
+    SkISize scaledCodecDimensions;
+    if (desiredScale > 0.5f) {
+        // sampleSize = 1
+        scaledCodecDimensions = fScanlineDecoder->getInfo().dimensions();
+    }
+    // sampleSize determines the step size between samples
+    // Ex: sampleSize = 2, sample every second pixel in x and y directions
+    int sampleSize = int(1 / desiredScale);
+
+    int scaledWidth = get_scaled_dimension(this->getInfo().width(), sampleSize);
+    int scaledHeight = get_scaled_dimension(this->getInfo().height(), sampleSize);
+
+    // Return the calculated output dimensions for the given scale
+    scaledCodecDimensions = SkISize::Make(scaledWidth, scaledHeight);
+
+    return best_scaled_dimensions(this->getInfo().dimensions(), nativeDimensions, 
+                                  scaledCodecDimensions, desiredScale);
+}
+
+// check if scaling to dstInfo size from srcInfo size using sampleSize is possible
+static bool scaling_supported(const SkImageInfo& dstInfo, const SkImageInfo& srcInfo,
+                              int sampleX, int sampleY) {
+     const int dstWidth = dstInfo.width();
+     const int dstHeight = dstInfo.height();
+     const int srcWidth = srcInfo.width();
+     const int srcHeight = srcInfo.height();
+      // only support down sampling, not up sampling
+     if (dstWidth > srcWidth || dstHeight  > srcHeight) {
+         return false;
+     }
+     // check that srcWidth is scaled down by an integer value
+     if (get_scaled_dimension(srcWidth, sampleX) != dstWidth) {
+         return false;
+     }
+     // check that src height is scaled down by an integer value
+     if (get_scaled_dimension(srcHeight, sampleY) != dstHeight) {
+         return false;
+     }
+     // sampleX and sampleY should be equal unless the original sampleSize requested was larger
+     // than srcWidth or srcHeight. If so, the result of this is dstWidth or dstHeight = 1.
+     // This functionality allows for tall thin images to still be scaled down by scaling factors.
+     if (sampleX != sampleY){
+         if (1 != dstWidth && 1 != dstHeight) {
+             return false;
+         }
+     }
+     return true;
+}
+
+// calculates sampleSize in x and y direction
+void SkScaledCodec::ComputeSampleSize(const SkImageInfo& dstInfo, const SkImageInfo& srcInfo,
+                                      int* sampleSizeX, int* sampleSizeY) {

[scroggo, 2015/08/13 18:01:21]

nit: I would probably name these "sampleXPtr" and "sampleYPtr". Not a big deal,
but it seems arbitrary that the difference between it and the local variable on
the stack is "Size".

+    int srcWidth = srcInfo.width();
+    int dstWidth = dstInfo.width();
+    int srcHeight = srcInfo.height();
+    int dstHeight = dstInfo.height();
+
+    int sampleX = srcWidth / dstWidth;
+    int sampleY = srcHeight / dstHeight;
+
+    // only support down sampling, not up sampling
+    SkASSERT(dstWidth <= srcWidth);
+    SkASSERT(dstHeight <= srcHeight);
+
+    // sampleX and sampleY should be equal unless the original sampleSize requested was
+    // larger than srcWidth or srcHeight.
+    // If so, the result of this is dstWidth or dstHeight = 1. This functionality
+    // allows for tall thin images to still be scaled down by scaling factors.
+
+    if (sampleX != sampleY){
+        if (1 != dstWidth && 1 != dstHeight) {
+
+            // rounding during onGetScaledDimensions can cause different sampleSizes
+            // Ex: srcWidth = 79, srcHeight = 20, sampleSize = 10
+            // dstWidth = 7, dstHeight = 2, sampleX = 79/7 = 11, sampleY = 20/2 = 10
+            // correct for this rounding by comparing width to sampleY and height to sampleX
+
+            if (get_scaled_dimension(srcWidth, sampleY) == dstWidth) {
+                sampleX = sampleY;
+            } else if (get_scaled_dimension(srcHeight, sampleX) == dstHeight) {
+                sampleY = sampleX;
+            }
+        }
+    }
+
+    if (sampleSizeX) {
+        *sampleSizeX = sampleX;
+    }
+    if (sampleSizeY) {
+        *sampleSizeY = sampleY;
+    }
+}
+
+// TODO: Implement subsetting in onGetPixels which works when and when not sampling 
+
+SkCodec::Result SkScaledCodec::onGetPixels(const SkImageInfo& requestedInfo, void* dst,
+                                           size_t rowBytes, const Options& options,
+                                           SkPMColor ctable[], int* ctableCount) {
+
+    if (options.fSubset) {
+        // Subsets are not supported.
+        return kUnimplemented;
+    } 
+
+    Result result = fScanlineDecoder->start(requestedInfo, &options, ctable, ctableCount);
+    if (kSuccess == result) {
+        // native decode supported
+        return fScanlineDecoder->getScanlines(dst, requestedInfo.height(), rowBytes);
+
+    }
+
+    if (kInvalidScale != result) {
+        // no scaling requested
+        return result;
+    }
+    
+    // scaling requested
+    int sampleX;
+    int sampleY;
+    ComputeSampleSize(requestedInfo, fScanlineDecoder->getInfo(), &sampleX, &sampleY);
+    if (!scaling_supported(requestedInfo, fScanlineDecoder->getInfo(), sampleX, sampleY)) {
+        return kInvalidScale;
+    }
+    // set first sample pixel in y direction
+    int Y0 = sampleY >> 1;
+
+    int dstHeight = requestedInfo.height();
+    int srcHeight = fScanlineDecoder->getInfo().height();
+    
+    SkImageInfo info = requestedInfo;
+    // use original height as scanlineDecoder does not support y sampling natively
+    info = info.makeWH(requestedInfo.width(), srcHeight);
+
+    // update scanlineDecoder with new info
+    result = fScanlineDecoder->start(info, &options, ctable, ctableCount);
+    if (kSuccess != result) {
+        return result;
+    }
+    
+    const bool requiresPostYSampling = fScanlineDecoder->requiresPostYSampling();
+
+    if (requiresPostYSampling) {
+        SkAutoMalloc storage(srcHeight * rowBytes);
+        uint8_t* storagePtr = static_cast<uint8_t*>(storage.get());
+        result = fScanlineDecoder->getScanlines(storagePtr, srcHeight, rowBytes);
+        if (kSuccess != result) {
+            return result;
+        }
+        storagePtr += Y0 * rowBytes;
+        for (int y = 0; y < dstHeight; y++) {
+            memcpy(dst, storagePtr, rowBytes);
+            storagePtr += sampleY * rowBytes;
+            dst = SkTAddOffset<void>(dst, rowBytes);
+        }
+    } else {
+        // does not require post y sampling
+        result = fScanlineDecoder->skipScanlines(Y0);
+        if (kSuccess != result) {
+            return result;
+        }
+        for (int y = 0; y < dstHeight; y++) {
+            result = fScanlineDecoder->getScanlines(dst, 1, rowBytes);
+            if (kSuccess != result) {
+                return result;
+            }
+            if (y < dstHeight - 1) {
+                result = fScanlineDecoder->skipScanlines(sampleY - 1);
+                if (kSuccess != result) {
+                    return result;
+                }
+            }
+            dst = SkTAddOffset<void>(dst, rowBytes);
+        }
+    }
+    return kSuccess;
+}