Add subsetting to SkScaledCodec

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 nullptr;
     }
     if (isWebp) {
         // Webp codec supports scaling and subsetting natively
-        return SkWebpCodec::NewFromStream(stream);  
+        return SkWebpCodec::NewFromStream(stream);
     }
 
     SkAutoTDelete<SkCodec> codec(SkCodec::NewFromStream(stream));
     if (nullptr == codec) {
         return nullptr;
     }
 
     // wrap in new SkScaledCodec
     return new SkScaledCodec(codec.detach());
 }
 
 SkCodec* SkScaledCodec::NewFromData(SkData* data) {
     if (!data) {
         return nullptr;
     }
     return NewFromStream(new SkMemoryStream(data));
 }
 
 SkScaledCodec::SkScaledCodec(SkCodec* codec)
     : INHERITED(codec->getInfo(), nullptr)
     , fCodec(codec)
 {}
 
 SkScaledCodec::~SkScaledCodec() {}
 
-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;
+static bool is_in_subset(int coord, int offset, int length) {

[scroggo, 2015/10/02 18:27:03]

I thought we had something like this function, but I may have been thinking of
SkTPin, which is not exactly what you want.

+    if (coord < offset || coord >= offset + length) {

[scroggo, 2015/10/02 18:27:03]

Why not:

return coord >= offset && coord < offset + length;

?

+        return false;
     }
+    return true;
+}
+
+static int get_sample_size(float scale) {

[scroggo, 2015/10/02 18:27:03]

Don't you define this somewhere else?

+    return SkScalarRoundToInt(1.0f / scale);
+}
+
+static bool use_native_scaling(const SkISize& origDims, const SkISize& nativeDims,
+        const SkISize& sampledDims, float desiredScale) {
+    if (nativeDims == sampledDims) {
+        // If both options are the same, choose native scaling.
+        return true;
+    }
+
     float idealWidth = origDims.width() * desiredScale;
     float idealHeight = origDims.height() * desiredScale;
 
-    // calculate difference between native dimensions and ideal dimensions
+    // Calculate difference between native scaling and ideal scaling.
     float nativeWDiff = SkTAbs(idealWidth - nativeDims.width());
     float nativeHDiff = SkTAbs(idealHeight - nativeDims.height());
     float nativeDiff = nativeWDiff + nativeHDiff;
 
     // Native scaling is preferred to sampling.  If we can scale natively to
     // within one of the ideal value, we should choose to scale natively.
     if (nativeWDiff < 1.0f && nativeHDiff < 1.0f) {
-        return nativeDims;
+        return true;
     }
 
-    // calculate difference between scaledCodec dimensions and ideal dimensions
-    float scaledCodecWDiff = SkTAbs(idealWidth - scaledCodecDims.width());
-    float scaledCodecHDiff = SkTAbs(idealHeight - scaledCodecDims.height());
-    float scaledCodecDiff = scaledCodecWDiff + scaledCodecHDiff;
+    // Calculate difference between native scaling and sampled scaling.
+    float sampledWDiff = SkTAbs(idealWidth - sampledDims.width());
+    float sampledHDiff = SkTAbs(idealHeight - sampledDims.height());
+    float sampledDiff = sampledWDiff + sampledHDiff;
 
-    // return dimensions closest to ideal dimensions.
-    // If the differences are equal, return nativeDims, as native scaling is more efficient.
-    return nativeDiff > scaledCodecDiff ? scaledCodecDims : nativeDims;
+    // Use native scaling if it is closer to the ideal scale.
+    return nativeDiff <= sampledDiff;
 }
 
 /*
  * Return a valid set of output dimensions for this decoder, given an input scale
  */
 SkISize SkScaledCodec::onGetScaledDimensions(float desiredScale) const {
-    SkISize nativeDimensions = fCodec->getScaledDimensions(desiredScale);
-    // support scaling down by integer numbers. Ex: 1/2, 1/3, 1/4 ...
-    SkISize scaledCodecDimensions;
-    if (desiredScale > 0.5f) {
-        // sampleSize = 1
-        scaledCodecDimensions = fCodec->getInfo().dimensions();
+    SkISize nativeDims = fCodec->getScaledDimensions(desiredScale);
+    int sampleSize = get_sample_size(desiredScale);
+    SkISize sampledDims = SkISize::Make(
+            get_scaled_dimension(this->getInfo().width(), sampleSize),
+            get_scaled_dimension(this->getInfo().height(), sampleSize));
+
+    if (use_native_scaling(this->getInfo().dimensions(), nativeDims, sampledDims, desiredScale)) {
+        return nativeDims;
     }
-    // sampleSize determines the step size between samples
-    // Ex: sampleSize = 2, sample every second pixel in x and y directions
-    int sampleSize = int ((1.0f / desiredScale) + 0.5f);
 
-    int scaledWidth = get_scaled_dimension(this->getInfo().width(), sampleSize);
-    int scaledHeight = get_scaled_dimension(this->getInfo().height(), sampleSize);
+    return sampledDims;
+}
 
-    // Return the calculated output dimensions for the given scale
-    scaledCodecDimensions = SkISize::Make(scaledWidth, scaledHeight);
+bool SkScaledCodec::onGetScaledSubsetDimensions(float desiredScale, Options* options) const {
 
-    return best_scaled_dimensions(this->getInfo().dimensions(), nativeDimensions,
-                                  scaledCodecDimensions, desiredScale);
+    SkISize nativeDims = fCodec->getScaledDimensions(desiredScale);
+    int sampleSize = get_sample_size(desiredScale);
+    SkISize sampledDims = SkISize::Make(
+            get_scaled_dimension(this->getInfo().width(), sampleSize),
+            get_scaled_dimension(this->getInfo().height(), sampleSize));
+
+    if (use_native_scaling(this->getInfo().dimensions(), nativeDims, sampledDims, desiredScale)) {
+        // Set the scaled dimensions and calculate subset size using native scaling.
+        options->fScaledDimensions = nativeDims;
+        float widthScale = ((float) nativeDims.width()) / ((float) this->getInfo().width());
+        float heightScale = ((float) nativeDims.height()) / ((float) this->getInfo().height());
+        // Notice that we may round the size of the subset up to 1.  This means that we must
+        // floor the left and top offsets to ensure that we do not suggest a subset that is
+        // off the edge of the image.
+        options->fScaledSubset = SkIRect::MakeXYWH(
+                int (((float) options->fSubset->left()) * widthScale),
+                int (((float) options->fSubset->top()) * heightScale),
+                SkTMax(1, SkScalarRoundToInt(((float) options->fSubset->width()) * widthScale)),
+                SkTMax(1, SkScalarRoundToInt(((float) options->fSubset->height()) * heightScale)));
+        return true;
+    }
+
+    // Set the scaled dimensions and calculate subset size using sampling.
+    options->fScaledDimensions = sampledDims;
+    options->fScaledSubset = SkIRect::MakeXYWH(
+            options->fSubset->left() / sampleSize,
+            options->fSubset->top() / sampleSize,
+            get_scaled_dimension(options->fSubset->width(), sampleSize),
+            get_scaled_dimension(options->fSubset->height(), sampleSize));
+    return true;
 }
 
 // check if scaling to dstInfo size from srcInfo size using sampleSize is possible
-static bool scaling_supported(const SkImageInfo& dstInfo, const SkImageInfo& srcInfo,
+static bool scaling_supported(const SkISize& dstSize, const SkISize& srcSize,
                               int* sampleX, int* sampleY) {
-    SkScaledCodec::ComputeSampleSize(dstInfo, srcInfo, sampleX, sampleY);
-    const int dstWidth = dstInfo.width();
-    const int dstHeight = dstInfo.height();
-    const int srcWidth = srcInfo.width();
-    const int srcHeight = srcInfo.height();
+    SkScaledCodec::ComputeSampleSize(dstSize, srcSize, sampleX, sampleY);
+    const int dstWidth = dstSize.width();
+    const int dstHeight = dstSize.height();
+    const int srcWidth = srcSize.width();
+    const int srcHeight = srcSize.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,
+void SkScaledCodec::ComputeSampleSize(const SkISize& dstSize, const SkISize& srcSize,
                                       int* sampleXPtr, int* sampleYPtr) {
-    int srcWidth = srcInfo.width();
-    int dstWidth = dstInfo.width();
-    int srcHeight = srcInfo.height();
-    int dstHeight = dstInfo.height();
+    int srcWidth = srcSize.width();
+    int dstWidth = dstSize.width();
+    int srcHeight = srcSize.height();
+    int dstHeight = dstSize.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;
             }
+            // FIXME (msarett): Should this never be reached?
         }
     }
 
     if (sampleXPtr) {
         *sampleXPtr = sampleX;
     }
     if (sampleYPtr) {
         *sampleYPtr = sampleY;
     }
 }
 
-// TODO: Implement subsetting in onGetPixels which works when and when not sampling 
+SkCodec::Result SkScaledCodec::onGetPixels(const SkImageInfo& scaledSubsetInfo, void* dst,
+        size_t rowBytes, const Options& options, SkPMColor ctable[], int* ctableCount,
+        int* rowsDecoded) {
 
-SkCodec::Result SkScaledCodec::onGetPixels(const SkImageInfo& requestedInfo, void* dst,
-                                           size_t rowBytes, const Options& options,
-                                           SkPMColor ctable[], int* ctableCount,
-                                           int* rowsDecoded) {
+    // There are various values of Rect, Size, and Info used in this function.  I think it is
+    // useful to go ahead and define what they mean.
+    // orig_:         Refers to the original image size.
+    // scaledSubset_: Refers to the size of the final output.  This can match the original
+    //                dimensions, be a subset of the original dimensions, be a scaled version
+    //                of the original dimensions, or be a scaled subset of the original dimensions.
+    // subset_:       Refers to the size of the unscaled subset in terms of the original image
+    //                dimensions.  If this is not a subset decode, this will match the original
+    //                image dimensions.
+    // scaled_:       Refers to the scaled size of the original image, ignoring any subsetting.
+    //                If we are not scaling, this will match the original dimensions.
+    SkISize origSize = this->getInfo().dimensions();
+    SkIRect subsetRect;
+    SkISize scaledSize;
+    SkIRect scaledSubsetRect;
+    if (nullptr == options.fSubset) {
+        // This is not a subset decode.
+        SkASSERT(options.fScaledDimensions.isZero());
+        SkASSERT(options.fScaledSubset.isEmpty());
 
-    if (options.fSubset) {
-        // Subsets are not supported.
-        return kUnimplemented;
+        // Set the "subset" to the full image dimensions.
+        subsetRect = SkIRect::MakeSize(origSize);
+
+        // This may be scaled or unscaled, depending on if scaledSize matches origSize.
+        scaledSize = scaledSubsetInfo.dimensions();
+        scaledSubsetRect = SkIRect::MakeSize(scaledSize);
+    } else {
+        // This is a subset decode.
+        if (!is_valid_subset(options.fSubset, origSize)) {
+            return kInvalidParameters;
+        }
+
+        subsetRect = *(options.fSubset);
+        if (options.fScaledDimensions.isZero()) {
+            // This is an unscaled subset decode.
+            SkASSERT(options.fScaledSubset.isEmpty());
+            SkASSERT(scaledSubsetInfo.dimensions() == subsetRect.size());
+
+            scaledSize = origSize;
+            scaledSubsetRect = subsetRect;
+        } else {
+            // This is a scaled subset decode.
+            SkASSERT(!options.fScaledSubset.isEmpty());
+            SkASSERT(scaledSubsetInfo.dimensions() == options.fScaledSubset.size());
+            if (!is_valid_subset(&options.fScaledSubset, options.fScaledDimensions)) {
+                return kInvalidParameters;
+            }
+
+            scaledSize = options.fScaledDimensions;
+            scaledSubsetRect = options.fScaledSubset;
+        }
     }
 
-    // FIXME: If no scaling/subsets are requested, we can call fCodec->getPixels.
-    Result result = fCodec->startScanlineDecode(requestedInfo, &options, ctable, ctableCount);
-    if (kSuccess == result) {
-        // native decode supported
-        switch (fCodec->getScanlineOrder()) {
-            case SkCodec::kTopDown_SkScanlineOrder:
-            case SkCodec::kBottomUp_SkScanlineOrder:
-            case SkCodec::kNone_SkScanlineOrder:
-                if (fCodec->getScanlines(dst, requestedInfo.height(), rowBytes) !=
-                        requestedInfo.height()) {
-                    // fCodec has already handled filling uninitialized memory.
-                    *rowsDecoded = requestedInfo.height();
-                    return kIncompleteInput;
-                }
-                return kSuccess;
-            case SkCodec::kOutOfOrder_SkScanlineOrder: {
-                for (int y = 0; y < requestedInfo.height(); y++) {
-                    int dstY = fCodec->nextScanline();
-                    void* dstPtr = SkTAddOffset<void>(dst, rowBytes * dstY);
+    // Reset the options for use by fCodec.  We will handle scaling and subsetting
+    // from this level, the native codec does not need to know about it.
+    Options newOptions = options;
+    newOptions.fSubset = nullptr;
+    newOptions.fScaledDimensions = SkISize::Make(0, 0);
+    newOptions.fScaledSubset = SkIRect::MakeEmpty();
+
+    // The native decoder needs the scaled size of the entire image to check if it can decode to
+    // the requested scale.
+    SkImageInfo scaledInfo = scaledSubsetInfo.makeWH(scaledSize.width(), scaledSize.height());
+    Result result = fCodec->startScanlineDecode(scaledInfo, &newOptions, ctable, ctableCount,
+                scaledSubsetRect.left(), scaledSubsetRect.width());
+    switch (result) {
+        case kSuccess:
+            return this->nativeDecode(scaledInfo, dst, rowBytes, scaledSubsetRect,
+                    options.fZeroInitialized, rowsDecoded);
+        case kInvalidScale:
+            // We will attempt to scale by sampling below.
+            break;
+        default:
+            return result;
+    }
+
+    // Try to provide the scale using sampling.
+    int sampleX;
+    int sampleY;
+    if (!scaling_supported(scaledSubsetInfo.dimensions(), subsetRect.size(), &sampleX, &sampleY)) {
+        return kInvalidScale;
+    }
+
+    // Create the image info that will be passed to fCodec.  We support scaling in the
+    // x-dimension, but we will perform scaling in the y-dimension here, so we pass the scaled
+    // width and the original height.
+    // FIXME: This is a bit confusing.  I think there is a plan to move scaling completely out
+    //        of the native codec.
+    SkImageInfo decodeInfo = scaledSubsetInfo.makeWH(scaledSize.width(), this->getInfo().height());
+
+    // When starting the fCodec, we pass the left offset based on the original image
+    // dimensions, but need the scaled version of the subset width.
+    // FIXME: This is a bit confusing.  I think there is a plan to move scaling completely out
+    //        of the native codec.
+    result = fCodec->startScanlineDecode(decodeInfo, &newOptions, ctable, ctableCount,
+            subsetRect.left(), scaledSubsetInfo.width());
+    if (kSuccess != result) {
+        SkASSERT(kInvalidScale != result);
+        return result;
+    }
+
+    return this->sampledDecode(scaledSubsetInfo, dst, rowBytes, subsetRect, scaledSubsetRect,
+            sampleX, sampleY, options.fZeroInitialized, rowsDecoded);
+}
+
+SkCodec::Result SkScaledCodec::nativeDecode(const SkImageInfo& scaledInfo, void* dst,
+        size_t rowBytes, const SkIRect& scaledSubsetRect, ZeroInitialized zeroInit,
+        int* rowsDecoded) {
+
+    int scaledSubsetTop = scaledSubsetRect.top();
+    int scaledSubsetHeight = scaledSubsetRect.height();
+    switch (fCodec->getScanlineOrder()) {
+        case SkCodec::kTopDown_SkScanlineOrder:
+        case SkCodec::kBottomUp_SkScanlineOrder:
+        case SkCodec::kNone_SkScanlineOrder: {
+            if (!fCodec->skipScanlines(scaledSubsetTop)) {
+                *rowsDecoded = 0;
+                return kIncompleteInput;
+            }
+
+            uint32_t decodedLines = fCodec->getScanlines(dst, scaledSubsetHeight,
+                    rowBytes);
+            if (decodedLines != scaledSubsetHeight) {
+                *rowsDecoded = decodedLines;
+                return kIncompleteInput;
+            }
+            return kSuccess;
+        }
+        case SkCodec::kOutOfOrder_SkScanlineOrder: {
+            for (int y = 0; y < scaledInfo.height(); y++) {
+                int dstY = fCodec->nextScanline();
+                if (is_in_subset(dstY, scaledSubsetTop, scaledSubsetHeight)) {
+                    void* dstPtr = SkTAddOffset<void>(dst, rowBytes * (dstY - scaledSubsetTop));
                     if (1 != fCodec->getScanlines(dstPtr, 1, rowBytes)) {
                         *rowsDecoded = y + 1;
                         return kIncompleteInput;
                     }
+                } else {
+                    if (!fCodec->skipScanlines(1)) {
+                        *rowsDecoded = y + 1;
+                        return kIncompleteInput;
+                    }
                 }
-                return kSuccess;
             }
+            return kSuccess;
         }
     }
+}
 
-    if (kInvalidScale != result) {
-        // no scaling requested
-        return result;
-    }
+SkCodec::Result SkScaledCodec::sampledDecode(const SkImageInfo& scaledSubsetInfo, void* dst,
+        size_t rowBytes, const SkIRect& subsetRect, const SkIRect& scaledSubsetRect, int sampleX,
+        int sampleY, ZeroInitialized zeroInit, int* rowsDecoded) {
 
-    // scaling requested
-    int sampleX;
-    int sampleY;
-    if (!scaling_supported(requestedInfo, fCodec->getInfo(), &sampleX, &sampleY)) {
-        return kInvalidScale;
-    }
-    // set first sample pixel in y direction
-    int Y0 = get_start_coord(sampleY);
-
-    int dstHeight = requestedInfo.height();
-    int srcHeight = fCodec->getInfo().height();
-    
-    SkImageInfo info = requestedInfo;
-    // use original height as codec does not support y sampling natively
-    info = info.makeWH(requestedInfo.width(), srcHeight);
-
-    // update codec with new info
-    // FIXME: The previous call to start returned kInvalidScale. This call may
-    // require a rewind. (skbug.com/4284)
-    result = fCodec->startScanlineDecode(info, &options, ctable, ctableCount);
-    if (kSuccess != result) {
-        return result;
-    }
-
+    // Set first sample pixel in y direction.
+    int y0 = get_start_coord(sampleY);
+    int scaledSubsetHeight = scaledSubsetRect.height();
     switch(fCodec->getScanlineOrder()) {
-        case SkCodec::kTopDown_SkScanlineOrder: {
-            if (!fCodec->skipScanlines(Y0)) {
+        case SkCodec::kTopDown_SkScanlineOrder:
+            if (!fCodec->skipScanlines(y0 + subsetRect.top())) {
                 *rowsDecoded = 0;
                 return kIncompleteInput;
             }
-            for (int y = 0; y < dstHeight; y++) {
+            for (int y = 0; y < scaledSubsetHeight; y++) {
                 if (1 != fCodec->getScanlines(dst, 1, rowBytes)) {
                     // The failed call to getScanlines() will take care of
                     // filling the failed row, so we indicate that we have
                     // decoded (y + 1) rows.
                     *rowsDecoded = y + 1;
                     return kIncompleteInput;
                 }
-                if (y < dstHeight - 1) {
+                if (y < scaledSubsetHeight - 1) {
                     if (!fCodec->skipScanlines(sampleY - 1)) {
                         *rowsDecoded = y + 1;
                         return kIncompleteInput;
                     }
                 }
                 dst = SkTAddOffset<void>(dst, rowBytes);
             }
             return kSuccess;
-        }
         case SkCodec::kBottomUp_SkScanlineOrder:
         case SkCodec::kOutOfOrder_SkScanlineOrder: {
             Result result = kSuccess;
             int y;
-            for (y = 0; y < srcHeight; y++) {
+            for (y = 0; y < this->getInfo().height(); y++) {
                 int srcY = fCodec->nextScanline();
-                if (is_coord_necessary(srcY, sampleY, dstHeight)) {
+                if (is_coord_necessary(srcY, sampleY, scaledSubsetHeight, subsetRect.top())) {
                     void* dstPtr = SkTAddOffset<void>(dst, rowBytes * get_dst_coord(srcY, sampleY));
                     if (1 != fCodec->getScanlines(dstPtr, 1, rowBytes)) {
                         result = kIncompleteInput;
                         break;
                     }
                 } else {
                     if (!fCodec->skipScanlines(1)) {
                         result = kIncompleteInput;
                         break;
                     }
                 }
             }
 
             // We handle filling uninitialized memory here instead of in the parent class.
             // The parent class does not know that we are sampling.
             if (kIncompleteInput == result) {
-                const SkImageInfo fillInfo = requestedInfo.makeWH(requestedInfo.width(), 1);
+                const SkImageInfo fillInfo = scaledSubsetInfo.makeWH(scaledSubsetRect.width(), 1);
                 const uint32_t fillValue = fCodec->getFillValue(fillInfo.colorType(),
                         fillInfo.alphaType());
-                for (; y < srcHeight; y++) {
+                for (; y < this->getInfo().height(); y++) {
                     int srcY = fCodec->outputScanline(y);
-                    if (is_coord_necessary(srcY, sampleY, dstHeight)) {
+                    if (is_coord_necessary(srcY, sampleY, scaledSubsetHeight, subsetRect.top())) {
                         void* dstRow = SkTAddOffset<void>(dst,
                                 rowBytes * get_dst_coord(srcY, sampleY));
-                        SkSwizzler::Fill(dstRow, fillInfo, rowBytes, fillValue,
-                                options.fZeroInitialized);
+                        SkSwizzler::Fill(dstRow, fillInfo, rowBytes, fillValue, zeroInit);
                     }
                 }
-                *rowsDecoded = dstHeight;
+                *rowsDecoded = scaledSubsetInfo.height();
             }
             return result;
         }
         case SkCodec::kNone_SkScanlineOrder: {
-            SkAutoMalloc storage(srcHeight * rowBytes);
+            SkAutoMalloc storage(subsetRect.height() * rowBytes);
             uint8_t* storagePtr = static_cast<uint8_t*>(storage.get());
-            int scanlines = fCodec->getScanlines(storagePtr, srcHeight, rowBytes);
-            storagePtr += Y0 * rowBytes;
-            scanlines -= Y0;
+            if (!fCodec->skipScanlines(subsetRect.top())) {
+                *rowsDecoded = 0;
+                return kIncompleteInput;
+            }
+            int scanlines = fCodec->getScanlines(storagePtr, subsetRect.height(), rowBytes);
+            scanlines -= y0;
+            storagePtr += y0 * rowBytes;
             int y = 0;
-            while (y < dstHeight && scanlines > 0) {
-                memcpy(dst, storagePtr, rowBytes);
+            while (y < scaledSubsetHeight && scanlines > 0) {
+                memcpy(dst, storagePtr, scaledSubsetInfo.minRowBytes());
                 storagePtr += sampleY * rowBytes;
                 dst = SkTAddOffset<void>(dst, rowBytes);
                 scanlines -= sampleY;
                 y++;
             }
-            if (y < dstHeight) {
+            if (y < scaledSubsetHeight) {
                 // fCodec has already handled filling uninitialized memory.
-                *rowsDecoded = dstHeight;
+                *rowsDecoded = scaledSubsetInfo.height();
                 return kIncompleteInput;
             }
             return kSuccess;
         }
         default:
             SkASSERT(false);
             return kUnimplemented;
     }
 }
 
 uint32_t SkScaledCodec::onGetFillValue(SkColorType colorType, SkAlphaType alphaType) const {
     return fCodec->onGetFillValue(colorType, alphaType);
 }
 
 SkCodec::SkScanlineOrder SkScaledCodec::onGetScanlineOrder() const {
     return fCodec->onGetScanlineOrder();
 }