SkScaledCodec class

This class does scaling by using a scanlineDecoder.
getScanlines and skipScanlines are used for y sampling,
the swizzler is used for x sampling

this class is currently only working for png and jpeg images
I will update other Codec types to work soon

For SkJpegCodec to implement width wise swizzling it now
uses a swizzler. I ran performance tests on this change.
Here are the performance test results:
https://docs.google.com/a/google.com/spreadsheets/d/1D7-Q_GXD_dI68LZO005NNvb8Wq2Ee0wEBEPG72671yw/edit?usp=sharing

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/0944100ac89f797714eeae0cf2875e2335ff52ee

Review URL: https://codereview.chromium.org/1260673002

3 files changed, 114 insertions, 0 deletions

diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index a0bd18a8e5..3465c1df5b 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -51,6 +51,8 @@ public:
      *  Return a size that approximately supports the desired scale factor.
      *  The codec may not be able to scale efficiently to the exact scale
      *  factor requested, so return a size that approximates that scale.
+     *  The returned value is the codec's suggestion for the closest valid
+     *  scale that it can natively support
      */
     SkISize getScaledDimensions(float desiredScale) const {
         return this->onGetScaledDimensions(desiredScale);
diff --git a/include/codec/SkScaledCodec.h b/include/codec/SkScaledCodec.h
new file mode 100644
index 0000000000..1bcdf085b2
--- /dev/null
+++ b/include/codec/SkScaledCodec.h
@@ -0,0 +1,70 @@
+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef SkScaledCodec_DEFINED
+#define SkScaledCodec_DEFINED
+
+#include "SkCodec.h"
+#include "SkScanlineDecoder.h"
+
+class SkScanlineDecoder;
+class SkStream;
+
+/**
+ * This class implements scaling, by sampling scanlines in the y direction.
+ * x-wise sampling is implemented in the swizzler, when getScanlines() is called.
+ */
+class SkScaledCodec : public SkCodec {
+public:
+    static SkCodec* NewFromStream(SkStream*);
+    static SkCodec* NewFromData(SkData*);
+
+    virtual ~SkScaledCodec();
+
+    /**
+     * returns whether a destination's dimensions are supported for down sampling
+     */
+    static bool DimensionsSupportedForSampling(const SkImageInfo& srcInfo, 
+                                               const SkImageInfo& dstInfo) {
+        // heights must be equal as no native y sampling is supported
+        if (dstInfo.height() != srcInfo.height()) {
+            return false;
+        }
+        // only support down sampling, dstWidth cannot be larger that srcWidth
+        if(dstInfo.width() > srcInfo.width()) {
+            return false;
+        }
+        return true;   
+    }
+
+    static void ComputeSampleSize(const SkImageInfo& dstInfo, const SkImageInfo& srcInfo,
+                                  int* sampleSizeX, int* sampleSizeY);
+
+protected:
+    /**
+     * Recommend a set of destination dimensions given a requested scale
+     */
+    SkISize onGetScaledDimensions(float desiredScale) const override;
+
+    Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*)
+            override;
+    SkEncodedFormat onGetEncodedFormat() const override {
+        return fScanlineDecoder->getEncodedFormat();
+    }
+
+    bool onReallyHasAlpha() const override {
+        return fScanlineDecoder->reallyHasAlpha();
+    }
+
+private:
+
+    SkAutoTDelete<SkScanlineDecoder> fScanlineDecoder;
+
+    explicit SkScaledCodec(SkScanlineDecoder*);
+
+    typedef SkCodec INHERITED;
+};
+#endif // SkScaledCodec_DEFINED
diff --git a/include/codec/SkScanlineDecoder.h b/include/codec/SkScanlineDecoder.h
index c233663dbd..c547f6701f 100644
--- a/include/codec/SkScanlineDecoder.h
+++ b/include/codec/SkScanlineDecoder.h
@@ -46,6 +46,18 @@ public:
     virtual ~SkScanlineDecoder() {}
 
     /**
+     *  Return a size that approximately supports the desired scale factor.
+     *  The codec may not be able to scale efficiently to the exact scale
+     *  factor requested, so return a size that approximates that scale.
+     *  The returned value is the codec's suggestion for the closest valid
+     *  scale that it can natively support
+     *  FIXME: share this with SkCodec
+     */
+    SkISize getScaledDimensions(float desiredScale) {
+        return this->onGetScaledDimensions(desiredScale);
+    }
+
+    /**
      *  Returns the default info, corresponding to the encoded data.
      */
     const SkImageInfo& getInfo() { return fSrcInfo; }
@@ -135,14 +147,44 @@ public:
         return this->onReallyHasAlpha();
     }
 
+    /**
+     *  Format of the encoded data.
+     */
+    SkEncodedFormat getEncodedFormat() const { return this->onGetEncodedFormat(); }
+
+    /**
+     * returns true if the image must be scaled, in the y direction, after reading, not during.
+     * To scale afterwards, we first decode every line and then sample the lines we want afterwards.
+     * An example is interlaced pngs, where calling getScanlines once (regardless of the count
+     * used) needs to read the entire image, therefore it is inefficient to call
+     * getScanlines more than once. Instead, it should only ever be called with all the
+     * rows needed.
+     */
+    bool requiresPostYSampling() {
+        return this->onRequiresPostYSampling();
+    }
+
 protected:
     SkScanlineDecoder(const SkImageInfo& srcInfo)
         : fSrcInfo(srcInfo)
         , fDstInfo()
         , fCurrScanline(0) {}
 
+    virtual SkISize onGetScaledDimensions(float /* desiredScale */) {
+        // By default, scaling is not supported.
+        return this->getInfo().dimensions();
+    }
+
+    virtual SkEncodedFormat onGetEncodedFormat() const = 0;
+
     virtual bool onReallyHasAlpha() const { return false; }
 
+    /**
+     * returns true if the image type is hard to sample and must be scaled after reading, not during
+     * An example is interlaced pngs, where the entire image must be read for each decode
+     */
+    virtual bool onRequiresPostYSampling() { return false; }
+
     const SkImageInfo& dstInfo() const { return fDstInfo; }
 
 private: