Make SkPngCodec decode progressively.

src/codec/SkSampler.h

 /*
  * 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 SkSampler_DEFINED
 #define SkSampler_DEFINED
 
 #include "SkCodec.h"
 #include "SkTypes.h"
 
 class SkSampler : public SkNoncopyable {
 public:
     /**
      *  Update the sampler to sample every sampleX'th pixel. Returns the
      *  width after sampling.
      */
     int setSampleX(int sampleX) {
         return this->onSetSampleX(sampleX);
     }
 
     /**
+     *  Update the sampler to sample every sampleY'th row.
+     */
+    void setSampleY(int sampleY) {
+        fSampleY = sampleY;
+    }
+
+    /**
+     *  Retrieve the value set for sampleY.
+     */
+    int sampleY() const {
+        return fSampleY;
+    }
+
+    /**
+     *  Based on fSampleY, return whether this row belongs in the output.
+     *  @param row Zero-based index into the subset.
+     */
+    bool rowNeeded(int row) const {
+        return row % fSampleY == 0;

[msarett, 2016/05/24 13:14:07]

Is this correct?  I think we don't start at row 0...
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/s...

Maybe this would be useful?
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/s...

[scroggo, 2016/05/24 14:24:50]

I believe it's used correctly by SkPngCodec, but it is unclear. Note how we
actually pass fLinesDecoded, before incrementing it, so we start with zero, even
though we may not be decoding row 0.
We could use that method, although from the current call site, I don't know the
scaledDim. I could plumb it in though. I think rowNeeded would work, but maybe
needs some better documentation? PTAL at the new comment. It might be a tad
circular though...

I guess the ugly part is that once we start supporting bottom up BMPs through
the new path we won't be able to just use fLinesDecoded; we'll need to something
more like is_coord_necessary.

[msarett, 2016/05/24 14:28:33]

Gotcha thanks.
New comment looks good.
It's fine with me if we figure this out when we get here.

+    }
+
+    /**
      * Fill the remainder of the destination with a single color
      *
      * @param info
      * Contains the color type of the rows to fill.
      * Contains the width of the destination rows to fill
      * Contains the number of rows that we need to fill.
      *
      * @param dst
      * The destination row to fill from.
      *
@@ skipping 13 matching lines @@
      */
     static void Fill(const SkImageInfo& info, void* dst, size_t rowBytes,
             uint32_t colorOrIndex, SkCodec::ZeroInitialized zeroInit);
 
     /**
      * Allow subclasses to implement unique versions of fill().
      */
     virtual void fill(const SkImageInfo& info, void* dst, size_t rowBytes,
             uint32_t colorOrIndex, SkCodec::ZeroInitialized zeroInit) {}
 
+    SkSampler()
+        : fSampleY(1)
+    {}
+
     virtual ~SkSampler() {}
 private:
+    int fSampleY;
 
     virtual int onSetSampleX(int) = 0;
 };
 
 #endif // SkSampler_DEFINED