Provides multiple implementations of Android's SkBitmapRegionDecoder

src/utils/SkBitmapRegionCanvas.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 "SkBitmapRegionCanvas.h"
+#include "SkCanvas.h"
+#include "SkScanlineDecoder.h"
+
+SkBitmapRegionCanvas::SkBitmapRegionCanvas(SkScanlineDecoder* decoder)
+    : INHERITED(decoder->getInfo().width(), decoder->getInfo().height())
+    , fDecoder(decoder)
+{}
+
+/*
+ * This has several key differences from the Android version:
+ *     Returns a Skia bitmap instead of an Android bitmap.
+ *     Android version attempts to reuse a recycled bitmap.
+ */
+SkBitmap* SkBitmapRegionCanvas::decodeRegion(int input_x, int input_y,
+                                             int input_w, int input_h,
+                                             int sampleSize,
+                                             SkColorType dstColorType) {
+    // Reject color types not supported by this method
+    if (kIndex_8_SkColorType == dstColorType || kGray_8_SkColorType == dstColorType) {
+        SkDebugf("Error: Color type not supported.\n");
+        return nullptr;
+    }
+
+    // Correct the output region if necessary
+    int left = SkTMax(0, input_x);
+    int leftOffset = left - input_x;
+    int top = SkTMax(0, input_y);
+    int topOffset = top - input_y;
+    int width = SkTMin(this->width() - left, input_w - leftOffset);
+    int height = SkTMin(this->height() - top, input_h - topOffset);
+    if (width <= 0 || height <= 0) {
+        SkDebugf("Error: Region must intersect part of the image.\n");
+        return nullptr;
+    }
+
+    // Create the image info the decode
+    SkAlphaType dstAlphaType = fDecoder->getInfo().alphaType();
+    if (kUnpremul_SkAlphaType == dstAlphaType) {
+        dstAlphaType = kPremul_SkAlphaType;
+    }
+    SkImageInfo decodeInfo = SkImageInfo::Make(this->width(), this->height(),
+            dstColorType, dstAlphaType);
+    
+    // Start the scanline decoder
+    SkCodec::Result r = fDecoder->start(decodeInfo);
+    if (SkCodec::kSuccess != r) {
+        SkDebugf("Error: Could not start scanline decoder.\n");
+        return nullptr;
+    }
+
+    // Allocate a bitmap for the unscaled decode
+    SkBitmap tmp;
+    SkImageInfo tmpInfo = decodeInfo.makeWH(this->width(), height);
+    if (!tmp.tryAllocPixels(tmpInfo)) {
+        SkDebugf("Error: Could not allocate pixels.\n");
+        return nullptr;
+    }
+
+    // Skip the unneeded rows
+    if (SkCodec::kSuccess != fDecoder->skipScanlines(top)) {
+        SkDebugf("Error: Failed to skip scanlines.\n");
+        return nullptr;
+    }
+
+    // Decode the necessary rows
+    SkCodec::Result result = fDecoder->getScanlines(tmp.getAddr(0, 0), height,

[scroggo, 2015/09/01 22:05:28]

So this won't work for all BMP, for example, but that's okay, since it only
needs to work for the big three, right?

[msarett, 2015/09/02 18:02:23]

Correct.  This was written before we had kBottomUp, kOutOfOrder etc. 
Theoretically, we could rewrite this to support all of the modes, but there's
not reason to unless we want to start supporting BMP, GIF, etc.

I'm adding an SkASSERT(kTopDown).

+            tmp.rowBytes());
+    switch (result) {
+        case SkCodec::kSuccess:
+        case SkCodec::kIncompleteInput:
+            break;
+        default:
+            SkDebugf("Error: Failed to get scanlines.\n");
+            return nullptr;
+    }
+
+    // Calculate the size of the output
+    int outWidth = SkTMax(1, input_w / sampleSize);
+    int outHeight = SkTMax(1, input_h / sampleSize);
+
+    // Initialize the destination bitmap
+    SkAutoTDelete<SkBitmap> bitmap(SkNEW(SkBitmap));
+    SkImageInfo dstInfo = decodeInfo.makeWH(outWidth, outHeight);
+    if (!bitmap->tryAllocPixels(dstInfo)) {
+        SkDebugf("Error: Could not allocate pixels.\n");
+        return nullptr;
+    }
+
+    // Zero the bitmap if the region is not completely within the image.
+    // TODO (msarett): Can we make this faster by implementing it to only
+    //                 zero parts of the image that we won't overwrite with
+    //                 pixels?
+    // TODO (msarett): This could be skipped if memory is zero initialized.
+    //                 This would matter if this code is moved to Android and
+    //                 uses Android bitmaps.
+    if (0 != leftOffset || 0 != topOffset ||
+            input_x + input_w > this->width() ||
+            input_y + input_h > this->height()) {
+        bitmap->eraseColor(0);
+    }
+
+    // Use a canvas to crop and scale to the destination bitmap
+    SkAutoTDelete<SkCanvas> canvas(SkNEW_ARGS(SkCanvas, (*bitmap)));
+    SkRect src = SkRect::MakeXYWH(left, 0, width, height);
+    SkRect dst = SkRect::MakeXYWH(leftOffset / sampleSize, topOffset / sampleSize,
+            SkTMax(1, width / sampleSize), SkTMax(1, height / sampleSize));
+    SkPaint* paint = new SkPaint();
+    // Overwrite the dst with the src pixels
+    paint->setXfermodeMode(SkXfermode::kSrc_Mode);
+    // TODO (msarett): Test multiple filter qualities.  kNone is the default.
+    canvas->drawBitmapRect(tmp, src, dst, paint);
+
+    return bitmap.detach();
+}