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)
+{}
+
+/*
+ * Chooses the correct image subset offsets and dimensions for the partial decode.
+ */
+static inline void set_subset_region(int regionOffset, int regionDimension,
+        int imageOriginalDimension, int* imageSubsetOffset, int* bitmapOffset,
+        int* imageSubsetDimension) {
+
+    // This must be at least zero, we can't start decoding the image at a negative coordinate.
+    *imageSubsetOffset = SkTMax(0, regionOffset);
+
+    // If regionOffset is less than zero, we decode to an offset location in the output bitmap.
+    *bitmapOffset = *imageSubsetOffset - regionOffset;
+
+    // Use imageSusetOffset to make sure we don't decode pixels past the edge of the image.
+    // Use bitmapOffset to make sure we don't decode pixels past the edge of the region.
+    *imageSubsetDimension = SkTMin(imageOriginalDimension - *imageSubsetOffset,
+            regionDimension - *bitmapOffset);
+}
+
+static int get_scaled_dimension(int dimension, int sampleSize) {
+    return SkTMax(1, dimension / sampleSize);
+}
+
+/*
+ * Three differences from the Android version:
+ *     Returns a Skia bitmap instead of an Android bitmap.
+ *     Android version attempts to reuse a recycled bitmap.
+ *     Removed the options object and used parameters for color type and
+ *     sample size.
+ */
+SkBitmap* SkBitmapRegionCanvas::decodeRegion(int regionLeft, int regionTop,

[scroggo, 2015/09/04 17:55:06]

I know Android started using this term "region", but I don't think it provides
any meaningful information. Maybe it fits because the name of this function is
"decodeRegion", but I liked the "input" names better (although they should be
camelCase e.g. inputX etc).

[msarett, 2015/09/04 18:54:10]

Will do.  And I'm replacing Left and Top in all of the names with X and Y.  Skia
uses the XYWH convention with SkRect.

+                                             int regionWidth, int regionHeight,
+                                             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;
+    }
+
+    // The client may not necessarily request a region that is fully within

[scroggo, 2015/09/04 17:55:07]

I find these comments very helpful :)

[msarett, 2015/09/04 18:54:10]

Great!  Third try is the charm haha.

+    // the image.  We may need to do some calculation to determine what part
+    // of the image to decode.
+
+    // The left offset of the portion of the image we want, where zero
+    // indicates the left edge of the image.
+    int imageSubsetLeft;
+    // The size of the output bitmap is determined by the size of the

[scroggo, 2015/09/04 17:55:07]

nit: blank line between code and large comment block:

int imageSubsetLeft;

// Comment block

[msarett, 2015/09/04 18:54:10]

Done.

+    // requested region, not by the size of the intersection of the region
+    // and the image dimensions.  If regionLeft is negative, we will need to
+    // place decoded pixels into the output bitmap starting at a left offset.
+    // If this is non-zero, imageSubsetLeft must be zero.
+    int bitmapLeft;

[scroggo, 2015/09/04 17:55:06]

Maybe this would be better named outLeft? Something that tells the reader this
applies to the output bitmap?

[msarett, 2015/09/04 18:54:10]

Yes agreed.

+    // The width of the portion of the image that we will write to the output
+    // bitmap.  If the region is not fully contained within the image, this
+    // will not be the same as regionWidth.
+    int imageSubsetWidth;
+    set_subset_region(regionLeft, regionWidth, this->width(), &imageSubsetLeft, &bitmapLeft,
+            &imageSubsetWidth);
+
+    // The top offset of the portion of the image we want, where zero
+    // indicates the top edge of the image.
+    int imageSubsetTop;
+    // The size of the output bitmap is determined by the size of the
+    // requested region, not by the size of the intersection of the region
+    // and the image dimensions.  If regionTop is negative, we will need to
+    // place decoded pixels into the output bitmap starting at a top offset.
+    // If this is non-zero, imageSubsetTop must be zero.
+    int bitmapTop;
+    // The height of the portion of the image that we will write to the output
+    // bitmap.  If the region is not fully contained within the image, this
+    // will not be the same as regionHeight.
+    int imageSubsetHeight;
+    set_subset_region(regionTop, regionHeight, this->height(), &imageSubsetTop, &bitmapTop,
+            &imageSubsetHeight);
+
+    if (imageSubsetWidth <= 0 || imageSubsetHeight <= 0) {
+        SkDebugf("Error: Region must intersect part of the image.\n");
+        return nullptr;
+    }
+
+    // Create the image info for 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(), imageSubsetHeight);
+    if (!tmp.tryAllocPixels(tmpInfo)) {
+        SkDebugf("Error: Could not allocate pixels.\n");
+        return nullptr;
+    }
+
+    // Skip the unneeded rows
+    if (SkCodec::kSuccess != fDecoder->skipScanlines(imageSubsetTop)) {
+        SkDebugf("Error: Failed to skip scanlines.\n");
+        return nullptr;
+    }
+
+    // Decode the necessary rows
+    SkCodec::Result result = fDecoder->getScanlines(tmp.getAddr(0, 0), imageSubsetHeight,
+            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
+    const int outWidth = get_scaled_dimension(regionWidth, sampleSize);
+    const int outHeight = get_scaled_dimension(regionHeight, sampleSize);
+
+    // Initialize the destination bitmap
+    SkAutoTDelete<SkBitmap> bitmap(new 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 != bitmapLeft || 0 != bitmapTop ||
+            regionLeft + regionWidth > this->width() ||
+            regionTop + regionHeight > this->height()) {
+        bitmap->eraseColor(0);
+    }
+
+    // Use a canvas to crop and scale to the destination bitmap
+    SkCanvas canvas(*bitmap);
+    SkRect src = SkRect::MakeXYWH(imageSubsetLeft, 0, imageSubsetWidth, imageSubsetHeight);
+    SkRect dst = SkRect::MakeXYWH(bitmapLeft / sampleSize, bitmapTop / sampleSize,
+            get_scaled_dimension(imageSubsetWidth, sampleSize),
+            get_scaled_dimension(imageSubsetHeight, sampleSize));
+    SkPaint paint;
+    // 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();
+}