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)
+{}
+
+/*
+ * Adjusts the image subset dimensions to safely fit within the original image,
+ * and to safely fit within the requested region size.
+ */
+static int get_safe_subset_dimension(int imageOffset, int imageDim, int regionOffset,

[scroggo, 2015/09/03 20:07:41]

These names are pretty opaque to me. They describe what gets passed to this
function, but not really what's done with them?

Really you just want to know the smaller of two differences?

Looking back at my comment, I see I said "Calculating width and height are very
similar. What do you think about a helper function?" But it's more than just
width and height - leftOffset and topOffset are parallels, along with left and
top. I was thinking a function to compute all of that. Maybe that would end up
being too messy though.

[msarett, 2015/09/03 22:10:30]

I'll add comments, I was hoping the names would make it more clear.

My original plan was to share more than this one line - but you'll notice that
left, leftOffset, top, topOffset are all used below.  I suppose we could pass
the address of local variable to a helper function, but I'm not sure that's an
improvement.

Edit: Tried sharing more code, let me know what you think.

+        int regionDim) {
+    int safeImageDim = imageDim - imageOffset;
+    int safeRegionDim = regionDim - regionOffset;
+    return SkTMin(safeImageDim, safeRegionDim);
+}
+
+static int get_scaled_dimension(int dimension, int sampleSize) {
+    return SkTMax(1, dimension / sampleSize);

[scroggo, 2015/09/03 20:07:41]

It seems like this is logically the same as SkScaledCodec.cpp's
get_scaled_dimension? Can we share? I guess the tricky part is finding the right
place for it?

[msarett, 2015/09/03 22:10:30]

Yeah I tried to share it in SkCodecPriv.h but that doesn't work.

I'm not aware of a good location - maybe there is one somewhere?  I'm not sure
it's worth creating a new file for?

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

Ah, because this is in utils? maybe declare it as an extern function? Either
way, it seems like we should decided on an implementation (not sure which is
better, but one does a divide no matter what, and then compares it against one,
and the other first compares the two numbers. I'd lean towards doing the check
first, but I could certainly be wrong.

If it's not an extern function, maybe add TODOs/FIXMEs to combine them?

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

I'll go with the original implementation in SkScaledCodec (that may soon be
moved to SkCodecPriv in another CL).

Actually I was able to include SkCodecPriv.h in this file by changing utils.gyp,
but then the compiler complained that all of the other functions in the
SkCodecPriv.h were defined but not used.

I have added a TODO to combine this with the other implementation.

+}
+
+/*
+ * 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
+    const int left = SkTMax(0, input_x);
+    const int leftOffset = left - input_x;
+    const int top = SkTMax(0, input_y);
+    const int topOffset = top - input_y;
+    const int width = get_safe_subset_dimension(left, this->width(), leftOffset, input_w);
+    const int height = get_safe_subset_dimension(top, this->height(), topOffset, input_h);
+    if (width <= 0 || height <= 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(), 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,
+            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(input_w, sampleSize);
+    const int outHeight = get_scaled_dimension(input_h, 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 != 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
+    SkCanvas canvas(*bitmap);
+    SkRect src = SkRect::MakeXYWH(left, 0, width, height);
+    SkRect dst = SkRect::MakeXYWH(leftOffset / sampleSize, topOffset / sampleSize,
+            get_scaled_dimension(width, sampleSize), get_scaled_dimension(height, 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();
+}