Implementation of SkBitmapRegionDecoder using SkAndroidCodec

tools/SkCodecTools.h

Index: tools/SkCodecTools.h
diff --git a/tools/SkCodecTools.h b/tools/SkCodecTools.h
index 097915b5d719711c37559f860791ca3b50936dd7..e798bda21dcdec346a909eddf20895b2bd305866 100644
--- a/tools/SkCodecTools.h
+++ b/tools/SkCodecTools.h
@@ -12,4 +12,52 @@ inline float get_scale_from_sample_size(uint32_t sampleSize) {
     return 1.0f / (float) sampleSize;
 }
 
+enum SubsetType {
+    kFullyInside_SubsetType,
+    kPartiallyInside_SubsetType,
+    kInvalid_SubsetType,

[scroggo, 2015/10/21 20:56:20]

Maybe kOutside?

[msarett, 2015/10/21 22:00:00]

Done.

+};
+
+/*
+ * Corrects image subset offsets and dimensions in order to perform a valid decode.
+ * Also indicates if the image subset should be placed at an offset within the
+ * output bitmap.
+ *
+ * Values of output variables are undefined if the SubsetType is kInvalid.
+ *
+ * @param imageDims Original image dimensions.
+ * @param subset    As input, the subset that the client requested.
+ *                  As output, the image subset that we will decode.
+ * @param outX      The left offset of the image subset within the output bitmap.
+ * @param outY      The top offset of the image subset within the output bitmap.
+ *
+ * @return An indication of how the subset is contained in the image.
+ *         If the return value is kInvalid, values of output variables are undefined.
+ */
+inline SubsetType adjust_subset_rect(const SkISize& imageDims, SkIRect* subset, int* outX,
+        int* outY) {
+    // These must be at least zero, we can't start decoding the image at a negative coordinate.
+    int left = SkTMax(0, subset->fLeft);
+    int top = SkTMax(0, subset->fTop);
+
+    // If input offsets are less than zero, we decode to an offset location in the output bitmap.
+    *outX = left - subset->fLeft;
+    *outY = top - subset->fTop;
+
+    // Make sure we don't decode pixels past the edge of the image or past the edge of the region.

[scroggo, 2015/10/21 20:56:20]

nit: region -> subset

Android called it BitmapRegionDecoder, but "Region" means something different in
Skia (SkRegion). It also means something different in Android (the same as in
Skia, actually). We're stuck with that name for the Java API (and I'm fine to
use it in e.g. BitmapRegionDecoderInterface etc), but I think we should say
"subset" here. (You could argue that subset need not be rectangular, but we
generally think of them as such.)

[msarett, 2015/10/21 22:00:00]

Yes thanks!

I generally try to say subset, but I've been slipping up in this class.

+    int width = SkTMin(imageDims.width() - left, subset->width() - *outX);
+    int height = SkTMin(imageDims.height() - top, subset->height() - *outY);
+    if (width <= 0 || height <= 0) {
+        return SubsetType::kInvalid_SubsetType;
+    }
+
+    subset->setXYWH(left, top, width, height);
+    if ((*outX != 0) || (*outY != 0) || (width != subset->width()) ||
+            (height != subset->height())) {
+        return SubsetType::kPartiallyInside_SubsetType;
+    }
+
+    return SubsetType::kFullyInside_SubsetType;
+}
+
 #endif // SkCodecTools_DEFINED