Add the ability to decode a subset to SkCodec.

tests/CodexTest.cpp

Index: tests/CodexTest.cpp
diff --git a/tests/CodexTest.cpp b/tests/CodexTest.cpp
index f4682055a42d8a67f17a578781774b6536d7e5cb..ca9eef0bdb4456f70d8ed199b798f2acf4dea2f8 100644
--- a/tests/CodexTest.cpp
+++ b/tests/CodexTest.cpp
@@ -9,6 +9,7 @@
 #include "SkBitmap.h"
 #include "SkCodec.h"
 #include "SkMD5.h"
+#include "SkRandom.h"
 #include "SkScanlineDecoder.h"
 #include "Test.h"
 
@@ -41,10 +42,27 @@ static void compare_to_good_digest(skiatest::Reporter* r, const SkMD5::Digest& g
     REPORTER_ASSERT(r, digest == goodDigest);
 }
 
+SkIRect generate_random_subset(SkRandom* rand, int w, int h) {
+    // If either dimension is zero, we will not be able to return a non-empty
+    // rectangle, resulting in an infinite loop.
+    SkASSERT(w > 1 && h > 1);

[emmaleer, 2015/07/21 15:24:55]

Should this be SkASSERT(w > 0 && h > 0) ?
Since the comment states "If either dimension is zero"
If it is already correct, we should update this comment so it's not conflicting
the code. 

[scroggo, 2015/07/21 16:48:56]

Ah, I meant either dimension of rect, down below, which will happen if w or h is
1 (since we call rand->nextULessThan(1), which can only return 0.

I agree that this comment is confusing....

I've also changed the function, in response to your comment regarding the early
exit for w or h of 1.

We *could* support a w or h of 1, we just need to change the call made on rand.
In fact, after rewriting the function, this assert is no longer needed (since it
can even support subsets of 1 x 1 sizes without going into the infinite loop).

+
+    SkIRect rect;
+    do {
+        rect.fLeft = rand->nextULessThan(w);
+        rect.fTop = rand->nextULessThan(h);
+        rect.fRight = rand->nextULessThan(w);
+        rect.fBottom = rand->nextULessThan(h);
+        rect.sort();
+    } while (rect.isEmpty());
+    return rect;
+}
+
 static void check(skiatest::Reporter* r,
                   const char path[],
                   SkISize size,
-                  bool supportsScanlineDecoding) {
+                  bool supportsScanlineDecoding,
+                  bool supportsSubsetDecoding) {
     SkAutoTDelete<SkStream> stream(resource(path));
     if (!stream) {
         SkDebugf("Missing resource '%s'\n", path);
@@ -102,53 +120,84 @@ static void check(skiatest::Reporter* r,
     } else {
         REPORTER_ASSERT(r, !scanlineDecoder);
     }
+
+    // Do not attempt to decode subsets of an image with a width and/or height of 1.

[emmaleer, 2015/07/21 15:24:55]

Why are we not attempting to decode subsets with a width or height of 1?
Is that because width and height must be even? Could you explain this briefly 

[scroggo, 2015/07/21 16:48:56]

Originally, I did not attempt to decode subsets of 1 x 1 images (i.e.
half-transparent-white-pixel.png) for two reasons:
- There is no meaningful subset of a 1 x 1 image
- generate_random_rect, as written, would go into an infinite loop on a 1 x 1
image

I then realized that generate_random_rect would *also* go into an infinite loop
for a 1 x h or w x 1 image, so I excluded those as well.

That said, I think it is meaningful to decode a subset of a one-dimensional
image; I just need to rewrite generate_random_rect to support that case.
(Although we do not actually have a one-dimensional image to test on - but we
could someday.) I've updated this (with comments) and fixed
generate_random_rect.

+    if (size.width() == 1 || size.height() == 1) {
+        return;
+    }
+
+    // Test decoding subsets. Decode an arbitrary number of random subsets.
+    SkRandom rand;
+    for (int i = 0; i < 5; i++) {
+        SkCodec::Options opts;
+        opts.fSubset = generate_random_subset(&rand, size.width(), size.height());
+        SkASSERT(!opts.fSubset.isEmpty());
+        const bool supported = codec->getValidSubset(&opts.fSubset);
+        REPORTER_ASSERT(r, supported == supportsSubsetDecoding);
+
+        SkImageInfo subsetInfo = info.makeWH(opts.fSubset.width(), opts.fSubset.height());
+        SkBitmap bm;
+        bm.allocPixels(subsetInfo);
+        const SkCodec::Result result = codec->getPixels(bm.info(), bm.getPixels(), bm.rowBytes(),
+                                                        &opts, NULL, NULL);
+
+        if (supportsSubsetDecoding) {
+            REPORTER_ASSERT(r, result == SkCodec::kSuccess);
+            // Webp is the only codec that supports subsets, and it will have modified the subset
+            // to have even left/top.
+            REPORTER_ASSERT(r, SkIsAlign2(opts.fSubset.fLeft) && SkIsAlign2(opts.fSubset.fTop));
+        } else {
+            // No subsets will work.
+            REPORTER_ASSERT(r, result == SkCodec::kUnimplemented);
+        }
+    }
 }
 
 DEF_TEST(Codec, r) {
     // WBMP
-    check(r, "mandrill.wbmp", SkISize::Make(512, 512), false);
+    check(r, "mandrill.wbmp", SkISize::Make(512, 512), false, false);
 
     // WEBP
-    check(r, "baby_tux.webp", SkISize::Make(386, 395), false);
-    check(r, "color_wheel.webp", SkISize::Make(128, 128), false);
-    check(r, "yellow_rose.webp", SkISize::Make(400, 301), false);
+    check(r, "baby_tux.webp", SkISize::Make(386, 395), false, true);
+    check(r, "color_wheel.webp", SkISize::Make(128, 128), false, true);
+    check(r, "yellow_rose.webp", SkISize::Make(400, 301), false, true);
 
     // BMP
-    check(r, "randPixels.bmp", SkISize::Make(8, 8), false);
+    check(r, "randPixels.bmp", SkISize::Make(8, 8), false, false);
 
     // ICO
     // These two tests examine interestingly different behavior:
     // Decodes an embedded BMP image
-    check(r, "color_wheel.ico", SkISize::Make(128, 128), false);
+    check(r, "color_wheel.ico", SkISize::Make(128, 128), false, false);
     // Decodes an embedded PNG image
-    check(r, "google_chrome.ico", SkISize::Make(256, 256), false);
+    check(r, "google_chrome.ico", SkISize::Make(256, 256), false, false);
 
     // GIF
-    check(r, "box.gif", SkISize::Make(200, 55), false);
-    check(r, "color_wheel.gif", SkISize::Make(128, 128), false);
-    check(r, "randPixels.gif", SkISize::Make(8, 8), false);
+    check(r, "box.gif", SkISize::Make(200, 55), false, false);
+    check(r, "color_wheel.gif", SkISize::Make(128, 128), false, false);
+    check(r, "randPixels.gif", SkISize::Make(8, 8), false, false);
 
     // JPG
-    check(r, "CMYK.jpg", SkISize::Make(642, 516), true);
-    check(r, "color_wheel.jpg", SkISize::Make(128, 128), true);
-    check(r, "grayscale.jpg", SkISize::Make(128, 128), true);
-    check(r, "mandrill_512_q075.jpg", SkISize::Make(512, 512), true);
-    check(r, "randPixels.jpg", SkISize::Make(8, 8), true);
+    check(r, "CMYK.jpg", SkISize::Make(642, 516), true, false);
+    check(r, "color_wheel.jpg", SkISize::Make(128, 128), true, false);
+    check(r, "grayscale.jpg", SkISize::Make(128, 128), true, false);
+    check(r, "mandrill_512_q075.jpg", SkISize::Make(512, 512), true, false);
+    check(r, "randPixels.jpg", SkISize::Make(8, 8), true, false);
 
     // PNG
-    check(r, "arrow.png", SkISize::Make(187, 312), true);
-    check(r, "baby_tux.png", SkISize::Make(240, 246), true);
-    check(r, "color_wheel.png", SkISize::Make(128, 128), true);
-    check(r, "half-transparent-white-pixel.png", SkISize::Make(1, 1), true);
-    check(r, "mandrill_128.png", SkISize::Make(128, 128), true);
-    check(r, "mandrill_16.png", SkISize::Make(16, 16), true);
-    check(r, "mandrill_256.png", SkISize::Make(256, 256), true);
-    check(r, "mandrill_32.png", SkISize::Make(32, 32), true);
-    check(r, "mandrill_512.png", SkISize::Make(512, 512), true);
-    check(r, "mandrill_64.png", SkISize::Make(64, 64), true);
-    check(r, "plane.png", SkISize::Make(250, 126), true);
-    check(r, "randPixels.png", SkISize::Make(8, 8), true);
-    check(r, "yellow_rose.png", SkISize::Make(400, 301), true);
+    check(r, "arrow.png", SkISize::Make(187, 312), true, false);
+    check(r, "baby_tux.png", SkISize::Make(240, 246), true, false);
+    check(r, "color_wheel.png", SkISize::Make(128, 128), true, false);
+    check(r, "half-transparent-white-pixel.png", SkISize::Make(1, 1), true, false);
+    check(r, "mandrill_128.png", SkISize::Make(128, 128), true, false);
+    check(r, "mandrill_16.png", SkISize::Make(16, 16), true, false);
+    check(r, "mandrill_256.png", SkISize::Make(256, 256), true, false);
+    check(r, "mandrill_32.png", SkISize::Make(32, 32), true, false);
+    check(r, "mandrill_512.png", SkISize::Make(512, 512), true, false);
+    check(r, "mandrill_64.png", SkISize::Make(64, 64), true, false);
+    check(r, "plane.png", SkISize::Make(250, 126), true, false);
+    check(r, "randPixels.png", SkISize::Make(8, 8), true, false);
+    check(r, "yellow_rose.png", SkISize::Make(400, 301), true, false);
 }
 
 static void test_invalid_stream(skiatest::Reporter* r, const void* stream, size_t len) {