Implement correct clipping for image filters.

Image filters in Skia currently clip the size of the the offscreen
bitmap used for filtering to the device clip bounds. This means that
any pixel-moving filter (e.g., blur) has edge artifacts at the clip
boundaries. This is problematic for tiling, where a single SkPicture
is played back with a clip set to the tile boundaries.

By implementing the onFilterBounds() traversal, and using it in
saveLayer() when a filter is present, we can clip the layer to the
expanded clip rect. Note that this requires that the traversal be
performed in reverse as compared to computeFastBounds().  (It's also
done in device space, unlike computeFastBounds()).

New test imagefiltersclipped tests pixel-moving filters when clipped
by various clip rects.
New test imageblurtiled tests tiled (compositor-style) rendering of
blurred text. There should be no artifacts at the tile boundaries.

BUG=337831
R=reed@google.com

Review URL: https://codereview.chromium.org/23011012

git-svn-id: http://skia.googlecode.com/svn/trunk@13323 2bbb7eff-a529-9590-31e7-b0007b416f81

1 files changed, 76 insertions, 0 deletions

diff --git a/gm/imageblurtiled.cpp b/gm/imageblurtiled.cpp
new file mode 100644
index 0000000000..96df4367b4
--- /dev/null
+++ b/gm/imageblurtiled.cpp
@@ -0,0 +1,76 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+#include "SkBlurImageFilter.h"
+#include "SkRandom.h"
+
+#define WIDTH 640
+#define HEIGHT 480
+
+namespace skiagm {
+
+class ImageBlurTiledGM : public GM {
+public:
+    ImageBlurTiledGM(SkScalar sigmaX, SkScalar sigmaY)
+        : fSigmaX(sigmaX), fSigmaY(sigmaY) {
+    }
+
+protected:
+    virtual SkString onShortName() {
+        return SkString("imageblurtiled");
+    }
+
+    virtual SkISize onISize() {
+        return make_isize(WIDTH, HEIGHT);
+    }
+
+    virtual void onDraw(SkCanvas* canvas) {
+        SkPaint paint;
+        paint.setImageFilter(new SkBlurImageFilter(fSigmaX, fSigmaY))->unref();
+        const SkScalar tile_size = SkIntToScalar(128);
+        SkRect bounds;
+        canvas->getClipBounds(&bounds);
+        for (SkScalar y = bounds.top(); y < bounds.bottom(); y += tile_size) {
+            for (SkScalar x = bounds.left(); x < bounds.right(); x += tile_size) {
+                canvas->save();
+                canvas->clipRect(SkRect::MakeXYWH(x, y, tile_size, tile_size));
+                canvas->saveLayer(NULL, &paint);
+                const char* str[] = {
+                    "The quick",
+                    "brown fox",
+                    "jumped over",
+                    "the lazy dog.",
+                };
+                SkPaint textPaint;
+                textPaint.setAntiAlias(true);
+                textPaint.setTextSize(SkIntToScalar(100));
+                int posY = 0;
+                for (unsigned i = 0; i < SK_ARRAY_COUNT(str); i++) {
+                    posY += 100;
+                    canvas->drawText(str[i], strlen(str[i]), SkIntToScalar(0),
+                                     SkIntToScalar(posY), textPaint);
+                }
+                canvas->restore();
+                canvas->restore();
+            }
+        }
+    }
+
+private:
+    SkScalar fSigmaX;
+    SkScalar fSigmaY;
+
+    typedef GM INHERITED;
+};
+
+//////////////////////////////////////////////////////////////////////////////
+
+static GM* MyFactory1(void*) { return new ImageBlurTiledGM(3.0f, 3.0f); }
+static GMRegistry reg1(MyFactory1);
+
+}