Fix rounding error with tiled bitmap draws

We were snapping the top-left of the destination rectangle to integers,
but using the original (fractional) size. This led to us losing rows or
columns of pixels at the first tile boundary.

Also added a GM that demonstrates the bug (now renders correctly).

BUG=skia:

Change-Id: I50629dab9dd90fedad2c7e3393a1b1d1c7a8d45e
Reviewed-on: https://skia-review.googlesource.com/8102
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>

gm/bitmaptiled.cpp

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2017 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"
+
+#if SK_SUPPORT_GPU
+#include "GrContext.h"
+
+// This test exercises Ganesh's drawing of tiled bitmaps. In particular, that the offsets and the
+// extents of the tiles don't causes gaps between tiles.
+static void draw_tile_bitmap_with_fractional_offset(SkCanvas* canvas, bool vertical) {
+    GrContext* context = canvas->getGrContext();
+    if (!context) {
+        skiagm::GM::DrawGpuOnlyMessage(canvas);
+        return;
+    }
+
+    // This should match kBmpSmallTileSize in SkGpuDevice.cpp. Note that our canvas size is tuned
+    // to this constant as well.
+    const int kTileSize = 1 << 10;
+
+    // We're going to draw a section of the bitmap that intersects 3 tiles (3x1 or 1x3).
+    // We need that to be < 50% of the total image, so our image is 7 tiles (7x1 or 1x7).
+    const int kBitmapLongEdge = 7 * kTileSize;
+    const int kBitmapShortEdge = 1 * kTileSize;
+
+    // To trigger tiling, we also need the image to be more than 50% of the cache, so we ensure the
+    // cache is sized to make that true.
+    const int kBitmapArea = kBitmapLongEdge * kBitmapShortEdge;
+    const size_t kBitmapBytes = kBitmapArea * sizeof(SkPMColor);
+
+    int oldMaxResources;
+    size_t oldMaxResourceBytes;
+    context->getResourceCacheLimits(&oldMaxResources, &oldMaxResourceBytes);
+
+    const size_t newMaxResourceBytes = kBitmapBytes + (kBitmapBytes / 2);
+    context->setResourceCacheLimits(oldMaxResources, newMaxResourceBytes);
+
+    // Construct our bitmap as either very wide or very tall
+    SkBitmap bmp;
+    bmp.allocN32Pixels(vertical ? kBitmapShortEdge : kBitmapLongEdge,
+                       vertical ? kBitmapLongEdge : kBitmapShortEdge, true);
+    bmp.eraseColor(SK_ColorWHITE);
+
+    // Draw ten strips with varying fractional offset to catch any rasterization issues with tiling
+    for (int i = 0; i < 10; ++i) {
+        float offset = i * 0.1f;
+        if (vertical) {
+            canvas->drawBitmapRect(bmp, SkRect::MakeXYWH(0.0f, (kTileSize - 50) + offset,
+                                                         32.0f, 1124.0f),
+                                   SkRect::MakeXYWH(37.0f * i, 0.0f, 32.0f, 1124.0f), nullptr);
+        } else {
+            canvas->drawBitmapRect(bmp, SkRect::MakeXYWH((kTileSize - 50) + offset, 0.0f,
+                                                         1124.0f, 32.0f),
+                                   SkRect::MakeXYWH(0.0f, 37.0f * i, 1124.0f, 32.0f), nullptr);
+        }
+    }
+
+    // Restore the cache
+    context->setResourceCacheLimits(oldMaxResources, oldMaxResourceBytes);
+}
+
+DEF_SIMPLE_GM_BG(bitmaptiled_fractional_horizontal, canvas, 1124, 365, SK_ColorBLACK) {
+    draw_tile_bitmap_with_fractional_offset(canvas, false);
+}
+
+DEF_SIMPLE_GM_BG(bitmaptiled_fractional_vertical, canvas, 1124, 365, SK_ColorBLACK) {
+    draw_tile_bitmap_with_fractional_offset(canvas, true);
+}
+
+#endif

gn/gm.gni

@@ -36,6 +36,7 @@ gm_sources = [
   "$_gm/bitmaprect.cpp",
   "$_gm/bitmaprecttest.cpp",
   "$_gm/bitmapshader.cpp",
+  "$_gm/bitmaptiled.cpp",
   "$_gm/bleed.cpp",
   "$_gm/blend.cpp",
   "$_gm/blurcircles.cpp",

src/gpu/SkGpuDevice.cpp

@@ -998,8 +998,7 @@ void SkGpuDevice::drawTiledBitmap(const SkBitmap& bitmap,
             tileR.roundOut(&iTileR);
             SkVector offset = SkPoint::Make(SkIntToScalar(iTileR.fLeft),
                                             SkIntToScalar(iTileR.fTop));
-            SkRect rectToDraw = SkRect::MakeXYWH(offset.fX, offset.fY,
-                                                 tileR.width(), tileR.height());
+            SkRect rectToDraw = tileR;
             dstMatrix.mapRect(&rectToDraw);
             if (GrSamplerParams::kNone_FilterMode != params.filterMode() || bicubic) {
                 SkIRect iClampRect;