have canvas send discard instead of retain if the draw would overwrite everything

src/core/SkCanvas.cpp

 /*
  * Copyright 2008 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCanvas.h"
 #include "SkCanvasPriv.h"
 #include "SkBitmapDevice.h"
 #include "SkColorFilter.h"
 #include "SkDraw.h"
 #include "SkDrawable.h"
 #include "SkDrawFilter.h"
 #include "SkDrawLooper.h"
 #include "SkErrorInternals.h"
 #include "SkImage.h"
 #include "SkMetaData.h"
 #include "SkNinePatchIter.h"
+#include "SkPaintPriv.h"
 #include "SkPathOps.h"
 #include "SkPatchUtils.h"
 #include "SkPicture.h"
 #include "SkRasterClip.h"
 #include "SkReadPixelsRec.h"
 #include "SkRRect.h"
 #include "SkSmallAllocator.h"
 #include "SkSurface_Base.h"
 #include "SkTemplates.h"
 #include "SkTextBlob.h"
 #include "SkTextFormatParams.h"
 #include "SkTLazy.h"
 #include "SkTraceEvent.h"
 #include "SkUtils.h"
 
 #if SK_SUPPORT_GPU
 #include "GrRenderTarget.h"
 #endif
 
+/*
+ *  Return true if the drawing this rect would hit every pixels in the canvas.
+ *
+ *  Returns false if
+ *  - rect does not contain the canvas' bounds
+ *  - paint is not fill
+ *  - paint would blur or otherwise change the coverage of the rect
+ */
+bool SkCanvas::willOverwriteAllPixels(const SkRect* rect, const SkPaint* paint,

[Justin Novosad, 2015/07/15 18:00:46]

"will" is ambiguous and can be interpreted as a declaration ("I am going
to..."). I would suggest using a name that makes it obvious that this function
is a question. Perhaps a name that starts with "is".

[reed1, 2015/07/15 21:41:02]

Done.

+                                      bool auxOpaque) const {
+    const SkISize size = this->getBaseLayerSize();
+    const SkRect bounds = SkRect::MakeIWH(size.width(), size.height());
+    if (!this->getClipStack()->quickContains(bounds)) {
+        return false;
+    }
+
+    if (rect) {
+        if (!this->getTotalMatrix().rectStaysRect()) {
+            return false; // conservative
+        }
+
+        SkRect devRect;
+        this->getTotalMatrix().mapRect(&devRect, *rect);
+
+        if (paint) {
+            SkPaint::Style paintStyle = paint->getStyle();
+            if (!(paintStyle == SkPaint::kFill_Style ||
+                  paintStyle == SkPaint::kStrokeAndFill_Style)) {
+                return false;
+            }
+            if (paint->getMaskFilter() || paint->getLooper()
+                || paint->getPathEffect() || paint->getImageFilter()) {
+                return false; // conservative
+            }
+        }
+
+        if (devRect.contains(bounds)) {
+            return false;
+        }
+    }
+    return true;
+}
+
+///////////////////////////////////////////////////////////////////////////////////////////////////
+
 static bool gIgnoreSaveLayerBounds;
 void SkCanvas::Internal_Private_SetIgnoreSaveLayerBounds(bool ignore) {
     gIgnoreSaveLayerBounds = ignore;
 }
 bool SkCanvas::Internal_Private_GetIgnoreSaveLayerBounds() {
     return gIgnoreSaveLayerBounds;
 }
 
 static bool gTreatSpriteAsBitmap;
 void SkCanvas::Internal_Private_SetTreatSpriteAsBitmap(bool spriteAsBitmap) {
@@ skipping 24 matching lines @@
     #define inc_layer()
     #define dec_layer()
     #define inc_rec()
     #define dec_rec()
     #define inc_canvas()
     #define dec_canvas()
 #endif
 
 typedef SkTLazy<SkPaint> SkLazyPaint;
 
-void SkCanvas::predrawNotify() {
+void SkCanvas::predrawNotify(bool completeOverwrite) {
     if (fSurfaceBase) {
-        fSurfaceBase->aboutToDraw(SkSurface::kRetain_ContentChangeMode);
+        fSurfaceBase->aboutToDraw(completeOverwrite ? SkSurface::kDiscard_ContentChangeMode
+                                                    : SkSurface::kRetain_ContentChangeMode);
     }
 }
 
+void SkCanvas::predrawNotify(const SkRect* rect, const SkPaint* paint, bool auxOpaque) {
+    if (fSurfaceBase) {
+        SkSurface::ContentChangeMode mode = SkSurface::kRetain_ContentChangeMode;
+        // Since willOverwriteAllPixels() may not be complete free to call, we only do so if
+        // there is an outstanding snapshot, since w/o that, there will be no copy-on-write
+        // and therefore we don't care which mode we're in.
+        //
+        if (fSurfaceBase->outstandingImageSnapshot()) {

[Justin Novosad, 2015/07/15 18:00:46]

+1  This is a an interesting optimization

+            if (this->willOverwriteAllPixels(rect, paint, auxOpaque)) {
+                mode = SkSurface::kDiscard_ContentChangeMode;
+            }
+        }
+        fSurfaceBase->aboutToDraw(mode);
+    }
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 static uint32_t filter_paint_flags(const SkSurfaceProps& props, uint32_t flags) {
     const uint32_t propFlags = props.flags();
     if (propFlags & SkSurfaceProps::kDisallowDither_Flag) {
         flags &= ~SkPaint::kDither_Flag;
     }
     if (propFlags & SkSurfaceProps::kDisallowAntiAlias_Flag) {
         flags &= ~SkPaint::kAntiAlias_Flag;
     }
@@ skipping 391 matching lines @@
     AutoDrawLooper  looper(this, fProps, paint, true);              \
     while (looper.next(type)) {                                     \
         SkDrawIter          iter(this);
 
 #define LOOPER_BEGIN(paint, type, bounds)                           \
     this->predrawNotify();                                          \
     AutoDrawLooper  looper(this, fProps, paint, false, bounds);     \
     while (looper.next(type)) {                                     \
         SkDrawIter          iter(this);
 
+#define LOOPER_BEGIN_CHECK_COMPLETE_OVERWRITE(paint, type, bounds, auxOpaque)  \
+    this->predrawNotify(bounds, &paint, auxOpaque);                 \
+    AutoDrawLooper  looper(this, fProps, paint, false, bounds);     \
+    while (looper.next(type)) {                                     \
+        SkDrawIter          iter(this);
+
 #define LOOPER_END    }
 
 ////////////////////////////////////////////////////////////////////////////
 
 void SkCanvas::resetForNextPicture(const SkIRect& bounds) {
     this->restoreToCount(1);
     fCachedLocalClipBounds.setEmpty();
     fCachedLocalClipBoundsDirty = true;
     fClipStack->reset();
     fMCRec->reset(bounds);
@@ skipping 298 matching lines @@
     if (x > 0) {
         x = 0;
     }
     if (y > 0) {
         y = 0;
     }
     // here x,y are either 0 or negative
     pixels = ((const char*)pixels - y * rowBytes - x * info.bytesPerPixel());
 
     // Tell our owning surface to bump its generation ID
-    this->predrawNotify();
+    const bool completeOverwrite = info.dimensions() == size;
+    this->predrawNotify(completeOverwrite);
 
     // The device can assert that the requested area is always contained in its bounds
     return device->writePixels(info, pixels, rowBytes, target.x(), target.y());
 }
 
 SkCanvas* SkCanvas::canvasForDrawIter() {
     return this;
 }
 
 //////////////////////////////////////////////////////////////////////////////
@@ skipping 1016 matching lines @@
         fSurfaceBase->aboutToDraw(SkSurface::kDiscard_ContentChangeMode);
     }
 }
 
 void SkCanvas::onDrawPaint(const SkPaint& paint) {
     TRACE_EVENT0("disabled-by-default-skia", "SkCanvas::drawPaint()");
     this->internalDrawPaint(paint);
 }
 
 void SkCanvas::internalDrawPaint(const SkPaint& paint) {
-    LOOPER_BEGIN(paint, SkDrawFilter::kPaint_Type, NULL)
+    LOOPER_BEGIN_CHECK_COMPLETE_OVERWRITE(paint, SkDrawFilter::kPaint_Type, NULL)
 
     while (iter.next()) {
         iter.fDevice->drawPaint(iter, looper.paint());
     }
 
     LOOPER_END
 }
 
 void SkCanvas::onDrawPoints(PointMode mode, size_t count, const SkPoint pts[],
                             const SkPaint& paint) {
@@ skipping 37 matching lines @@
         // To prevent accidental rejecting at this stage, we have to sort it before we check.
         SkRect tmp(r);
         tmp.sort();
 
         bounds = &paint.computeFastBounds(tmp, &storage);
         if (this->quickReject(*bounds)) {
             return;
         }
     }
 
-    LOOPER_BEGIN(paint, SkDrawFilter::kRect_Type, bounds)
+    LOOPER_BEGIN_CHECK_COMPLETE_OVERWRITE(paint, SkDrawFilter::kRect_Type, bounds)
 
     while (iter.next()) {
         iter.fDevice->drawRect(iter, r, looper.paint());
     }
 
     LOOPER_END
 }
 
 void SkCanvas::onDrawOval(const SkRect& oval, const SkPaint& paint) {
     TRACE_EVENT0("disabled-by-default-skia", "SkCanvas::drawOval()");
@@ skipping 845 matching lines @@
     }
 
     if (matrix) {
         canvas->concat(*matrix);
     }
 }
 
 SkAutoCanvasMatrixPaint::~SkAutoCanvasMatrixPaint() {
     fCanvas->restoreToCount(fSaveCount);
 }