Add a Canvas::SaveAndUnscale() method for pixel-accurate drawing.

ui/gfx/canvas.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/canvas.h"
 
 #include <cmath>
 #include <limits>
 
 #include "base/i18n/rtl.h"
@@ skipping 162 matching lines @@
            paint);
   DrawRect(Rect(rect.x(), rect.y(), 1, rect.height()), paint);
   DrawRect(Rect(rect.x() + rect.width() - 1, rect.y(), 1, rect.height()),
            paint);
 }
 
 void Canvas::Save() {
   canvas_->save();
 }
 
+float Canvas::SaveAndUnscale() {

[sky, 2015/10/13 22:23:11]

How expensive is save? The common case is scale=1. Is it worth optimizing that?

[Peter Kasting, 2015/10/13 22:34:42]

Save() is very cheap.  Also we can't get rid of it when the scale factor is 1
because callers are going to unconditionally Restore().

scale(1, 1) is pretty cheap as well, so I didn't bother to try to optimize it
away.

+  Save();
+  SkScalar scale_factor = 1.0f / image_scale_;

[sky, 2015/10/13 22:23:11]

I'm not familiar enough with what we want to happen, but did you consider
unscaling by the current transform?

[Peter Kasting, 2015/10/13 22:34:42]

I did, but I'm not convinced that's what we actually want.  We want to undo the
"device pixel -> DIP" transformation, but imagine if some other caller, e.g. a
"magnifying glass effect" implementation, had added an additional scale atop
this.  We'd still want that scale to apply to the current canvas.

+  canvas_->scale(scale_factor, scale_factor);
+  return image_scale_;
+}
+
 void Canvas::SaveLayerAlpha(uint8 alpha) {
   canvas_->saveLayerAlpha(NULL, alpha);
 }
 
 void Canvas::SaveLayerAlpha(uint8 alpha, const Rect& layer_bounds) {
   SkRect bounds(RectToSkRect(layer_bounds));
   canvas_->saveLayerAlpha(&bounds, alpha);
 }
 
 void Canvas::Restore() {
@@ skipping 428 matching lines @@
   // by the paint).
   SkPaint p(paint);
   p.setFilterQuality(filter ? kLow_SkFilterQuality : kNone_SkFilterQuality);
   p.setShader(shader.get());
 
   // The rect will be filled by the bitmap.
   canvas_->drawRect(dest_rect, p);
 }
 
 }  // namespace gfx