add newImage API

src/image/SkImage.cpp

Index: src/image/SkImage.cpp
diff --git a/src/image/SkImage.cpp b/src/image/SkImage.cpp
index 97e7475a34ff28dc49b6499d5ecaef86c3c8d7fa..dc2fd7dfd85cda5d57dfac2546a62042e0a20985 100644
--- a/src/image/SkImage.cpp
+++ b/src/image/SkImage.cpp
@@ -94,6 +94,22 @@ const char* SkImage::toString(SkString* str) const {
     return str->c_str();
 }
 
+SkImage* SkImage::newImage(int newWidth, int newHeight, const SkIRect* subset,
+                           SkFilterQuality quality) const {
+    if (newWidth < 0 || newHeight < 0) {

[scroggo, 2015/01/22 20:05:14]

Did you make a decision on whether to allow SkImages with width/height of zero?

[reed1, 2015/01/22 21:52:52]

Good question... I'll give you an answer soon.

[reed1, 2015/01/22 21:58:24]

Zero is illegal, will update this check.

+        return NULL;
+    }
+    if (subset && !SkIRect::MakeWH(this->width(), this->height()).contains(*subset)) {
+        return NULL;
+    }
+
+    if (NULL == subset && this->width() == newWidth && this->height() == newHeight) {

[scroggo, 2015/01/22 20:05:14]

What if the subset == this->dimensions?

(Silly use case, I know...)

[reed1, 2015/01/22 21:52:52]

Ah, good catch. Will add that check.

+        return SkRef(const_cast<SkImage*>(this));
+    }
+
+    return as_IB(this)->onNewImage(newWidth, newHeight, subset, quality);
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 static bool raster_canvas_supports(const SkImageInfo& info) {
@@ -126,3 +142,32 @@ bool SkImage_Base::onReadPixels(const SkImageInfo& dstInfo, void* dstPixels, siz
 
     return true;
 }
+
+SkImage* SkImage_Base::onNewImage(int newWidth, int newHeight, const SkIRect* subset,
+                                  SkFilterQuality quality) const {
+    const bool opaque = this->isOpaque();
+    const SkImageInfo info = SkImageInfo::Make(newWidth, newHeight, kN32_SkColorType,
+                                               opaque ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
+    SkAutoTUnref<SkSurface> surface(this->newSurface(info, NULL));
+    if (!surface.get()) {
+        return NULL;
+    }
+
+    SkRect src;
+    if (subset) {
+        src.set(*subset);
+    } else {
+        src = SkRect::MakeIWH(this->width(), this->height());
+    }
+
+    surface->getCanvas()->scale(newWidth / src.width(), newHeight / src.height());
+    surface->getCanvas()->translate(-src.x(), -src.y());
+
+    SkPaint paint;
+    paint.setXfermodeMode(SkXfermode::kSrc_Mode);
+    paint.setFilterQuality(quality);
+    surface->getCanvas()->drawImage(this, 0, 0, &paint);
+    return surface->newImageSnapshot();
+}
+
+