PDF: Add (low-memory) SkPDFBitmap class

src/pdf/SkPDFBitmap.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef SkPDFBitmap_DEFINED
+#define SkPDFBitmap_DEFINED
+
+#include "SkPDFTypes.h"
+#include "SkBitmap.h"
+
+/**
+ * This is a replacement for SkPDFImage.  As of now, it only supports

[mtklein, 2015/02/12 00:21:46]

Let's write this comment first as if SkPDFImage didn't exist, then follow up
with "This is a more memory-efficient replacement for SkPDFImage when we're
working with 8888 bitmaps, the most common format."  Reading this comment as it
stands doesn't help me when I don't know what SkPDFImage is.

[hal.canary, 2015/02/12 21:28:03]

Done.

+ * 8888 bitmaps (the most common case).  This onbect takes very little

[mtklein, 2015/02/12 00:21:46]

object

+ * extra memory (aside from refing the bitmap's pixels), and its
+ * emitObject() does not cache any data.
+ *
+ * The SkPDFBitmap::Create function will check the canon for duplicates.
+ */
+class SkPDFBitmap : public SkPDFObject {
+public:
+    // Returns NULL on unsupported bitmap;
+    // TODO(halcanary): support other bitmap colortypes and replace
+    // SkPDFImage.
+    static SkPDFBitmap* Create(const SkBitmap&,
+                               const SkIRect& subset);

[mtklein, 2015/02/12 00:21:46]

Does passing a subset here have any different effect from subsetting the bitmap
before you pass it in?  If so, explain the difference?  If not, remove the
feature?

[hal.canary, 2015/02/12 21:28:03]

It's  only here to match the calling site.

+    ~SkPDFBitmap();
+    void emitObject(SkWStream*, SkPDFCatalog*) SK_OVERRIDE;
+    void addResources(SkTSet<SkPDFObject*>* resourceSet,
+                      SkPDFCatalog* catalog) const SK_OVERRIDE;
+    bool equals(const SkBitmap& other) const {
+        return fBitmap.getGenerationID() == other.getGenerationID() &&
+               fBitmap.pixelRefOrigin() == other.pixelRefOrigin() &&
+               fBitmap.dimensions() == other.dimensions() && !fIsAlpha;

[mtklein, 2015/02/12 00:21:46]

What's the deal with && !fIsAlpha?  That deserves a comment about why it's
weird.

[hal.canary, 2015/02/12 21:28:03]

When IsAlpha is true, the SkPDFBitmap represents that bitmap's alpha layer. 
Otherwise, the SkPDFBitmap represents the entire bitmap.

+    }
+
+private:
+    SkBitmap fBitmap;
+    SkAutoTUnref<SkPDFBitmap> fSMask;
+    bool fIsAlpha;
+    SkPDFBitmap(const SkBitmap&, bool isAlpha, SkPDFBitmap* smask);
+};
+
+#endif  // SkPDFBitmap_DEFINED