use some helper Make functions to initialize SkImageInfo

include/core/SkImageInfo.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImageInfo_DEFINED
 #define SkImageInfo_DEFINED
 
 #include "SkTypes.h"
+#include "SkSize.h"
 
 class SkFlattenableWriteBuffer;
 class SkFlattenableReadBuffer;
 
 /**
  *  Describes how to interpret the alpha compoent of a pixel.
  */
 enum SkAlphaType {
     /**
      *  All pixels should be treated as opaque, regardless of the value stored
@@ skipping 80 matching lines @@
 
 /**
  *  Describe an image's dimensions and pixel type.
  */
 struct SkImageInfo {
     int         fWidth;
     int         fHeight;
     SkColorType fColorType;
     SkAlphaType fAlphaType;
 
+    static SkImageInfo Make(int width, int height, SkColorType ct, SkAlphaType at) {
+        SkASSERT(width >= 0);
+        SkASSERT(height >= 0);
+        SkImageInfo info = {
+            width, height, ct, at
+        };
+        return info;
+    }
+
+    /**
+     *  Sets colortype to the native ARGB32 type.
+     */
+    static SkImageInfo MakeN32(int width, int height, SkAlphaType at) {
+        SkASSERT(width >= 0);
+        SkASSERT(height >= 0);
+        SkImageInfo info = {
+            width, height, kPMColor_SkColorType, at
+        };
+        return info;
+    }
+
+    /**
+     *  Sets colortype to the native ARGB32 type, and the alphatype to premul.
+     */
+    static SkImageInfo MakeN32Premul(int width, int height) {
+        SkASSERT(width >= 0);
+        SkASSERT(height >= 0);
+        SkImageInfo info = {
+            width, height, kPMColor_SkColorType, kPremul_SkAlphaType
+        };
+        return info;
+    }
+    
+    /**
+     *  Sets colortype to the native ARGB32 type, and the alphatype to premul.
+     */
+    static SkImageInfo MakeN32Premul(const SkISize& size) {
+        return MakeN32Premul(size.width(), size.height());
+    }
+    
+    static SkImageInfo MakeA8(int width, int height) {
+        SkASSERT(width >= 0);
+        SkASSERT(height >= 0);
+        SkImageInfo info = {
+            width, height, kAlpha_8_SkColorType, kPremul_SkAlphaType
+        };
+        return info;
+    }
+    
     bool isOpaque() const {
         return SkAlphaTypeIsOpaque(fAlphaType);
     }
 
     int bytesPerPixel() const {
         return SkColorTypeBytesPerPixel(fColorType);
     }
 
     size_t minRowBytes() const {
         return fWidth * this->bytesPerPixel();
@@ skipping 11 matching lines @@
 
     size_t getSafeSize(size_t rowBytes) const {
         if (0 == fHeight) {
             return 0;
         }
         return (fHeight - 1) * rowBytes + fWidth * this->bytesPerPixel();
     }
 };
 
 #endif