image readers: ability to check if image has alpha before full read

Conflicts:
	src/webp_reader.cpp

include/mapnik/image_reader.hpp

@@ -57,6 +57,7 @@ struct MAPNIK_DECL image_reader : private mapnik::noncopyable
 {
     virtual unsigned width() const=0;
     virtual unsigned height() const=0;
+    virtual bool has_alpha() const=0;
     virtual bool premultiplied_alpha() const=0;
     virtual void read(unsigned x,unsigned y,image_data_32& image)=0;
     virtual ~image_reader() {}

src/jpeg_reader.cpp

@@ -80,6 +80,7 @@ public:
     ~jpeg_reader();
     unsigned width() const;
     unsigned height() const;
+    inline bool has_alpha() const { return false; }
     inline bool premultiplied_alpha() const { return true; }
     void read(unsigned x,unsigned y,image_data_32& image);
 private:

src/png_reader.cpp

@@ -69,12 +69,14 @@ private:
     unsigned height_;
     int bit_depth_;
     int color_type_;
+    bool has_alpha_;
 public:
     explicit png_reader(std::string const& file_name);
     png_reader(char const* data, std::size_t size);
     ~png_reader();
     unsigned width() const;
     unsigned height() const;
+    inline bool has_alpha() const { return has_alpha_; }
     bool premultiplied_alpha() const { return false; } //http://www.libpng.org/pub/png/spec/1.1/PNG-Rationale.html
     void read(unsigned x,unsigned y,image_data_32& image);
 private:
@@ -129,7 +131,8 @@ png_reader<T>::png_reader(std::string const& file_name)
       width_(0),
       height_(0),
       bit_depth_(0),
-      color_type_(0)
+      color_type_(0),
+      has_alpha_(false)
 {
     if (!source_.is_open()) throw image_reader_exception("PNG reader: cannot open file '"+ file_name + "'");
     if (!stream_) throw image_reader_exception("PNG reader: cannot open file '"+ file_name + "'");
@@ -143,7 +146,8 @@ png_reader<T>::png_reader(char const* data, std::size_t size)
       width_(0),
       height_(0),
       bit_depth_(0),
-      color_type_(0)
+      color_type_(0),
+      has_alpha_(false)
 {
 
     if (!stream_) throw image_reader_exception("PNG reader: cannot open image stream");
@@ -193,7 +197,7 @@ void png_reader<T>::init()
 
     png_uint_32  width, height;
     png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth_, &color_type_,0,0,0);
-
+    has_alpha_ = (color_type_ & PNG_COLOR_MASK_ALPHA) || png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS);
     width_=width;
     height_=height;
 

src/tiff_reader.cpp

@@ -139,6 +139,7 @@ public:
     virtual ~tiff_reader();
     unsigned width() const;
     unsigned height() const;
+    inline bool has_alpha() const { return false; /*FIXME*/ }
     bool premultiplied_alpha() const;
     void read(unsigned x,unsigned y,image_data_32& image);
 private:

src/webp_reader.cpp

@@ -108,12 +108,14 @@ private:
     size_t size_;
     unsigned width_;
     unsigned height_;
+    bool has_alpha_;
 public:
     explicit webp_reader(char const* data, std::size_t size);
     explicit webp_reader(std::string const& filename);
     ~webp_reader();
     unsigned width() const;
     unsigned height() const;
+    inline bool has_alpha() const { return has_alpha_; }
     bool premultiplied_alpha() const { return false; }
     void read(unsigned x,unsigned y,image_data_32& image);
 private:
@@ -143,7 +145,8 @@ template <typename T>
 webp_reader<T>::webp_reader(char const* data, std::size_t size)
     : buffer_(new buffer_policy_type(reinterpret_cast<uint8_t const*>(data), size)),
       width_(0),
-      height_(0)
+      height_(0),
+      has_alpha_(false)
 {
     init();
 }
@@ -153,7 +156,8 @@ webp_reader<T>::webp_reader(std::string const& filename)
     : buffer_(nullptr),
       size_(0),
       width_(0),
-      height_(0)
+      height_(0),
+      has_alpha_(false)
 {
     std::ifstream file(filename.c_str(), std::ios::binary);
     if (!file)
@@ -199,6 +203,10 @@ void webp_reader<T>::init()
         height_ = config.input.height;
         has_alpha_ = config.input.has_alpha;
     }
+    else
+    {
+        throw image_reader_exception("WEBP reader: WebPGetFeatures failed");
+    }
 }
 
 template <typename T>