+ fix reading a region from JPEG file

+ re-use jpeg lib structures
+ avoid resource leaking (store FILE in boost::shared_ptr with custom deleter)

src/jpeg_reader.cpp

@@ -32,6 +32,7 @@ extern "C"
 }
 
 // boost
+#include <boost/shared_ptr.hpp>
 #include <boost/scoped_array.hpp>
 
 // std
@@ -41,13 +42,22 @@ namespace mapnik
 {
 class JpegReader : public image_reader, mapnik::noncopyable
 {
+    struct file_closer
+    {
+       void operator() (FILE * file)
+       {
+           if (file != 0) fclose(file);
+       }
+    };
+    typedef boost::shared_ptr<FILE> file_ptr;
+
 private:
     std::string fileName_;
     unsigned width_;
     unsigned height_;
     jpeg_decompress_struct cinfo;
     jpeg_error_mgr jerr;
-    FILE *fp;
+    file_ptr file_;
 public:
     explicit JpegReader(std::string const& fileName);
     ~JpegReader();
@@ -74,16 +84,13 @@ JpegReader::JpegReader(std::string const& fileName)
     : fileName_(fileName),
       width_(0),
       height_(0),
-      fp(NULL)
+      file_()
 {
     init();
 }
 
-JpegReader::~JpegReader() {
-    if (fp)
-    {
-        fclose(fp);
-    }
+JpegReader::~JpegReader()
+{
     jpeg_destroy_decompress(&cinfo);
 }
 
@@ -101,22 +108,28 @@ void JpegReader::on_error_message(j_common_ptr cinfo)
 
 void JpegReader::init()
 {
-    fp = fopen(fileName_.c_str(),"rb");
+    FILE * fp = fopen(fileName_.c_str(),"rb");
     if (!fp) throw image_reader_exception("JPEG Reader: cannot open image file " + fileName_);
-
+    file_ = boost::shared_ptr<FILE>(fp,file_closer());
     cinfo.err = jpeg_std_error(&jerr);
     jerr.error_exit = on_error;
     jerr.output_message = on_error_message;
     jpeg_create_decompress(&cinfo);
-    jpeg_stdio_src(&cinfo, fp);
+    jpeg_stdio_src(&cinfo, &*file_);
     jpeg_read_header(&cinfo, TRUE);
     jpeg_start_decompress(&cinfo);
     width_ = cinfo.output_width;
     height_ = cinfo.output_height;
-    // if enabled: "Application transferred too few scanlines"
-    //jpeg_finish_decompress(&cinfo);
-    jpeg_destroy_decompress(&cinfo);
-    fclose(fp);
+
+    if (cinfo.out_color_space == JCS_UNKNOWN)
+    {
+        throw image_reader_exception("JPEG Reader: failed to read unknown color space in " + fileName_);
+    }
+    if (cinfo.output_width == 0 || cinfo.output_height == 0)
+    {
+        jpeg_destroy_decompress (&cinfo);
+        throw image_reader_exception("JPEG Reader: failed to read image size of " + fileName_);
+    }
 }
 
 unsigned JpegReader::width() const
@@ -131,59 +144,42 @@ unsigned JpegReader::height() const
 
 void JpegReader::read(unsigned x0, unsigned y0, image_data_32& image)
 {
-    fp = fopen(fileName_.c_str(),"rb");
-    if (!fp) throw image_reader_exception("JPEG Reader: cannot open image file " + fileName_);
-
-    cinfo.err = jpeg_std_error(&jerr);
-
-    jpeg_create_decompress(&cinfo);
-    jpeg_stdio_src(&cinfo, fp);
-
-    jpeg_read_header(&cinfo, TRUE);
-    if (cinfo.out_color_space == JCS_UNKNOWN)
-        throw image_reader_exception("JPEG Reader: failed to read unknown color space in " + fileName_);
-
-    jpeg_start_decompress(&cinfo);
-
-    if (cinfo.output_width == 0) {
-        jpeg_destroy_decompress (&cinfo);
-        throw image_reader_exception("JPEG Reader: failed to read image size of " + fileName_);
-    }
-
     JSAMPARRAY buffer;
     int row_stride;
     unsigned char a,r,g,b;
     row_stride = cinfo.output_width * cinfo.output_components;
     buffer = (*cinfo.mem->alloc_sarray) ((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);
 
-    unsigned w = std::min(unsigned(image.width()),width_);
-    unsigned h = std::min(unsigned(image.height()),height_);
+    unsigned w = std::min(unsigned(image.width()),width_ - x0);
+    unsigned h = std::min(unsigned(image.height()),height_ - y0);
 
     boost::scoped_array<unsigned int> out_row(new unsigned int[w]);
-    // TODO - handle x0
-    for (unsigned i=0;i<h;++i)
+    unsigned row = 0;
+    while (cinfo.output_scanline < cinfo.output_height)
     {
         jpeg_read_scanlines(&cinfo, buffer, 1);
-        if (i>=y0 && i<h)
+        if (row >= y0 && row < y0 + h)
         {
-            for (unsigned int x=0; x<w; x++)
+            for (unsigned int x = 0; x < w; ++x)
             {
+                unsigned col = x + x0;
                 a = 255; // alpha not supported in jpg
-                r = buffer[0][cinfo.output_components * x];
+                r = buffer[0][cinfo.output_components * col];
                 if (cinfo.output_components > 2)
                 {
-                    g = buffer[0][cinfo.output_components*x+1];
-                    b = buffer[0][cinfo.output_components*x+2];
+                    g = buffer[0][cinfo.output_components * col + 1];
+                    b = buffer[0][cinfo.output_components * col + 2];
                 } else {
                     g = r;
                     b = r;
                 }
                 out_row[x] = color(r, g, b, a).rgba();
             }
-            image.setRow(i-y0, out_row.get(), w);
+            image.setRow(row - y0, out_row.get(), w);
         }
+        ++row;
     }
-    jpeg_finish_decompress(&cinfo);
     jpeg_destroy_decompress(&cinfo);
 }
+
 }