Added Canvas::blur()

will add a better implementation later when I have time

src/blur.cc

@@ -0,0 +1,74 @@
+
+//
+// blur.cc
+//
+// Copyright (c) 2010 LearnBoost <tj@learnboost.com>
+//
+
+#include <string.h>
+#include "Canvas.h"
+
+void
+Canvas::blur(cairo_surface_t *surface, int radius) {
+  // Steve Hanov, 2009
+  // Released into the public domain.
+  
+  // get width, height
+  int width = cairo_image_surface_get_width( surface );
+  int height = cairo_image_surface_get_height( surface );
+  unsigned char* dst = (unsigned char*)malloc(width*height*4);
+  unsigned* precalc = 
+      (unsigned*)malloc(width*height*sizeof(unsigned));
+  unsigned char* src = cairo_image_surface_get_data( surface );
+  double mul=1.f/((radius*2)*(radius*2));
+  int channel;
+  
+  // The number of times to perform the averaging. According to wikipedia,
+  // three iterations is good enough to pass for a gaussian.
+  const int MAX_ITERATIONS = 3; 
+  int iteration;
+
+  memcpy( dst, src, width*height*4 );
+
+  for ( iteration = 0; iteration < MAX_ITERATIONS; iteration++ ) {
+      for( channel = 0; channel < 4; channel++ ) {
+          int x,y;
+
+          // precomputation step.
+          unsigned char* pix = src;
+          unsigned* pre = precalc;
+
+          pix += channel;
+          for (y=0;y<height;y++) {
+              for (x=0;x<width;x++) {
+                  int tot=pix[0];
+                  if (x>0) tot+=pre[-1];
+                  if (y>0) tot+=pre[-width];
+                  if (x>0 && y>0) tot-=pre[-width-1];
+                  *pre++=tot;
+                  pix += 4;
+              }
+          }
+
+          // blur step.
+          pix = dst + (int)radius * width * 4 + (int)radius * 4 + channel;
+          for (y=radius;y<height-radius;y++) {
+              for (x=radius;x<width-radius;x++) {
+                  int l = x < radius ? 0 : x - radius;
+                  int t = y < radius ? 0 : y - radius;
+                  int r = x + radius >= width ? width - 1 : x + radius;
+                  int b = y + radius >= height ? height - 1 : y + radius;
+                  int tot = precalc[r+b*width] + precalc[l+t*width] - 
+                      precalc[l+b*width] - precalc[r+t*width];
+                  *pix=(unsigned char)(tot*mul);
+                  pix += 4;
+              }
+              pix += (int)radius * 2 * 4;
+          }
+      }
+      memcpy( src, dst, width*height*4 );
+  }
+
+  free( dst );
+  free( precalc );
+}