Re: Image Scaling

On Sun, Jun 16, 2013 at 11:42:35AM +0200, Sebastian Geerken wrote:
> On Sa, Jun 15, 2013, Johannes Hofmann wrote:
> > On Fri, Jun 14, 2013 at 08:01:00PM +0200, Sebastian Geerken wrote:
> > > Hi!
> > > 
> > > I've just push some changes for a better image scaling. See these two
> > > examples for the effect:
> > > 
> > > - http://www.dillo.org/~sgeerken/grid.html
> > > - http://www.dillo.org/~sgeerken/tenniel.html
> > > 
> > > I'm not completely sure about the side-effects. It should be slightly
> > > slower (see comment: it can be improved), and it will not work with
> > > other that RGB and RGBA data (for the case that Imgbuf is used for
> > > other representations). The old code is still there; just look at
> > > FltkImgbuf::scaleRow.
> > 
> > Nice! BTW, I have this page bookmarked for quite some time:
> > http://www.4p8.com/eric.brasseur/gahttp://www.4p8.com/eric.brasseur/gamma.htmlmma.html
> > I don't know whether this makes a difference in real life, but the
> > examples are interesting.
> 
> Interesting!
> 
> I didn't read the whole article, but it made me wonder what effect a
> gamma correction has on image scaling. See the test page
> <http://www.4p8.com/eric.brasseur/gamma_dalai_lama.html> with:
> 
> 1. dillo 3.0.3,
> 2. current version from the hg repository, and
> 3. latter with the attached patch applied.
> 
> The patch uses floating point arithmetics; perhaps there is a faster
> algorithm based on integer arithmetics.

I don't know, but we could cache the values as in attached patch.

  Johannes

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diff -r 91fdbc0feb68 dw/fltkimgbuf.cc
--- a/dw/fltkimgbuf.cc	Sun Jun 16 21:23:19 2013 +0200
+++ b/dw/fltkimgbuf.cc	Sun Jun 16 22:27:23 2013 +0200
@@ -22,6 +22,7 @@
 #include "../lout/misc.hh"
 
 #include <FL/fl_draw.H>
+#include <math.h>
 
 #define IMAGE_MAX_AREA (6000 * 6000)
 
@@ -210,6 +211,33 @@
    }
 }
 
+static int gamma (int x, double gamma, int table[256])
+{
+   if (x < 0 || x >= 256) {
+      assert (0);
+      return 0;
+   }
+
+   if (table[x] <= 0)
+      table[x] =  256 * pow((double)x / 256, gamma);
+
+   return table[x];
+}
+
+static int gamma_22 (int x)
+{
+   static int table[256] = {0};
+
+   return gamma (x, 2.2, table);
+}
+
+static int gamma_1_div_22 (int x)
+{
+   static int table[256] = {0};
+
+   return gamma (x, 1 / 2.2, table);
+}
+
 /**
  * General method to scale an image buffer. Used to scale single lines
  * in scaleRowBeautiful.
@@ -247,12 +275,12 @@
             for(int yo = yo1; yo < yo2; yo++) {
                const core::byte *ps = src + bpp * (yo * srcWidth + xo);
                for(int i = 0; i < bpp; i++)
-                  v[i] += ps[i];
+                  v[i] += gamma_22 (ps[i]);
             }
          
          core::byte *pd = dest + bpp * (y * destWidth + x);
          for(int i = 0; i < bpp; i++)
-            pd[i] = v[i] / n;
+            pd[i] = gamma_1_div_22 (v[i] / n);
       }
 }