Re: Image Scaling

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.

Sebastian

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diff -r 5fbb1dd423a6 dw/fltkimgbuf.cc
--- a/dw/fltkimgbuf.cc	Sun Jun 16 10:39:27 2013 +0200
+++ b/dw/fltkimgbuf.cc	Sun Jun 16 11:41:42 2013 +0200
@@ -22,6 +22,7 @@
 #include "../lout/misc.hh"
 
 #include <FL/fl_draw.H>
+#include <math.h>
 
 #define IMAGE_MAX_AREA (6000 * 6000)
 
@@ -206,6 +207,11 @@
    }
 }
 
+static int gamma (int x, double gamma)
+{
+   return 256 * pow((double)x / 256, gamma);
+}
+
 /**
  * General method to scale an image buffer. Used to scale single lines
  * in scaleRowBeautiful.
@@ -243,12 +249,12 @@
             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 (ps[i], 2.2);
             }
          
          core::byte *pd = dest + bpp * (y * destWidth + x);
          for(int i = 0; i < bpp; i++)
-            pd[i] = v[i] / n;
+            pd[i] = gamma (v[i] / n, 1 / 2.2);
       }
 }