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  Fastest method of referencing all pixels in a line?  (Read 3073 times)
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Offline counterp

Senior Devvie


Medals: 11



« Posted 2011-07-15 17:31:00 »

Currently, to find all pixels in a line I am using Bresenham's line algorithm. I converted the method from pseudo-code (on wikipedia) to java, but it's rather slow. Here is the code:

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   private void getPixelsInLine(int x0, int y0, int x1, int y1) {
      boolean steep = Math.abs(y1 - y0) > Math.abs(x1 - x0);
      if (steep) {
         int tmp = x0;
         x0 = y0;
         y0 = tmp;
         tmp = x1;
         x1 = y1;
         y1 = tmp;
      }
      if (x0 > x1) {
         int tmp = x0;
         x0 = x1;
         x1 = tmp;
         tmp = y0;
         y0 = y1;
         y1 = tmp;
      }
      int deltax = x1 - x0;
      int deltay = Math.abs(y1 - y0);
      int error = deltax / 2;
      int ystep = -1;
      int y = y0;
      if (y0 < y1)
         ystep = 1;
      for (int x = x0; x <= x1; x++) {
         if (steep)
            mark(y, x);
         else
            mark(x, y);
         error -= deltay;
         if (error < 0) {
            y += ystep;
            error += deltax;
         }
      }
   }


I was wondering if there is a faster method of obtaining all pixels in a line/better implementation?
Offline avm1979
« Reply #1 - Posted 2011-07-15 19:22:10 »

That should be very fast. Fast enough that you could do it thousands of times a second and not worry about at all.

Unless you're doing something hideously slow in the "mark" function. If this were actually slow, that's where I'd look...

Offline Roquen
« Reply #2 - Posted 2011-07-15 20:00:07 »

You're leaving out the most important piece of information: what do you really want to do?
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Offline counterp

Senior Devvie


Medals: 11



« Reply #3 - Posted 2011-07-15 23:47:35 »

yes it is fast, it seems the problem is linked to the other thread I posted (with the images)
Offline pitbuller
« Reply #4 - Posted 2011-07-16 09:02:34 »

int error = deltax / 2;

Could be:

int error = deltax >>1;



I have allways thinked do Java found these kind of bit shift optimizations automatically?
Offline Roquen
« Reply #5 - Posted 2011-07-16 09:44:54 »

Not the same if negative.
Offline zoto

Senior Devvie


Medals: 4



« Reply #6 - Posted 2011-07-16 23:27:24 »

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int error = deltax >>>1;

http://en.wikipedia.org/wiki/Arithmetic_shift
http://en.wikipedia.org/wiki/Logical_shift
Offline counterp

Senior Devvie


Medals: 11



« Reply #7 - Posted 2011-07-17 05:19:19 »


see post above yours
Offline Roquen
« Reply #8 - Posted 2011-07-17 07:28:32 »

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  int i = -1;
  int r0 = i/2;
  int r1 = i>>1;


Shifting (alone) isnt't the same for negative.  The result depends on the sub-bit, so compilers won't convert division into a shift unless it can statically determine that all values are positive or zero.
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