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 Fastest method of referencing all pixels in a line?  (Read 3187 times) 0 Members and 1 Guest are viewing this topic.
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:

 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37 `   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?
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...

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?
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)
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?
Roquen
 « Reply #5 - Posted 2011-07-16 09:44:54 »

Not the same if negative.
zoto

Senior Devvie

Medals: 4

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

 1 `int error = deltax >>>1;`

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

Senior Devvie

Medals: 11

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

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

 1  2  3 `  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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