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akiko on 10/24/12

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fast bresenham bitmap scaler in C

/ Published in: C

simple and fast bresenham image scaler, no interpolation

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// This code snippet demonstrates the simplest bresenham image scaling algorithm.
// I use it for fast, non-interpolated image upsampling on an 16bit 20MHz MCU.
// You will notice that all inner loops are quite easy on cpu - there are
// only additions and a few cmp's.
// Also, all memory addressing (in inner loops) is done indirectly, which is
// in general much faster than recalculating absolute addresses.
// Note that in my case pixels are not 24 bit RGB triplets, but 8bit 422 packed
// format. You can easily extend this to 3 lookups and writes instead of 1 (for
// genuine 24bit RGB).
//
// Hope it helps :)
// written by akiko / agt, 2012
// http://dev.modmancer.com

#define DEMO_WIDTH 64
#define DEMO_HEIGHT 64
#define FB_WIDTH 320
#define FB_HEIGHT 240

unsigned int pixels_written = 0;
UINT8 *src = framebuffer;	// input buffer of size DEMO_WIDTH * DEMO_HEIGHT
UINT8 *dst = buffer;		// output buffer of size FB_WIDTH * FB_HEIGHT

int xint_part = DEMO_WIDTH / FB_WIDTH;
int xfrac_part = DEMO_WIDTH % FB_WIDTH;

int yint_part = DEMO_HEIGHT / FB_HEIGHT;
int yfrac_part = DEMO_HEIGHT % FB_HEIGHT;
int ye = 0;

int dsty = 0;
int xe = 0;

while (dsty < FB_HEIGHT)
{
	int num_pixels = FB_WIDTH;
	while (num_pixels-- > 0)
	{
		*dst++ = *src;
		src += xint_part;
		xe += xfrac_part;
		if (xe >= FB_WIDTH)
		{
			xe -= FB_WIDTH;
			src++;
		}
	}

// use bresenham to increase line counter
	dsty += yint_part;
	ye += yfrac_part;
	if(ye >= FB_HEIGHT)
	{
		ye -= FB_HEIGHT;
		dsty++;
	}
}

URL: bresenham scaling

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