Problem reading 8 bit per channel image

[dmaitra]

Hi,

I'm new to MagickWand, and am trying to read an 8-bit per channel image using the code I wrote (see below). While it seems to be able to read 16-bits per channel images perfectly, it appears to convert the 8-bit data into 16-bits. Am I missing something?

I'm using a test image at http://www.astro.lsa.umich.edu/~dmaitra/tt.tif

Here's the convert output

Code: Select all

$ convert tt.tif txt:-
# ImageMagick pixel enumeration: 3,2,255,rgb
0,0: (  0,  0,  0)  #000000  black
1,0: (  1,  0,  0)  #010000  rgb(1,0,0)
2,0: (  2,  0,  0)  #020000  rgb(2,0,0)
0,1: (255,255,  0)  #FFFF00  yellow
1,1: (255,255,255)  #FFFFFF  white
2,1: (  0,  0,  0)  #000000  black
$

and here's the output of my code

Code: Select all

$ ./minimal tt.tif 
wd=3 ht=2 depth=8
0 0 => 0 0 0
1 0 => 257 0 0
2 0 => 514 0 0
0 1 => 65535 65535 0
1 1 => 65535 65535 65535
2 1 => 0 0 0
$

This is the code

Code: Select all

/*
   gcc `MagickWand-config --cflags --cppflags` -o minimal minimal.c    \
   `MagickWand-config --ldflags --libs` -lm -Wall -W -Wextra  -g \
*/

/* Includes and functions */ /*{{{*/
#include <math.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <wand/MagickWand.h>

/* For some reason this is not defined in the .h headers!!! */
void PixelGetMagickColor (const PixelWand *, MagickPixelPacket *);
/*}}}*/

int main(int argc,char **argv)/*{{{*/
{

/* Define ThrowWandException */ /*{{{*/
#define ThrowWandException(wand) \
{ \
  char \
    *description; \
 \
  ExceptionType \
    severity; \
 \
  description=MagickGetException(wand,&severity); \
  (void) fprintf(stderr,"%s %s %lu %s\n",GetMagickModule(),description); \
  description=(char *) MagickRelinquishMemory(description); \
  exit(-1); \
}/*}}}*/

/* Declare variables and stuff */ /*{{{*/
unsigned long wd, ht, x, y;
int dp;
MagickBooleanType status;
MagickPixelPacket pixel;
MagickWand *image_wand;
PixelIterator *iterator;
PixelWand **pixels;
/*}}}*/

if (argc != 2){/*{{{*/
	(void) fprintf(stdout,"Usage: %s image\n",argv[0]);
	exit(0);
}/*}}}*/

/* Read an image, get dimensions */ /*{{{*/
MagickWandGenesis();
image_wand=NewMagickWand();  
status=MagickReadImage(image_wand,argv[1]);
if (status == MagickFalse) ThrowWandException(image_wand);

wd = (unsigned long)MagickGetImageWidth(image_wand);
ht = (unsigned long)MagickGetImageHeight(image_wand);
dp = (int)MagickGetImageDepth(image_wand);
printf("wd=%lu ht=%lu depth=%d\n",wd,ht,dp);
/*}}}*/

/* Get pixel values */ /*{{{*/
iterator=NewPixelIterator(image_wand);
if (iterator == (PixelIterator *)NULL) ThrowWandException(image_wand);

for (y=0; y<ht; y++){
	pixels=PixelGetNextIteratorRow(iterator,&wd);
	if (pixels == (PixelWand **)NULL) break;
	for (x=0; x<wd; x++){
		PixelGetMagickColor(pixels[x],&pixel);
		printf("%lu %lu => %g %g %g\n",x,y,pixel.red,
				pixel.green,pixel.blue);
	}

}/*}}}*/

iterator=DestroyPixelIterator(iterator);

/* Clean up */ /*{{{*/
image_wand=DestroyMagickWand(image_wand);
MagickWandTerminus();
/*}}}*/

return 0;
}/*}}}*/

I'd appreciate any help.
Thanks,
Dipankar

[magick]

Internally, pixels are stored at the quantum depth specified when ImageMagick is compiled. Your version is Q16 or 16-bits per pixel component (0-65535). You can rebuild ImageMagick at 8-bits or divide the 16-bit pixels by 257 or use MagickExportImagePixels() to export an array of 8-bit pixel values.

[dmaitra]

Awesome! Thanks magick!

I had suspected something like that but couldn't convince myself that I hadn't made a coding error.

[anthony]

The only reason the txt: output image was 8 bit and not 16 bit, was that the input image was 8-bit
If you had reset the specific images depth using +depth the txt: image would have been 16 bit (or whatever the compile time quality of your IM library was).