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August 2007
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2007.08.07 03:46 "bilevel image scrambling when size not divisible by 8", by Lee Cooper
I'm developing an image processing toolbox in C/C++ based on the TIFF
library and I have one function that converts grayscale TIFF data to
B&W by thresholding and another function to write the result to disk.
When the number of pixels in the image isn't evenly divisible by 8 the
result TIFF's contents are scrambled somewhere in the writing process.
For example a 200x100 image of a single narrow vertical strip writes
OK, the same image at 201x100 appears as a series of diagonal bands
and some seemingly random garbage at the bottom. The thresholding
portion seems to be working OK, I have verified this by writing its
result in ASCII and checking manually in Matlab.
Apparently I have some misunderstanding about how bilevel images are
handled in TIFF but I can't find much info on the subject. When the
number of pixels isn't a multiple of eight how is the remainder of the
byte handled? I feel like this will probably be a simple answer but
I'm stuck and really need to make some progress. I've included some
code snippets below.
----------------------------------------------------------------------------------------------------------------------
int TIFFwriteBW(uint8* image, char* filename, uint32 width, uint32 height) {
TIFF* out = TIFFOpen(filename, "w");
TIFFSetField (out, TIFFTAG_IMAGEWIDTH, width);
TIFFSetField(out, TIFFTAG_IMAGELENGTH, height);
TIFFSetField(out, TIFFTAG_SAMPLESPERPIXEL, 1);
TIFFSetField(out, TIFFTAG_BITSPERSAMPLE, 1);
TIFFSetField(out, TIFFTAG_FILLORDER, FILLORDER_MSB2LSB);
TIFFSetField(out, TIFFTAG_ORIENTATION,
ORIENTATION_TOPLEFT); TIFFSetField(out, TIFFTAG_PLANARCONFIG,
PLANARCONFIG_CONTIG);
TIFFSetField(out, TIFFTAG_PHOTOMETRIC, PHOTOMETRIC_MINISBLACK);
TIFFSetField(out, TIFFTAG_COMPRESSION, COMPRESSION_NONE);
TIFFSetField(out, TIFFTAG_ROWSPERSTRIP, height);
TIFFWriteEncodedStrip(out, 0, image, (uint32)ceil((height * width) / 8.0);
TIFFClose(out);
return(1);
}
uint8* gray2BW(uint8* image, uint32 npixels, uint8 thresh) {
uint8* bw = (uint8*) malloc((uint32) ceil(npixels / 8.0) * sizeof(uint8));
if(bw != NULL) {
uint8 *grayptr = image;
int i, j;
for(i = 0; i < npixels; i++) {
*(bw + i/8) <<= 1; //shift byte contents left by one
if(*grayptr >= thresh) { //test threshold each pixel
*(bw + i/8) |= 01;
}
grayptr++;
}
return(bw);
}
else {
return(NULL);
}
}