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March 1994
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1994.03.31 19:30 "Re: A little PackBits bug", by Dan Mccoy
You pointed out the bug in 3.3 packbits code:
> if (n == 1 && -op[-2] == 1 && ...
back in 3.2, op was declared "char" and the test read:
> if (n == 1 && op[-2] == (char)-1 && ...
which was a little better.
Updating the (char) to (tidata_t) would probably have worked better
than the changes introduced in 3.3.
>I think the most portable way is to test: op[-2] == 255.
> ...
>The solution: - (char)op[-2] == 1 works on machines where
>char is signed char.
How about the solution: op[-2] == (tidata_t) -1
>Second I am using a "private" TIFF-format for temporary storing
>images generated by our GKS-software in the following format
>Separate planes
Here's your problem, separate planes.
If you use LZW for separate planes, you have to write all of each
plane before moving on to the next plane.
Personally, I'm fond of contiguous planes.
>TIFFWriteScanline:Compression algorithm does not support random access
>I am writing the image with the following simplified code fragment :
> for (i = 0; i < numscans; i++) {
> /* small loop over planes (or) samples */
> for (sample = 0; sample < samperpix; sample++ ) {
> /* start byte in data stream = startdata */
> startdata = (char *) &data[iofp] + startbyte;
> (void) TIFFWriteScanline(out, (tdata_t) startdata, tmprow,
> (tsample_t) sample);
> iofp += planelen;
> } /* end loop over samples */
> } /* end loop over scans */
I do not understand why this is direct access.
Because when rowsperstrip > 1, it wants to buffer two strips of
plane 0 before it can move on to plane 1.
The easiest way to get your code working would be to reorder your loops
(you may have to fix up the buffer indexing):
/* small loop over planes (or) samples */
for (sample = 0; sample < samperpix; sample++ ) {
for (i = 0; i < numscans; i++) {
/* start byte in data stream = startdata */
startdata = (char *) &data[iofp] + startbyte;
(void) TIFFWriteScanline(out, (tdata_t) startdata, tmprow,
(tsample_t) sample);
iofp += planelen;
} /* end loop over scans */
} /* end loop over samples */
I hope that helps,
Dan McCoy Pixar mccoy@pixar.com