1994.03.31 10:31 "A little PackBits bug", by Roland Nahser

Dear TIFF-Developers,

I am an user of your fantastic libtiff Version 3.30beta. With 3.3 it is much easier to port to different environments. But there seems to be a little bug in PackBitsEncode in the following if statement:

                case LITERAL_RUN:       /* literal followed by a run */
                        /*
                         * Check to see if previous run should
                         * be converted to a literal, in which
                         * case we convert literal-run-literal
                         * to a single literal.
                         */
                        if (n == 1 && -op[-2] == 1 && *lastliteral < 126) {
                                state = (((*lastliteral) += 2) == 127 ?
                                    BASE : LITERAL);
                                op[-2] = op[-1];        /* replicate */

variable op is declared as tidata_t (unsigned char *). Using it as -op[-2] does not always result in 1 as expected but in -255. (binary representation of -1 as u_char = 255 (on nearly every machine I know from). This bug arose on DEC Ultrix and SunOS with gcc. 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. Using explicit signed char is not possible on all machines (I think Ultrix is one of them).

---

Second I am using a "private" TIFF-format for temporary storing images generated by our GKS-software in the following format Separate planes

SamplePerPixel = 3 or 4
BitsperSample = 1
Compression = PACKBITS or LZW

when I try to use RowsperStrip > 1 (for better performance and compression with LZW) I get the errormessage:

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.

Are there any plns to support real direct access in libtiff? I am looking for a format for intermediate storage of tiled images within rasterization of very large high resolution images for phototype setters.

--
Roland Nahser