What causes "%s: can't set sample factors " in DrKLO/Telegram?

Use the exact format HxV per component separated by commas, each factor 1..4, e.g. -sample 2x2 or -sample 2x2,1x1,1x1.

%s: can't set sample factors

Error message

%s: can't set sample factors

What it means

cjpeg calls set_sample_factors() to parse -sample (format HxV[,HxV,...] per component). It returns FALSE if sscanf fails to read H x V, if the separator is not 'x'/'X' between H and V or not ',' between pairs, or if H or V is outside 1..4. The wrapper prints this message and calls usage().

Source

Thrown at TMessagesProj/jni/mozjpeg/cjpeg.c:636

        fprintf(stderr, "%s: can't set quality ratings\n", progname);
        usage();
      }

    if (qtablefile != NULL)     /* process -qtables if it was present */
      if (! read_quant_tables(cinfo, qtablefile, force_baseline)) {
        fprintf(stderr, "%s: can't read qtable file\n", progname);
        usage();
      }

    if (qslotsarg != NULL)      /* process -qslots if it was present */
      if (!set_quant_slots(cinfo, qslotsarg))
        usage();

    /* set_quality_ratings sets default subsampling, so the explicit
       subsampling must be set after it */
    if (samplearg != NULL)      /* process -sample if it was present */
      if (! set_sample_factors(cinfo, samplearg)) {
        fprintf(stderr, "%s: can't set sample factors\n", progname);
        usage();
      }

#ifdef C_PROGRESSIVE_SUPPORTED
    if (simple_progressive)     /* process -progressive; -scans can override */
      jpeg_simple_progression(cinfo);
#endif

#ifdef C_MULTISCAN_FILES_SUPPORTED
    if (scansarg != NULL)       /* process -scans if it was present */
      if (!read_scan_script(cinfo, scansarg))
        usage();
#endif
  }

  return argn;                  /* return index of next arg (file name) */
}

View on GitHub (pinned to 45ab8f4308)

Solutions

Use the exact format HxV per component separated by commas, each factor 1..4, e.g. -sample 2x2 or -sample 2x2,1x1,1x1.
Validate the input matches ^[1-4]x[1-4](,[1-4]x[1-4])*$ before passing.
Remember fewer entries than components are backfilled with 1x1.

Example fix

# before
cjpeg -sample 2-2 input.ppm > out.jpg
# after
cjpeg -sample 2x2 input.ppm > out.jpg

Defensive patterns

Strategy: validation

Validate before calling

#!/bin/sh
# Validate -sample: HxV[,HxV,...] each in 1..4
SAMPLE='2x2'
printf '%s' "$SAMPLE" | grep -Eq '^[1-4]x[1-4](,[1-4]x[1-4])*$' || { echo 'bad -sample' >&2; exit 1; }
cjpeg -sample "$SAMPLE" in.ppm > out.jpg

Type guard

// n/a: string-format validation in the caller.

Try / catch

if ! cjpeg -sample "$SAMPLE" in.ppm > out.jpg 2>/tmp/e; then
  grep -q "can't set sample factors\|sampling factors must be" /tmp/e && { echo 'fix -sample (HxV, 1..4)'; exit 1; }
fi

Prevention

Restrict -sample input with the documented regex before invoking cjpeg.
Document the 1..4 factor limit prominently for users configuring subsampling.
Test the wrapper with edge values (1x1, 4x4, 2x2,1x1,1x1).

When it happens

Trigger: Passing -sample with malformed syntax: -sample 2 (missing xV), -sample 2-2 (wrong separator), -sample 4x2x2 (extra), or a factor of 5 or 0, e.g. -sample 5x1.

Common situations: Confusing the HxV notation with NxN or WxH conventions used elsewhere; typos; values from an encoder GUI that uses a different representation; exceeding the libjpeg 1..4 sampling-factor limit.

Related errors

AI-assisted analysis of DrKLO/Telegram@45ab8f4308 (2026-08-14). Data as JSON: /api/errors/56a6b0888c9fffcd. Report an issue: GitHub.