percentage slider setup

Is there an existing issue for this?

• I checked and did not find my issue in the already reported ones

Describe the bug

Following up on a discussion with @dterrahe regarding sliders.

AFAIK, there is no bug on the current UI; however, there is a problem in the framework, and potential incorrect usage.
In bauhaus.c, the following function has a heuristic to set the number of digits. If a naïve dev sets up a slider as:

// suppose the model is a real number, hard_max is 10 or below;
// dev wants to set up single-digit percentages, e.g. 12.3%
dt_bauhaus_slider_set_digits(slider, 1);
dt_bauhaus_slider_set_format(slider, "%);

then the actual code sets digits = -1:

void dt_bauhaus_slider_set_format(GtkWidget *widget,
                                  const char *format)
{
  dt_bauhaus_widget_t *w = DT_BAUHAUS_WIDGET(widget);
  if(w->type != DT_BAUHAUS_SLIDER) return;
  dt_bauhaus_slider_data_t *d = &w->slider;

  d->format = g_intern_string(format);

  if(strstr(format,"%") && fabsf(d->hard_max) <= 10)
  {
    if(d->factor == 1.0f) d->factor = 100;
    d->digits -= 2;
  }
}

Questions:

• do we want to prevent this, or it will be caught during testing, anyway?
• should we prevent negative digits in dt_bauhaus_slider_set_format
• is setting the digits recommended before or after? Before: sets the real number digits (e.g. 4 means 0.1234, 12.34), but is subject to the 'max 10' heuristics; after: explicit.
  useless.c uses before:

  dt_bauhaus_slider_set_digits(g->factor, 2);
  // Additional parameters determine how the value will be shown.
  dt_bauhaus_slider_set_format(g->factor, "%");
  

I've extracted the digit/format pairs, where the digits are set first (hope I got them all).

sliders.txt

Steps to reproduce

No reproducible bug

Expected behavior

No response

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

darktable.org / GitHub release

darktable version

all versions

What OS are you using?

Linux

What is the version of your OS?

all platforms

Describe your system

No response

Are you using OpenCL GPU in darktable?

None

If yes, what is the GPU card and driver?

No response

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

Start in bauhaus.c at dt_bauhaus_slider_set_format and inspect how digits, factor, hard_max, and percentage formats interact. Compare the digit/format pairs in sliders.txt with the setup in useless.c; this issue is done when the desired ordering and handling of negative digits are agreed and the affected slider configurations behave consistently.