Slider unable to produce range of odd values

[ExpertOfNil]

Description

When defining the egui::Slider bounds with a range having odd bounds (e.g. 3..=31) and step size of 2_f64, instead of having selectable values {3, 5, 7, ..., 29, 31}, values {4, 6, 8, ..., 30, 31} are provided. This is not remedied with .clamp_to_range(true). Similarly, providing a range 3..=501 and a step size of 2_f64 produces values {4, 10, 20, 30, ..., 490, 500, 501}. NOTE this scenario is also not remedied by using ranges starting at 1.0, either.

To Reproduce

egui::Grid::new("CameraConfigGroup")
    .num_columns(2)
    .spacing([40.0, 4.0])
    .striped(true)
    .show(ui, |ui| {
        ui.add(egui::Label::new("Value 1:"));
        ui.add(
            egui::Slider::new(&mut self.slider_value1, 3.0..=31.0)
                .step_by(2_f64)
                .fixed_decimals(1),
        );
        ui.end_row();
        ui.add(egui::Label::new("Value 2:"));
        ui.add(
            egui::Slider::new(&mut self.canny_edges.threshold2, 3.0..=501.0)
                .step_by(2_f64)
                .fixed_decimals(1),
        );
    });

Expected Behavior

• Value 1: Selectable values {3.0, 5.0, 7.0, ..., 29.0, 31.0}
• Value 2: Selectable values {3.0, 5.0, 7.0, ..., 499.0, 501.0}

Actual Behavior

• Value 1: Slider may start at 3.0, but once moved, it will only provide values {4.0, 6.0, 8.0, ..., 30.0, 31.0}, even when manually editing the text value
• Value 2: Slider may start at 3.0, but once moved, it will only provide values {4.0, 10.0, 20.0, ..., 490.0, 500.0, 501.0}. Manually editing the text will only allow even numbers, with the exception of 501.0.

Screenshots

Hypothesis

I believe this may be the cause of the issue (slider.rs, lines 501-514):

fn set_value(&mut self, mut value: f64) {
    if self.clamp_to_range {
        let start = *self.range.start();
        let end = *self.range.end();
        value = value.clamp(start.min(end), start.max(end));
    }
    if let Some(max_decimals) = self.max_decimals {
        value = emath::round_to_decimals(value, max_decimals);
    }
    if let Some(step) = self.step {
        value = (value / step).round() * step;
        // should be: value = (value + step).round()?
    }
    set(&mut self.get_set_value, value);
}

Desktop

• OS: Ubuntu 20.04
• Eframe: v0.23.0
  • Also tried using eframe = { git = "https://github.com/emilk/egui", branch = "master", features = ["persistence"] }

[YgorSouza]

The solution is probably to take the start of the range into account when rounding.

if let Some(step) = self.step {
    let start = self.range.start();
    value = start + ((value - start) / step).round() * step;
}

[ExpertOfNil]

If I'm understanding what's going on in Slider::set_value correctly, it can be mocked up using Slider::from_get_set:

let range = 3.0..=31.0;
let start = *range.start();
ui.add(egui::Slider::from_get_set(
    range,
    |v: Option<f64>| -> f64 {
        if let Some(v) = v {
            self.slider_value = start + ((v - start) / 2_f64).round() * 2_f64;
        }
        return self.slider_value;
    },
));

Using @YgorSouza suggestion, I get the desired behavior.

[emilk]

Yeah, I agree that the step should start at the start value, and @YgorSouza's solution seems sounds - please make a PR 🙏