How to measure char "stride" for monospaced font at current zoom level?

[barries]

Hi!

How can I determine a monospaced font's effective char-to-char "stride" (char width + char spacing) at the current zoom level, so that I can do the usual layout (indentation, for example) and pointer-pos-to-char-cell mapping?

I've read trough the online docs for various Font* structs and Painter, but nothing I've found seems to offer the char_width or char_spacing getters. I suspect I just haven't found my way to the lower level APIs that I need, and the one's I vaguely recall seeing don't seem to take the zoom level into account--they're at the raw font level.

For reference, I hacked up this worky but inefficient and inelegant code and found that the char_stride works well for my needs, but isn't a simple metric that scales with zoom: repeatedly zooming in from 1.0 with Ctrl-+, results in strides like 9.0, 8.18, 8.33, ..., 8.0, 8.125, .... So, it seems like zoom level is factored and the char size (and spacing) are scaled to the requested point size +0.0/-1.0, probably to optimize for pixel alignment and visual clarity.

Thanks!

let text_style = egui::TextStyle::Monospace;

let text_format = TextFormat {                                           
    color:   ui.visuals().override_text_color.unwrap_or(Color32::BLACK), 
    font_id: text_style.resolve(ui.style()),                         
    ..TextFormat::default()                                              
};                                                                       

if self.prev_zoom_factor != ui.ctx().zoom_factor() { // Lay out some text to determine the char_stride; I can't find a way to determine the inter-character spacing for a monospace font in egui 
    self.prev_zoom_factor = ui.ctx().zoom_factor();                                                                                                                                       
                                                                                                                                                                                          
    ui.ctx().fonts(|fonts| {                                                                                                                                                              
        let mut job = LayoutJob::default();                                                                                                                                               
        job.append("01234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890", 0.0, text_format.clone());                                    
        let galley = fonts.layout_job(job);                                                                                                                                               
        let glyphs = &galley.rows.first().unwrap().glyphs;                                                                                                                                
        self.char_stride = (glyphs.last().unwrap().pos.x - glyphs.first().unwrap().pos.x) / (glyphs.len() - 1) as f32;
        println!("{}", self.char_stride);
    });                                                                                                                                                                                   
}                                                                                                                                                                                         

[mattfbacon]

https://docs.rs/egui/latest/egui/text/struct.Fonts.html#method.glyph_width and https://docs.rs/egui/latest/egui/text/struct.Fonts.html#method.row_height may be close to what you want.

[mattfbacon]

https://docs.rs/egui/latest/egui/struct.Galley.html#method.cursor_from_pos may also be useful if this is a bit of an XY problem since you mentioned you want to map from pointer pos to char cell.

[barries]

Ok, thanks to your answer, I dug in a bit more.

What happens in the galley layout code is that each character is aligned to a pixel so that it renders clearly, so the actual achieved spacing (what I call the stride) is, on average, a bit off, and ends up being an integer.

So, the floating point glyph.advance_width (what the font metrics you linked to yields, many thanks!!!) is the ideal advance width; the slight rounding off of each glyph's pos.x causes the actual advance width to be an integer. Here, at the moment, glyph.advance_width is 8.639999, and the "stride" measured by the above code is 9.000000. Using the stride works perfectly for char cell <-> cursor X mapping So, I'll keep the code above because I don't want to couple tightly to egui's font layout code.

I still don't understand how the zoom factor interacts with this, but I don't need to. The stride works perfectly at all zoom levels (so far--who knows if it'll trip over a corner case one day...).

Thanks again for cluebatting me, it made me dig in deeper.

• Barrie

P.S. Here's the code in egui that does the rounding (text_layout.rs line 185 in my current egui version):

paragraph.cursor_x = font.round_to_pixel(paragraph.cursor_x);