[BUG] GIF output has poor palette selection

[simon-frankau]

Describe the bug

When I tried to produce a plot with a single highlighted point using the gif backend (as part of an animated gif), the highlighted point did not come out in the colour I expected - I asked for bright red, and got a very dark red.

My theory is that the gif is generated by the gif package, which uses the NeuQuant algorithm, which is primarily intended to quantize images with many colours. It uses a "samping factor" to choose what fraction of pixels to sample, which I suspect makes it possible to ignore or under-weigh bright little highlights when constructing the palette.

Plotters uses a "speed" of 10 (https://docs.rs/plotters-bitmap/0.3.0/src/plotters_bitmap/bitmap.rs.html#86), as recommended. I suspect this is just the sampling factor, although I've not traced it through to the gif crate source. This makes it possible to miss sampling areas of ~10 pixels.

I imagine this might seem like a bit of a corner case, but it's surprisingly confusing when you create a plot with a single highlighted point and it just isn't highlighted!

I see a few possible responses:

• Call it an issue with the upstream gif crate, and wash your hands of it. (Fair enough! :)
• Make it possible to set "speed" to 1 (I've only tested on my example below, not in general):
  • Set it to 1 in the source, and potentially surprise people with a slowdown when they create a gif with the new version
  • Expose "speed" to the user, potentially making the interface surprisingly complex
  • Count the colours used in the image, and if they're < 256 set speed to 1, hoping it gives a perfect mapping.
• Work around the shortcoming by identifying images with < 256 colours and converting to a paletted image before passing to the library.

You may see others. To be honest, I'd probably go with calling it an issue with the gif crate, but I want to give you first refusal. :)

If any of the other solutions appeal, I'd be happy to work on the PR.

To Reproduce

Minimised code:

use plotters::prelude::*;                                                                                                         

const OUT_FILE_NAME: &str = "out.gif";                                                                                            
const FRAME_DELAY: u32 = 10;                                                                                                      

fn main() -> Result<(), Box<dyn std::error::Error>> {                                                                             
    let backend = BitMapBackend::gif(OUT_FILE_NAME, (1024, 1024), FRAME_DELAY)?;                                                  
    let drawing_area = backend.into_drawing_area();                                                                               
    drawing_area.fill(&WHITE)?;                                                                                                   
    let mut scatter_ctx = ChartBuilder::on(&drawing_area)                                                                         
        .x_label_area_size(40)                                                                                                    
        .y_label_area_size(40)                                                                                                    
        .build_cartesian_2d(-1f64..1f64, -1f64..1f64)?;                                                                           
    scatter_ctx                                                                                                                   
        .configure_mesh()                                                                                                         
        .disable_x_mesh()                                                                                                         
        .disable_y_mesh()                                                                                                         
        .draw()?;                                                                                                                 
    scatter_ctx.draw_series([Circle::new(     
![out](https://user-images.githubusercontent.com/6563758/126066032-a82efac8-97c1-47f4-b2a6-e2b6caf428ce.gif)

        (0.25, 0.1),                                                                                                              
        5,                                                                                                                        
        RED.filled(),                                                                                                             
    )])?;                                                                                                                         
    drawing_area.present()?;                                                                                                      
    Ok(())                                                                                                                        
}

Output gif attached:

Version Information Not quite sure how best to answer this, but my Cargo.lock has:

[[package]]
name = "plotters"
version = "0.3.1"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "32a3fd9ec30b9749ce28cd91f255d569591cdf937fe280c312143e3c4bad6f2a"

Thanks, Simon.

[simon-frankau]

I've also raised https://github.com/image-rs/image-gif/issues/109, in case they feel it should be fixed at that layer.

[simon-frankau]

This has been fixed upstream in image-gif, and I've just checked that cargo update picks it up. It does, so it's all done.