Add sixel backend

src/image_backends/mod.rs

@@ -2,6 +2,8 @@ use image::DynamicImage;
 
 #[cfg(target_os = "linux")]
 mod kitty;
+#[cfg(target_os = "linux")]
+mod sixel;
 
 pub trait ImageBackend {
     fn add_image(&self, lines: Vec<String>, image: &DynamicImage) -> String;
@@ -11,6 +13,8 @@ pub trait ImageBackend {
 pub fn get_best_backend() -> Option<Box<dyn ImageBackend>> {
     if kitty::KittyBackend::supported() {
         Some(Box::new(kitty::KittyBackend::new()))
+    } else if sixel::SixelBackend::supported() {
+        Some(Box::new(sixel::SixelBackend::new()))
     } else {
         None
     }

src/image_backends/sixel.rs

@@ -0,0 +1,161 @@
+use image::{
+    imageops::{colorops, FilterType},
+    math::nq::NeuQuant,
+    DynamicImage, GenericImageView, ImageBuffer, Pixel, Rgb,
+};
+use libc::{
+    c_void, ioctl, poll, pollfd, read, tcgetattr, tcsetattr, termios, winsize, ECHO, ICANON,
+    POLLIN, STDIN_FILENO, STDOUT_FILENO, TCSANOW, TIOCGWINSZ,
+};
+use std::time::Instant;
+
+pub struct SixelBackend {}
+
+impl SixelBackend {
+    pub fn new() -> Self {
+        Self {}
+    }
+
+    pub fn supported() -> bool {
+        // save terminal attributes and disable canonical input processing mode
+        let old_attributes = unsafe {
+            let mut old_attributes: termios = std::mem::zeroed();
+            tcgetattr(STDIN_FILENO, &mut old_attributes);
+
+            let mut new_attributes = old_attributes;
+            new_attributes.c_lflag &= !ICANON;
+            new_attributes.c_lflag &= !ECHO;
+            tcsetattr(STDIN_FILENO, TCSANOW, &new_attributes);
+            old_attributes
+        };
+
+        // ask for the primary device attribute string
+        println!("\x1B[c");
+
+        let start_time = Instant::now();
+        let mut stdin_pollfd = pollfd {
+            fd: STDIN_FILENO,
+            events: POLLIN,
+            revents: 0,
+        };
+        let mut buf = Vec::<u8>::new();
+        loop {
+            // check for timeout while polling to avoid blocking the main thread
+            while unsafe { poll(&mut stdin_pollfd, 1, 0) < 1 } {
+                if start_time.elapsed().as_millis() > 50 {
+                    unsafe {
+                        tcsetattr(STDIN_FILENO, TCSANOW, &old_attributes);
+                    }
+                    return false;
+                }
+            }
+            let mut byte = 0;
+            unsafe {
+                read(STDIN_FILENO, &mut byte as *mut _ as *mut c_void, 1);
+            }
+            buf.push(byte);
+            if buf.starts_with(&[0x1B, b'[', b'?']) && buf.ends_with(&[b'c']) {
+                unsafe {
+                    tcsetattr(STDIN_FILENO, TCSANOW, &old_attributes);
+                }
+                return true;
+            }
+        }
+    }
+}
+
+impl super::ImageBackend for SixelBackend {
+    fn add_image(&self, lines: Vec<String>, image: &DynamicImage) -> String {
+        let tty_size = unsafe {
+            let tty_size: winsize = std::mem::zeroed();
+            ioctl(STDOUT_FILENO, TIOCGWINSZ, &tty_size);
+            tty_size
+        };
+        let width_ratio = tty_size.ws_col as f64 / tty_size.ws_xpixel as f64;
+        let height_ratio = tty_size.ws_row as f64 / tty_size.ws_ypixel as f64;
+
+        // resize image to fit the text height with the Lanczos3 algorithm
+        let image = image.resize(
+            u32::max_value(),
+            (lines.len() as f64 / height_ratio) as u32,
+            FilterType::Lanczos3,
+        );
+        let image_columns = width_ratio * image.width() as f64;
+        let image_rows = height_ratio * image.height() as f64;
+
+        let rgba_image = image.to_rgba(); // convert the image to rgba samples
+        let flat_samples = rgba_image.as_flat_samples();
+        let mut rgba_image = rgba_image.clone();
+        // reduce the amount of colors using dithering
+        colorops::dither(
+            &mut rgba_image,
+            &NeuQuant::new(10, 16, flat_samples.image_slice().unwrap()),
+        );
+        let rgb_image = ImageBuffer::from_fn(rgba_image.width(), rgba_image.height(), |x, y| {
+            let rgba_pixel = rgba_image.get_pixel(x, y);
+            let mut rgb_pixel = rgba_pixel.to_rgb();
+            for subpixel in &mut rgb_pixel.0 {
+                *subpixel = (*subpixel as f32 / 255.0 * rgba_pixel[3] as f32) as u8;
+            }
+            rgb_pixel
+        });
+
+        let mut image_data = Vec::<u8>::new();
+        image_data.extend("\x1BPq".as_bytes()); // start sixel data
+        image_data.extend(format!("\"1;1;{};{}", image.width(), image.height()).as_bytes());
+
+        let mut colors = std::collections::HashMap::<Rgb<u8>, u8>::new();
+        // subtract 1 -> divide -> add 1 to round up the integer division
+        for i in 0..((rgb_image.height() - 1) / 6 + 1) {
+            let sixel_row = rgb_image.view(
+                0,
+                i * 6,
+                rgb_image.width(),
+                std::cmp::min(6, rgb_image.height() - i * 6),
+            );
+            for (_, _, pixel) in sixel_row.pixels() {
+                if !colors.contains_key(&pixel) {
+                    // sixel uses percentages for rgb values
+                    let color_multiplier = 100.0 / 255.0;
+                    image_data.extend(
+                        format!(
+                            "#{};2;{};{};{};",
+                            colors.len(),
+                            (pixel[0] as f32 * color_multiplier) as u32,
+                            (pixel[1] as f32 * color_multiplier) as u32,
+                            (pixel[2] as f32 * color_multiplier) as u32
+                        )
+                        .as_bytes(),
+                    );
+                    colors.insert(pixel, colors.len() as u8);
+                }
+            }
+            for (color, color_index) in &colors {
+                let mut sixel_samples = Vec::<u8>::with_capacity(sixel_row.width() as usize);
+                sixel_samples.resize(sixel_row.width() as usize, 0);
+                for (x, y, pixel) in sixel_row.pixels() {
+                    if color == &pixel {
+                        sixel_samples[x as usize] = sixel_samples[x as usize] | (1 << y);
+                    }
+                }
+                image_data.extend(format!("#{}", color_index).bytes());
+                image_data.extend(sixel_samples.iter().map(|x| x + 0x3F));
+                image_data.push('$' as u8);
+            }
+            image_data.push('-' as u8);
+        }
+        image_data.extend("\x1B\\".as_bytes());
+
+        image_data.extend(format!("\x1B[{}A", image_rows as u32 + 2).as_bytes()); // move cursor to top-left corner
+        image_data.extend(format!("\x1B[{}C", image_columns as u32 + 1).as_bytes()); // move cursor to top-right corner of image
+        let mut i = 0;
+        for line in &lines {
+            image_data.extend(format!("\x1B[s{}\x1B[u\x1B[1B", line).as_bytes());
+            i += 1;
+        }
+        image_data
+            .extend(format!("\n\x1B[{}B", lines.len().max(image_rows as usize) - i).as_bytes()); // move cursor to end of image
+
+        String::from_utf8(image_data).unwrap()
+    }
+}