implement histogram correction from CCCB_Ledwand

src/brightness.rs

@@ -1,6 +1,6 @@
-use servicepoint::{Brightness, Command, Connection};
-use log::info;
 use crate::cli::BrightnessCommand;
+use log::info;
+use servicepoint::{Brightness, Command, Connection};
 
 pub(crate) fn brightness(connection: &Connection, brightness_command: BrightnessCommand) {
     match brightness_command {
@@ -17,4 +17,4 @@ pub(crate) fn brightness_set(connection: &Connection, brightness: Brightness) {
         .send(Command::Brightness(brightness))
         .expect("Failed to set brightness");
     info!("set brightness to {brightness:?}");
-}
+}

src/cli.rs

@@ -114,7 +114,12 @@ pub enum StreamCommand {
 
 #[derive(clap::Parser, std::fmt::Debug, Clone)]
 pub struct StreamScreenOptions {
-    #[arg(long, short, default_value_t = false, help = "Disable dithering - improves performance")]
+    #[arg(
+        long,
+        short,
+        default_value_t = false,
+        help = "Disable dithering - improves performance"
+    )]
     pub no_dither: bool,
 
     #[arg(

src/ledwand_dither.rs

@@ -0,0 +1,127 @@
+//! Based on https://github.com/WarkerAnhaltRanger/CCCB_Ledwand
+
+use image::GrayImage;
+use servicepoint::{PIXEL_HEIGHT, PIXEL_WIDTH};
+
+pub struct LedwandDither {
+    options: LedwandDitherOptions,
+    tmpbuf: GrayImage,
+}
+
+#[derive(Debug, Default)]
+pub struct LedwandDitherOptions {
+    pub size: Option<(u32, u32)>,
+}
+
+type GrayHistogram = [usize; 256];
+
+struct HistogramCorrection {
+    pre_offset: f32,
+    post_offset: f32,
+    factor: f32,
+}
+
+impl LedwandDither {
+    pub fn new(options: LedwandDitherOptions) -> Self {
+        let (width, height) = options
+            .size
+            .unwrap_or((PIXEL_WIDTH as u32, PIXEL_HEIGHT as u32));
+        Self {
+            tmpbuf: GrayImage::new(width, height),
+            options,
+        }
+    }
+
+    pub fn histogram_correction(image: &mut GrayImage) {
+        let histogram = Self::make_histogram(image);
+        let correction = Self::determine_histogram_correction(image, histogram);
+        Self::apply_histogram_correction(image, correction)
+    }
+
+    fn make_histogram(image: &GrayImage) -> GrayHistogram {
+        let mut histogram = [0; 256];
+        for pixel in image.pixels() {
+            histogram[pixel.0[0] as usize] += 1;
+        }
+        histogram
+    }
+
+    fn determine_histogram_correction(
+        image: &GrayImage,
+        histogram: GrayHistogram,
+    ) -> HistogramCorrection {
+        let adjustment_pixels = image.len() / 100;
+
+        let mut num_pixels = 0;
+        let mut brightness = 0;
+
+        let mincut = loop {
+            num_pixels += histogram[brightness as usize] as usize;
+            brightness += 1;
+            if num_pixels >= adjustment_pixels {
+                break u8::min(brightness, 20);
+            }
+        };
+
+        let minshift = loop {
+            num_pixels += histogram[brightness as usize] as usize;
+            brightness += 1;
+            if num_pixels >= 2 * adjustment_pixels {
+                break u8::min(brightness, 64);
+            }
+        };
+
+        brightness = u8::MAX;
+        num_pixels = 0;
+        let maxshift = loop {
+            num_pixels += histogram[brightness as usize] as usize;
+            brightness -= 1;
+            if num_pixels >= 2 * adjustment_pixels {
+                break u8::max(brightness, 192);
+            }
+        };
+
+        let pre_offset = -(mincut as f32 / 2.);
+        let post_offset = -(minshift as f32);
+        let factor = (255.0 - post_offset) / maxshift as f32;
+        HistogramCorrection {
+            pre_offset,
+            post_offset,
+            factor,
+        }
+    }
+
+    fn apply_histogram_correction(image: &mut GrayImage, correction: HistogramCorrection) {
+        let midpoint = image.width() / 2;
+        for (x, _, pixel) in image.enumerate_pixels_mut() {
+            if x > midpoint {
+                continue;
+            }
+
+            let pixel = &mut pixel.0[0];
+            let value = (*pixel as f32 + correction.pre_offset) * correction.factor
+                + correction.post_offset;
+            *pixel = value.clamp(0f32, u8::MAX as f32) as u8;
+        }
+    }
+
+    pub fn median_brightness(image: &GrayImage) -> u8 {
+        let histogram = Self::make_histogram(image);
+        let midpoint = image.len() / 2;
+
+        debug_assert_eq!(
+            image.len(),
+            histogram.iter().copied().map(usize::from).sum()
+        );
+
+        let mut num_pixels = 0;
+        for brightness in u8::MIN..=u8::MAX {
+            num_pixels += histogram[brightness as usize] as usize;
+            if num_pixels >= midpoint {
+                return brightness;
+            }
+        }
+
+        unreachable!("Somehow less pixels where counted in the histogram than exist in the image")
+    }
+}

src/main.rs

@@ -11,6 +11,7 @@ use servicepoint::{Brightness, Connection};
 
 mod brightness;
 mod cli;
+mod ledwand_dither;
 mod pixels;
 mod stream_stdin;
 mod stream_window;

src/pixels.rs

@@ -1,6 +1,6 @@
-use servicepoint::{BitVec, Command, CompressionCode, Connection, PIXEL_COUNT};
-use log::info;
 use crate::cli::PixelCommand;
+use log::info;
+use servicepoint::{BitVec, Command, CompressionCode, Connection, PIXEL_COUNT};
 
 pub(crate) fn pixels(connection: &Connection, pixel_command: PixelCommand) {
     match pixel_command {
@@ -31,4 +31,4 @@ pub(crate) fn pixels_off(connection: &Connection) {
         .send(Command::Clear)
         .expect("failed to clear pixels");
     info!("reset pixels");
-}
+}

src/stream_stdin.rs

@@ -72,6 +72,7 @@ impl App {
 
     fn single_line(&mut self, line: &str) {
         let mut line_grid = CharGrid::new(TILE_WIDTH, 1);
+        line_grid.fill(' ');
         Self::line_onto_grid(&mut line_grid, 0, line);
         Self::line_onto_grid(&mut self.mirror, self.y, line);
         self.connection

src/stream_window.rs

@@ -1,4 +1,5 @@
 use crate::cli::StreamScreenOptions;
+use crate::ledwand_dither::{LedwandDither, LedwandDitherOptions};
 use image::{
     imageops::{dither, resize, BiLevel, FilterType},
     DynamicImage, ImageBuffer, Luma, Rgb, Rgba,
@@ -26,9 +27,18 @@ pub fn stream_window(connection: &Connection, options: StreamScreenOptions) {
     let mut bitmap = Bitmap::new(PIXEL_WIDTH, PIXEL_HEIGHT);
     info!("now starting to stream images");
     loop {
-        let frame = get_next_frame(&capturer, options.no_dither);
+        let mut frame = get_next_frame(&capturer);
+
+        LedwandDither::histogram_correction(&mut frame);
+        let cutoff = if options.no_dither {
+            LedwandDither::median_brightness(&frame)
+        } else {
+            dither(&mut frame, &BiLevel);
+            u8::MAX / 2
+        };
+
         for (mut dest, src) in bitmap.iter_mut().zip(frame.pixels()) {
-            *dest = src.0[0] > u8::MAX / 2;
+            *dest = src.0[0] > cutoff;
         }
 
         connection
@@ -41,21 +51,18 @@ pub fn stream_window(connection: &Connection, options: StreamScreenOptions) {
     }
 }
 
-fn get_next_frame(capturer: &Capturer, no_dither: bool) -> ImageBuffer<Luma<u8>, Vec<u8>> {
+/// returns next frame from the capturer, resized and grayscale
+fn get_next_frame(capturer: &Capturer) -> ImageBuffer<Luma<u8>, Vec<u8>> {
     let frame = capturer.get_next_frame().expect("failed to capture frame");
     let frame = frame_to_image(frame);
     let frame = frame.grayscale().to_luma8();
-    let mut frame = resize(
+
+    resize(
         &frame,
         PIXEL_WIDTH as u32,
         PIXEL_HEIGHT as u32,
         FilterType::Nearest,
-    );
-
-    if !no_dither {
-        dither(&mut frame, &BiLevel);
-    }
-    frame
+    )
 }
 
 fn start_capture(options: &StreamScreenOptions) -> Option<Capturer> {