ledmatrix: gamma correction for smoother brightness ramps

fl16-inputmodules/build.rs

@@ -0,0 +1,29 @@
+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::path::Path;
+
+fn main() {
+    let out_dir = env::var("OUT_DIR").unwrap();
+    let gamma_path = Path::new(&out_dir).join("gamma.rs");
+    let mut f = File::create(gamma_path).unwrap();
+
+    // Determined empirically for a PVT panel. May need to become conditional
+    // on features, TBD.
+    const GAMMA: f32 = 3.2;
+
+    let corrected: [f32; 256] =
+        std::array::from_fn(|i| f32::powf((i as f32) / 255., GAMMA) * 255. + 0.5);
+
+    writeln!(f, "const GAMMA: [u8; 256] = [").unwrap();
+
+    const LINE_LEN: usize = 8;
+    for line in corrected.chunks(LINE_LEN) {
+        write!(f, "   ").unwrap();
+        for element in line {
+            write!(f, " {:>3},", *element as u8).unwrap();
+        }
+        writeln!(f).unwrap();
+    }
+    writeln!(f, "];").unwrap();
+}

fl16-inputmodules/src/fl16.rs

@@ -678,6 +678,8 @@ where
         Ok(())
     }
 
+    /// Fills the matrix with a _raw_ brightness value, i.e. without gamma
+    /// correction, to show the native PWM values.
     pub fn fill_brightness(&mut self, brightness: u8) -> Result<(), Error<I2cError>> {
         for x in 0..self.device.width {
             for y in 0..self.device.height {

fl16-inputmodules/src/patterns.rs

@@ -310,7 +310,8 @@ pub fn double_gradient() -> Grid {
 pub fn _fill_grid(grid: &Grid, matrix: &mut Foo) {
     for y in 0..HEIGHT {
         for x in 0..WIDTH {
-            matrix.device.pixel(x as u8, y as u8, grid.0[x][y]).unwrap();
+            let p = gamma::correct(grid.0[x][y]);
+            matrix.device.pixel(x as u8, y as u8, p).unwrap();
         }
     }
 }
@@ -327,9 +328,11 @@ pub fn fill_grid_pixels(state: &LedmatrixState, matrix: &mut Foo) {
     for y in 0..HEIGHT {
         for x in 0..WIDTH {
             let (register, page) = (matrix.device.calc_pixel)(x as u8, y as u8);
-            brightnesses[(page as usize) * 0xB4 + (register as usize)] =
+            let uncorrected =
                 ((state.grid.0[x][y] as u64) * (state.brightness as u64)
                     / (BRIGHTNESS_LEVELS as u64)) as u8;
+            brightnesses[(page as usize) * 0xB4 + (register as usize)] =
+                gamma::correct(uncorrected);
         }
     }
     matrix.device.fill_matrix(&brightnesses).unwrap();
@@ -384,3 +387,11 @@ pub fn every_nth_col(n: usize) -> Grid {
 
     grid
 }
+
+pub mod gamma {
+    pub const fn correct(value: u8) -> u8 {
+        GAMMA[value as usize]
+    }
+
+    include!(concat!(env!("OUT_DIR"), "/gamma.rs"));
+}