fl16-inputmodules: add gamma correction.

The brightness values sent to the LED controller actually control a PWM
duty cycle. LEDs emit light roughly in proportion to the PWM duty cycle,
but human vision perceives brightness on an exponential curve --
generally it takes 2x the physical luminous flux for the eye to perceive
something as one step brighter.

As a result, brightness ramps (like the one generated by
--all-brightnesses) were rapidly brightening to what appeared to be max,
and then flattening.

This change adds gamma correction, which maps the linear input
brightness values to a non-linear function of PWM duty cycles, causing
the ramp to look far more ramp-y.

The gamma value I've chosen here is arguably a preference, I messed with
output on a PVT panel until I found something that looked approximately
right.

Author: cbiffle

fl16-inputmodules/src/patterns.rs

@@ -313,7 +313,8 @@ 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();
         }
     }
 }
@@ -330,9 +331,11 @@ 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();
@@ -387,3 +390,11 @@ 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"));
+}