Added test of ToU8ForWriting. Found a bug.

jxl/src/image.rs

@@ -189,7 +189,7 @@ pub mod debug_tools {
 
     impl ToU8ForWriting for u16 {
         fn to_u8_for_writing(self) -> u8 {
-            (self as u32 * 255 / 65535) as u8
+            (self as u32 >> 8) as u8
         }
     }
 
@@ -201,7 +201,7 @@ pub mod debug_tools {
 
     impl ToU8ForWriting for u32 {
         fn to_u8_for_writing(self) -> u8 {
-            (self as f32 / (2.0f32.powi(32) - 1.0)).to_u8_for_writing()
+            (self >> 24) as u8
         }
     }
 
@@ -255,4 +255,38 @@ mod test {
         assert!(image.as_rect().to_pgm().starts_with(b"P5\n32 32\n255\n"));
         Ok(())
     }
+
+    #[cfg(feature = "debug_tools")]
+    #[test]
+    fn to_u8() {
+        use super::debug_tools::ToU8ForWriting;
+        let mut want_u8 = 0u8;
+        let mut source_u16 = 0x00ffu16;
+        for _ in 0..256 {
+            assert!(source_u16.to_u8_for_writing() == want_u8);
+            want_u8 = want_u8.wrapping_add(1);
+            source_u16 = source_u16.wrapping_add(0x0100);
+        }
+        want_u8 = 0;
+        let mut source_f32 = 1f32 / 512f32;
+        for _ in 0..256 {
+            assert!(source_f32.to_u8_for_writing() == want_u8);
+            want_u8 = want_u8.wrapping_add(1);
+            source_f32 = source_f32 + (1f32 / 256f32);
+        }
+        want_u8 = 0;
+        let mut source_u32 = 0x00ffffffu32;
+        for _ in 0..256 {
+            assert!(source_u32.to_u8_for_writing() == want_u8);
+            want_u8 = want_u8.wrapping_add(1);
+            source_u32 = source_u32.wrapping_add(0x01000000);
+        }
+        want_u8 = 0;
+        let mut source_f16 = half::f16::from_f32(1f32 / 512f32);
+        for _ in 0..256 {
+            assert!(source_f16.to_u8_for_writing() == want_u8);
+            want_u8 = want_u8.wrapping_add(1);
+            source_f16 = source_f16 + half::f16::from_f32(1f32 / 256f32);
+        }
+    }
 }