Added test of ToU8ForWriting. Found a bug.

AUTHORS

@@ -21,3 +21,4 @@ Ewout ter Hoeven <[email protected]>
 Sami Boukortt
 Tomáš Král <[email protected]>
 Wonwoo Choi <[email protected]>
+Martin Bruse <[email protected]>

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 * 0xff + 0x8000) / 0xffff) 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 as u64 * 0xff + 0x80000000) / 0xffffffff) as u8
         }
     }
 
@@ -247,12 +247,63 @@ mod test {
         assert_eq!(image.as_rect_mut().row(30)[30], 1);
         Ok(())
     }
+}
+
+#[cfg(test)]
+#[cfg(feature = "debug_tools")]
+mod debug_tools_test {
+    use super::debug_tools::ToU8ForWriting;
+    use super::Image;
+    use crate::error::Result;
 
-    #[cfg(feature = "debug_tools")]
     #[test]
     fn to_pgm() -> Result<()> {
         let image = Image::<u8>::new(32, 32)?;
         assert!(image.as_rect().to_pgm().starts_with(b"P5\n32 32\n255\n"));
         Ok(())
     }
+
+    #[test]
+    fn u16_to_u8() {
+        let mut left_source_u16 = 0xffffu16 / 510;
+        for want_u8 in 0x00u8..0xffu8 {
+            assert!(left_source_u16.to_u8_for_writing() == want_u8);
+            assert!((left_source_u16 + 1).to_u8_for_writing() == want_u8 + 1);
+            // Since we have 256 u8 values, but 0x00 and 0xff only have half the
+            // range, we actually get whole ranges of size 0xffff / 255.
+            left_source_u16 = left_source_u16.wrapping_add(0xffff / 255);
+        }
+    }
+
+    #[test]
+    fn f32_to_u8() {
+        let epsilon = 1e-4f32;
+        for want_u8 in 0x00u8..0xffu8 {
+            let threshold = 1f32 / 510f32 + (1f32 / 255f32) * (want_u8 as f32);
+            assert!((threshold - epsilon).to_u8_for_writing() == want_u8);
+            assert!((threshold + epsilon).to_u8_for_writing() == want_u8 + 1);
+        }
+    }
+
+    #[test]
+    fn u32_to_u8() {
+        let mut left_source_u32 = 0xffffffffu32 / 510;
+        for want_u8 in 0x00u8..0xffu8 {
+            assert!(left_source_u32.to_u8_for_writing() == want_u8);
+            assert!((left_source_u32 + 1).to_u8_for_writing() == want_u8 + 1);
+            // Since we have 256 u8 values, but 0x00 and 0xff only have half the
+            // range, we actually get whole ranges of size 0xffffffff / 255.
+            left_source_u32 = left_source_u32.wrapping_add(0xffffffffu32 / 255);
+        }
+    }
+
+    #[test]
+    fn f16_to_u8() {
+        let epsilon = half::f16::from_f32(1e-3f32);
+        for want_u8 in 0x00u8..0xffu8 {
+            let threshold = half::f16::from_f32(1f32 / 510f32 + (1f32 / 255f32) * (want_u8 as f32));
+            assert!((threshold - epsilon).to_u8_for_writing() == want_u8);
+            assert!((threshold + epsilon).to_u8_for_writing() == want_u8 + 1);
+        }
+    }
 }