add constant test

jxl/src/image.rs

@@ -184,6 +184,13 @@ impl<T: ImageDataType> Image<T> {
         Ok(img)
     }
 
+    #[cfg(test)]
+    pub fn new_constant(size: (usize, usize), val: T) -> Result<Image<T>> {
+        let mut img = Self::new(size)?;
+        img.data.iter_mut().for_each(|x| *x = val);
+        Ok(img)
+    }
+
     pub fn size(&self) -> (usize, usize) {
         self.size
     }

jxl/src/render/stages/upsample.rs

@@ -98,6 +98,24 @@ mod test {
         )
     }
 
+    #[test]
+    fn upsample2x_constant() -> Result<()> {
+        let ups_factors = CustomTransformData::default();
+        let image_size = (238, 412);
+        let input_size = (image_size.0 / 2, image_size.1 / 2);
+        let val = 0.777f32;
+        let input = Image::new_constant(input_size, val)?;
+        let stage = Upsample2x::new(&ups_factors, 0);
+        let output: Vec<Image<f32>> =
+            make_and_run_simple_pipeline(stage, &[input], image_size, 123)?.1;
+        for x in 0..image_size.0 {
+            for y in 0..image_size.1 {
+                assert!((output[0].as_rect().row(y)[x] - val).abs() <= 0.0000001);
+            }
+        }
+        Ok(())
+    }
+
     #[test]
     fn test_upsample2() -> Result<()> {
         let mut input = Image::new((7, 7))?;