Pictures: Set Transparent Color refuses a spot that is already transparent (#1217)

crates/engine/src/cmd/recolor.rs

@@ -179,6 +179,11 @@ fn picture_pixel_at(s: &Session, p: &Value, at: &Value) -> Result<(ShapeId, Rgba
         let px = ((u * f64::from(w)) as u32).min(w.saturating_sub(1));
         let py = ((v * f64::from(h)) as u32).min(h.saturating_sub(1));
         let Some(c) = img.get_pixel_checked(px, py) else { continue };
+        // A transparent pixel keeps a colour nobody sees (often black): clearing it would clear
+        // that colour where it is drawn.
+        if c[3] == 0 {
+            return Err(bad(CMD, "that spot of the picture is already transparent"));
+        }
         return Ok((id, Rgba::rgb(c[0], c[1], c[2])));
     }
     Err(bad(CMD, "the point is not on a selected picture"))

crates/engine/src/tests.rs

@@ -2380,6 +2380,15 @@ fn transparent_color_at_a_point_takes_the_pictures_own_pixel_there() {
     // Beside the picture there is no colour to take, and nothing changes.
     assert!(s.execute("picture.transparentColor", &json!({"ids": [id], "at": [50, 50]})).is_err());
     assert_eq!(clear(&s), Some(blue));
+    // A spot that is already transparent hides a colour (black here, as in most PNGs): taking it
+    // would clear the black drawn elsewhere in the picture.
+    let img = image::RgbaImage::from_fn(4, 2, |x, _| if x < 2 { image::Rgba([0, 0, 0, 0]) } else { image::Rgba([0, 0, 0, 255]) });
+    let mut png = vec![];
+    image::DynamicImage::ImageRgba8(img).write_to(&mut std::io::Cursor::new(&mut png), image::ImageFormat::Png).unwrap();
+    let logo =
+        s.execute("insert.picture", &json!({"data": cmd::base64_encode(&png), "name": "b.png", "rect": [100, 400, 400, 200]})).unwrap()["id"].clone();
+    assert!(s.execute("picture.transparentColor", &json!({"ids": [logo], "at": [150, 500]})).is_err());
+    s.execute("picture.transparentColor", &json!({"ids": [logo], "at": [450, 500]})).unwrap();
 }
 
 #[test]