fix draw text with alpha (#720)

* fix: correct text alpha blending

Author: var4yn

Cargo.toml

@@ -21,7 +21,7 @@ text = ["dep:ab_glyph"]
 [dependencies]
 ab_glyph = { version = "0.2.23", default-features = false, features = ["std"], optional = true}
 approx = { version = "0.5", default-features = false }
-image = { version = "0.25.0", default-features = false }
+image = { version = "0.25.8", default-features = false }
 itertools = { version = "0.14.0", default-features = false, features = [
     "use_std",
 ] }
@@ -31,7 +31,7 @@ rand = { version = "0.9.0", default-features = false, features = [
     "thread_rng"
 ] }
 rand_distr = { version = "0.5.0", default-features = false }
-rayon = { version = "1.8.0", optional = true, default-features = false }
+rayon = { version = "1.10.0", optional = true, default-features = false }
 sdl2 = { version = "0.38.0", optional = true, default-features = false, features = [
     "bundled",
 ] }

src/drawing/text.rs

@@ -98,10 +98,18 @@ pub fn draw_text_mut<C>(
             if (0..image_width).contains(&image_x) && (0..image_height).contains(&image_y) {
                 let image_x = image_x as u32;
                 let image_y = image_y as u32;
-                let pixel = canvas.get_pixel(image_x, image_y);
+                let mut pixel = canvas.get_pixel(image_x, image_y);
                 let gv = gv.clamp(0.0, 1.0);
-                let weighted_color = weighted_sum(pixel, color, 1.0 - gv, gv);
-                canvas.draw_pixel(image_x, image_y, weighted_color);
+
+                if C::Pixel::HAS_ALPHA {
+                    let color = color.map_with_alpha(|f| f, |g| Clamp::clamp(g.into() * gv));
+
+                    pixel.blend(&color);
+                } else {
+                    pixel = weighted_sum(pixel, color, 1.0 - gv, gv);
+                }
+
+                canvas.draw_pixel(image_x, image_y, pixel);
             }
         })
     });

tests/data/truth/text_alpha.png

@@ -1000,3 +1000,24 @@ fn test_draw_text() {
         }
     }
 }
+
+#[test]
+#[cfg(feature = "text")]
+fn test_draw_text_with_alpha() {
+    fn draw_text_with_alpha(img: &RgbaImage) -> RgbaImage {
+        let mut img = img.clone();
+
+        let font_bytes = include_bytes!("data/fonts/DejaVuSans.ttf");
+        let font = ab_glyph::FontRef::try_from_slice(font_bytes).unwrap();
+
+        let text = "Hello world!";
+        let scale = 30.0;
+        let (x, y) = (50, 100);
+        let color_text = Rgba([255, 255, 255, 85]);
+        imageproc::drawing::draw_text_mut(&mut img, color_text, x, y, scale, &font, text);
+
+        img
+    }
+
+    compare_to_truth("elephant.png", "text_alpha.png", draw_text_with_alpha);
+}