Add support for transparency (#31)

* Add support for alpha channel

* Fix compatibility with shadow

* Small fix

* Update README

* Use RGBA instead of ARGB

README.md

@@ -65,7 +65,7 @@ pikaur -S silicon
 sudo pacman -S --needed pkgconf freetype2 fontconfig libxcb xclip
 ```
 
-## Basic Usage
+## Examples
 
 Read code from file
 
@@ -94,7 +94,15 @@ silicon main.rs -o main.png --highlight-lines '1; 3-4'
 Custom the image
 
 ```bash
-silicon ./target/test.rs -o test.png --shadow-color '#555555' --background '#ffffff' --shadow-blur-radius 30 --no-window-controls
+silicon ./target/test.rs -o test.png \
+    --shadow-color '#555555' --background '#ffffff' \
+    --shadow-blur-radius 30 --no-window-controls
+```
+
+Transparent background
+
+```bash
+silicon ./target/test.rs -o test.png --background '#ffffff00'
 ```
 
 see `silicon --help` for detail

src/blur.rs

@@ -284,7 +284,7 @@ fn box_blur_horz(
             val_r += isize::from(bb[0]) - isize::from(fv[0]);
             val_g += isize::from(bb[1]) - isize::from(fv[1]);
             val_b += isize::from(bb[2]) - isize::from(fv[2]);
-            val_a += isize::from(bb[2]) - isize::from(fv[3]);
+            val_a += isize::from(bb[3]) - isize::from(fv[3]);
 
             frontbuf[ti] = [
                 round(val_r as f32 * iarr) as u8,

src/utils.rs

@@ -15,20 +15,30 @@ pub fn init_syntect() -> (SyntaxSet, ThemeSet) {
     )
 }
 
-pub(crate) trait ToRgba {
+pub trait ToRgba {
     type Target;
     fn to_rgba(&self) -> Self::Target;
 }
 
+/// Parse hex color (#RRGGBB or #RRGGBBAA)
 impl ToRgba for str {
     type Target = Result<Rgba<u8>, std::num::ParseIntError>;
     fn to_rgba(&self) -> Self::Target {
-        let rgb = u32::from_str_radix(&self[1..], 16)?;
+        let mut rgb = u32::from_str_radix(&self[1..], 16)?;
+
+        let alpha = if self.len() <= 7 {
+            0xff
+        } else {
+            let alpha = (rgb & 0xff) as u8;
+            rgb >>= 8;
+            alpha
+        };
+
         Ok(Rgba([
             ((rgb >> 16) & 0xff) as u8,
             ((rgb >> 8) & 0xff) as u8,
             (rgb & 0xff) as u8,
-            0xff,
+            alpha,
         ]))
     }
 }
@@ -285,3 +295,15 @@ pub(crate) fn draw_filled_circle_mut<I>(
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::utils::ToRgba;
+    use image::Rgba;
+
+    #[test]
+    fn to_rgba() {
+        assert_eq!("#abcdef".to_rgba().unwrap(), Rgba([0xab, 0xcd, 0xef, 0xff]));
+        assert_eq!("#abcdef00".to_rgba().unwrap(), Rgba([0xab, 0xcd, 0xef, 0x00]));
+    }
+}