[cli] Handle all color types in png decoder

Add conversions for all color types in PNG decoder used in CLI. (While at it, also made few minor cleanups.)

Fixes #971.

codecs/png/src/lib.rs

@@ -1,5 +1,7 @@
-use std::io::Cursor;
-
+use rgb::{
+    alt::{GRAY8, GRAYA8},
+    AsPixels, FromSlice, RGB8, RGBA8,
+};
 use wasm_bindgen::prelude::*;
 use wasm_bindgen::Clamped;
 
@@ -18,38 +20,57 @@ extern "C" {
     ) -> ImageData;
 }
 
-#[wasm_bindgen(catch)]
+#[wasm_bindgen]
 pub fn encode(data: &[u8], width: u32, height: u32) -> Vec<u8> {
-    let mut buffer = Cursor::new(Vec::<u8>::new());
+    let mut buffer = Vec::new();
 
     {
         let mut encoder = png::Encoder::new(&mut buffer, width, height);
         encoder.set_color(png::ColorType::RGBA);
         encoder.set_depth(png::BitDepth::Eight);
-        let mut writer = encoder.write_header().unwrap();
-        writer.write_image_data(data).unwrap();
+        let mut writer = encoder.write_header().unwrap_throw();
+        writer.write_image_data(data).unwrap_throw();
     }
 
-    buffer.into_inner()
+    buffer
 }
 
-#[wasm_bindgen(catch)]
-pub fn decode(data: &[u8]) -> ImageData {
-    let mut decoder = png::Decoder::new(Cursor::new(data));
+// Convert pixels in-place within buffer containing source data but preallocated
+// for entire [num_pixels * sizeof(RGBA)].
+// This works because all the color types are <= RGBA by size.
+fn expand_pixels<Src: Copy>(buf: &mut [u8], to_rgba: impl Fn(Src) -> RGBA8)
+where
+    [u8]: AsPixels<Src> + FromSlice<u8>,
+{
+    assert!(std::mem::size_of::<Src>() <= std::mem::size_of::<RGBA8>());
+    let num_pixels = buf.len() / 4;
+    for i in (0..num_pixels).rev() {
+        let src_pixel = buf.as_pixels()[i];
+        buf.as_rgba_mut()[i] = to_rgba(src_pixel);
+    }
+}
+
+#[wasm_bindgen]
+pub fn decode(mut data: &[u8]) -> ImageData {
+    let mut decoder = png::Decoder::new(&mut data);
     decoder.set_transformations(png::Transformations::EXPAND);
-    let (info, mut reader) = decoder.read_info().unwrap();
+    let (info, mut reader) = decoder.read_info().unwrap_throw();
     let num_pixels = (info.width * info.height) as usize;
     let mut buf = vec![0; num_pixels * 4];
-    reader.next_frame(&mut buf).unwrap();
+    reader.next_frame(&mut buf).unwrap_throw();
 
-    // Transformations::EXPAND will make sure color_type is either
-    // RGBA or RGB. If it’s RGB, we need inject an alpha channel.
-    if info.color_type == png::ColorType::RGB {
-        for i in (0..num_pixels).rev() {
-            buf[i * 4 + 0] = buf[i * 3 + 0];
-            buf[i * 4 + 1] = buf[i * 3 + 1];
-            buf[i * 4 + 2] = buf[i * 3 + 2];
-            buf[i * 4 + 3] = 255;
+    // Transformations::EXPAND will expand indexed palettes and lower-bit
+    // grayscales to higher color types, but we still need to transform
+    // the rest to RGBA.
+    match info.color_type {
+        png::ColorType::RGBA => {}
+        png::ColorType::RGB => expand_pixels(&mut buf, RGB8::into),
+        png::ColorType::GrayscaleAlpha => expand_pixels(&mut buf, GRAYA8::into),
+        png::ColorType::Grayscale => {
+            expand_pixels(&mut buf, |gray: GRAY8| GRAYA8::from(gray).into())
+        }
+        png::ColorType::Indexed => {
+            unreachable!("Found indexed color type, but expected it to be already expanded")
         }
     }