slightly improve printing and explanations

src/main.rs

@@ -62,7 +62,8 @@ fn encode_standard(pixels: &[[u8; 4]], width: u32, height: u32) -> Vec<u8> {
     w
 }
 
-/// Simple program to optimise palette based PNGs
+/// Simple program to optimise palette based PNGs and substitute certain rgb values
+/// Assumes that there is only alpha 0 and alpha 255
 #[derive(Parser, Debug)]
 #[command(version, about, long_about = None)]
 struct Args {
@@ -79,14 +80,14 @@ struct Args {
 
 fn parse_rgb(hex: String) -> Result<[u8; 4], String> {
     if hex.len() != 6 {
-        return Err("expected rrggbb".into());
+        return Err("Expected the format rrggbb".into());
     }
 
     let bytes: Vec<u8> = (0..6)
         .step_by(2)
         .map(|i| u8::from_str_radix(&hex[i..i + 2], 16))
         .collect::<Result<_, _>>()
-        .map_err(|_| "invalid hex color")?;
+        .map_err(|_| "Invalid hex color")?;
 
     Ok([bytes[0], bytes[1], bytes[2], 0xff])
 }
@@ -109,9 +110,9 @@ fn main() {
         .map(|[a, b]| (*a, *b))
         .collect();
     if asubs_len == 0 {
-        println!("no substitutions selected, only re-encoding")
+        println!("No substitutions selected, only re-encoding")
     } else {
-        println!("substitutions to use: {:?}", pairs);
+        println!("Using substitutions: {:?}", pairs);
     }
 
     let decoder = PngDecoder::new(BufReader::new(File::open(in_path).unwrap())).unwrap();
@@ -127,8 +128,11 @@ fn main() {
     let plte_data = encode_plte(image_data.as_slice(), width, height);
     let std_data = encode_standard(image_data.as_slice(), width, height);
 
-    println!("plte data has length {}", plte_data.len(),);
-    println!("std data has length {}", std_data.len(),);
+    println!(
+        "Palette data has length {}, standard has {}",
+        plte_data.len(),
+        std_data.len()
+    );
 
     let mut file = File::create(out_path).unwrap();
     if plte_data.len() <= std_data.len() {