Restore streaming output writes

During the upgrade from image 0.23 to 0.24, `ImageBuffer::write_to()`
was changed to require a seekable writer.
This doesn't work when writing to stdout, so we were forced to buffer
the output before writing it out in one go.

It turns out that most individual encoders don't actually require
seeking, and neither of the two we use does.
Restore streaming behavior by invoking encoders directly.

See: image-rs/image#1922

src/main.rs

@@ -5,14 +5,16 @@
 use std::env;
 use std::ffi::CString;
 use std::fs::File;
-use std::io::{self, Write};
+use std::io;
 use std::path::Path;
 use std::process;
 use std::time;
 
 use getopts::Options;
+use image::codecs;
 use image::GenericImage;
 use image::GenericImageView;
+use image::ImageOutputFormat;
 use image::Rgba;
 use image::RgbaImage;
 use x11::xlib;
@@ -97,8 +99,8 @@ fn run() -> i32 {
         .unwrap_or("png".to_string())
         .to_lowercase();
     let output_format = match output_ext.as_ref() {
-        "png" => image::ImageOutputFormat::Png,
-        "pam" => image::ImageOutputFormat::Pnm(image::codecs::pnm::PnmSubtype::ArbitraryMap),
+        "png" => ImageOutputFormat::Png,
+        "pam" => ImageOutputFormat::Pnm(codecs::pnm::PnmSubtype::ArbitraryMap),
         _ => {
             eprintln!("Invalid image format specified");
             return 1;
@@ -234,25 +236,30 @@ fn run() -> i32 {
         }
     };
 
-    if path == "-" {
-        let mut buf = Vec::new();
-        image
-            .write_to(&mut io::Cursor::new(&mut buf), output_format)
-            .expect("Encoding failed");
-        io::stdout()
-            .write_all(buf.as_slice())
-            .expect("Writing to stdout failed");
+    let writer: Box<dyn io::Write> = if path == "-" {
+        Box::new(io::stdout())
     } else {
         match File::create(&Path::new(&path)) {
-            Ok(mut f) => image
-                .write_to(&mut f, output_format)
-                .expect("Writing to file failed"),
+            Ok(f) => Box::new(f),
             Err(e) => {
                 eprintln!("Failed to create {}: {}", path, e);
                 return 1;
             }
         }
+    };
+
+    match output_format {
+        ImageOutputFormat::Png => {
+            let encoder = codecs::png::PngEncoder::new(writer);
+            util::write_image_buffer_with_encoder(&image, encoder)
+        }
+        ImageOutputFormat::Pnm(subtype) => {
+            let encoder = codecs::pnm::PnmEncoder::new(writer).with_subtype(subtype);
+            util::write_image_buffer_with_encoder(&image, encoder)
+        }
+        _ => unreachable!(),
     }
+    .expect("Failed to write output");
 
     0
 }

src/util.rs

@@ -49,3 +49,21 @@ pub fn parse_int<T: num_traits::Num>(string: &str) -> Result<T, T::FromStrRadixE
         _ => T::from_str_radix(string, 10),
     }
 }
+
+use image::EncodableLayout;
+pub fn write_image_buffer_with_encoder<P, Container>(
+    image: &image::ImageBuffer<P, Container>,
+    encoder: impl image::ImageEncoder,
+) -> image::ImageResult<()>
+where
+    P: image::PixelWithColorType,
+    [P::Subpixel]: image::EncodableLayout,
+    Container: core::ops::Deref<Target = [P::Subpixel]>,
+{
+    encoder.write_image(
+        image.as_raw().as_bytes(),
+        image.width(),
+        image.height(),
+        P::COLOR_TYPE,
+    )
+}