Change the JPEG encoder to work on a GenericImageView instead of a by…

…te slice

This change allows users to encode jpeg images from other sources
than memory, thus finally allowing the encoding of images that
do not fit in memory.

This also allows users to make the encoder work in a streaming fashion.
(see image-rs#1219)

This also finally allows encoding images from other pixel formats than
8-bit RGB. The conversion to 8bit YCbCr (used in JPEG) is done on the fly
during encoding.

src/color.rs

@@ -1,6 +1,7 @@
-use num_traits::{NumCast, ToPrimitive, Zero};
 use std::ops::{Index, IndexMut};
 
+use num_traits::{NumCast, ToPrimitive, Zero};
+
 use crate::traits::{Pixel, Primitive};
 
 /// An enumeration over supported color types and bit depths
@@ -57,6 +58,16 @@ impl ColorType {
         }
     }
 
+    /// Returns false if the color scheme is grayscale, true otherwise.
+    pub fn has_color(self) -> bool {
+        use ColorType::*;
+        match self {
+            L8 | L16 | La8 | La16 => false,
+            Rgb8 | Bgr8 | Rgb16 | Rgba8 | Bgra8 | Rgba16 => true,
+            __NonExhaustive(marker) => match marker._private {},
+        }
+    }
+
     /// Returns the number of bits contained in a pixel of `ColorType` ```c``` (which will always be
     /// a multiple of 8).
     pub fn bits_per_pixel(self) -> u16 {
@@ -1126,7 +1137,7 @@ impl<T: Primitive> Invert for Bgr<T> {
 
 #[cfg(test)]
 mod tests {
-    use super::{Luma, LumaA, Pixel, Rgb, Rgba, Bgr, Bgra};
+    use super::{Bgr, Bgra, Luma, LumaA, Pixel, Rgb, Rgba};
 
     #[test]
     fn test_apply_with_alpha_rgba() {

src/dynimage.rs

@@ -738,9 +738,8 @@ impl DynamicImage {
             }
             #[cfg(feature = "jpeg")]
             image::ImageOutputFormat::Jpeg(quality) => {
-                let mut j = jpeg::JPEGEncoder::new_with_quality(w, quality);
-
-                j.encode(&bytes, width, height, color)?;
+                let j = jpeg::JPEGEncoder::new_with_quality(w, quality);
+                j.write_image(&bytes, width, height, color)?;
                 Ok(())
             }