Support DynamicImage in canny and remove imperative .to_luma8() in docs and README

[twixan]

Problem

Calling the old canny signature as shown in the documentation example fails to compile because the trait bound could never be satisfied:

error[E0277]: the trait bound `image::buffer_::ImageBuffer<image::color::Luma<u8>, Vec<u8>>:
    From<ImageBuffer<Luma<u8>, Vec<u8>>>` is not satisfied
   --> src\main.rs:7:9
    |
6   |     let detection = edge_detection::canny(
    |                     --------------------- required by a bound introduced by this call
7   |         source_image,
    |         ^^^^^^^^^^^^ the trait `From<ImageBuffer<Luma<u8>, Vec<u8>>>` is not implemented for `image::buffer_::ImageBuffer<image::color::Luma<u8>, Vec<u8>>`
    |
    = help: the trait `From<ImageBuffer<Luma<u8>, Vec<u8>>>` is not implemented for `image::buffer_::ImageBuffer<image::color::Luma<u8>, Vec<u8>>`
            but trait `From<image::dynimage::DynamicImage>` is implemented for it
    = help: for that trait implementation, expected `image::dynimage::DynamicImage`, found `ImageBuffer<Luma<u8>, Vec<u8>>`
    = note: required for `ImageBuffer<Luma<u8>, Vec<u8>>` to implement `Into<image::buffer_::ImageBuffer<image::color::Luma<u8>, Vec<u8>>>`

196 | pub fn canny<T: Into<image::GrayImage>>(
    |                 ^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `canny`

Changes

• Signature update
  • Changed

    - pub fn canny<T: Into<GrayImage>>(…)
    + pub fn canny<T: Into<DynamicImage>>(…)

  • Internally convert to gray with

    let dyn_img: DynamicImage = image.into();
    let gs_image: GrayImage    = dyn_img.into_luma8();

• Documentation update
  • Removed the mandatory .to_luma8() call from the example in lib.rs.
  • Removed the mandatory .to_luma8() call from the example in README.md

Rationale

• The old bound T: Into<GrayImage> was never satisfied by any concrete type, leading to compilation errors, the example provided by the documentation simply didn't work.
• Accepting Into<DynamicImage> leverages existing From<Buffer> for DynamicImage implementations, allowing all common image buffers (RGB, RGBA, Luma, etc.) to be passed in.
• Conversion to 8‑bit grayscale remains unchanged internally, preserving the algorithm’s assumptions.

Usage Examples

// 1) Explicit grayscale conversion:
let source_image = image::open("testdata/line-simple.png")
    .expect("failed to read image")
    .to_luma8();

let detection = edge_detection::canny(
    source_image,
    1.2,  // sigma
    0.2,  // strong threshold
    0.01, // weak threshold
);

// 2) Pass DynamicImage directly:
let source_image = image::open("testdata/line-simple.png")
    .expect("failed to read image");

let detection = edge_detection::canny(
    source_image,
    1.2,  // sigma
    0.2,  // strong threshold
    0.01, // weak threshold
);

[polyfloyd]

Thanks!

[polyfloyd]

Merged by hand