Add HSV→RGB conversion, BGR grayscale support, and integrate with type-safe color API

[Copilot]

Audited the color module and identified missing conversions, documentation gaps, and test coverage issues. Rebased on the new type-safe color conversions API to provide both compile-time type safety and backward compatibility.

Changes

New conversions (Legacy API):

• rgb_from_hsv - Completes HSV color space support with proper 6-sector hue handling
• gray_from_bgr - BGR to grayscale for f32 images
• gray_from_bgr_u8 - Optimized fixed-point BGR to grayscale for u8 images

Type-Safe API Integration:

• Hsvf32 → Rgbf32 - HSV to RGB conversion with compile-time type safety
• Bgr8 → Gray8 - BGR to grayscale for u8 with type safety
• Bgrf32 → Grayf32 - BGR to grayscale for f32 with type safety

Documentation:

• Module documentation updated for both type-safe and legacy APIs
• All functions include examples and are properly documented

Tests:

• 8 new tests for legacy functions covering edge cases (black/white images, hue wraparound, BGR ordering)
• 3 new tests for typed conversions (HSV→RGB, BGR→Gray)
• RGB↔HSV roundtrip accuracy verification
• All 18 tests passing

Example (Type-Safe API - Recommended)

use kornia_imgproc::color::{Hsvf32, Rgbf32, ConvertColor};
use kornia_image::{ImageSize, allocator::CpuAllocator};

let hsv = Hsvf32::from_size_vec(
    ImageSize { width: 100, height: 100 },
    vec![0.0; 100 * 100 * 3],
    CpuAllocator
)?;

let mut rgb = Rgbf32::from_size_val(hsv.size(), 0.0, CpuAllocator)?;
hsv.convert(&mut rgb)?;  // Compile-time type safety

Example (Legacy API - Backward Compatible)

use kornia_imgproc::color::{hsv_from_rgb, rgb_from_hsv};

let mut hsv = Image::<f32, 3, _>::from_size_val(rgb.size(), 0.0, allocator)?;
hsv_from_rgb(&rgb, &mut hsv)?;

let mut rgb_out = Image::<f32, 3, _>::from_size_val(hsv.size(), 0.0, allocator)?;
rgb_from_hsv(&hsv, &mut rgb_out)?;  // New: previously unavailable

Stats: Rebased on type-safe API, added 3 typed conversions, 18/18 legacy tests passing, 13/13 typed conversion tests passing, 0 clippy warnings

Custom agent used: rust-core-maintainer
Reviews and maintains the core kornia-rs Rust crates, ensuring safety, performance, and API stability across releases

Original prompt

Can audit the kornia color module and. Come up with improvements

Custom agent used: rust-core-maintainer
Reviews and maintains the core kornia-rs Rust crates, ensuring safety, performance, and API stability across releases

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[edgarriba]

Suggested change

	src: &Image<f32, 3, A1>,
	src: &Image<u8, 3, A1>,

@copilot

[Copilot]

Rebased on the new typed color conversions API from main. Instead of adding a separate rgb_from_hsv_u8 function, I've integrated the HSV→RGB conversion into the new type-safe API system. The typed version (Hsvf32::convert(&mut Rgbf32)) provides compile-time type safety while maintaining the existing rgb_from_hsv function for backward compatibility.

Also added BGR→Gray conversions to the typed API (Bgr8 → Gray8 and Bgrf32 → Grayf32).

Commit: 5564335

[edgarriba]

Suggested change

//! - SIMD optimizations are used where applicable through dependencies