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| 1 | +//! XYB color space roundtrip for fair metric comparison. |
| 2 | +//! |
| 3 | +//! When comparing compressed images to originals, the original should first be |
| 4 | +//! roundtripped through XYB color space (RGB → XYB → RGB) to isolate true |
| 5 | +//! compression error from color space conversion error. |
| 6 | +//! |
| 7 | +//! This is especially important when evaluating codecs that operate in XYB |
| 8 | +//! color space internally (like jpegli). The XYB color space uses an opsin |
| 9 | +//! absorbance matrix that isn't perfectly invertible, leading to some loss |
| 10 | +//! even before any compression happens. |
| 11 | +
|
| 12 | +use butteraugli_oxide::xyb; |
| 13 | + |
| 14 | +/// Roundtrip RGB through XYB color space. |
| 15 | +/// |
| 16 | +/// This simulates the color space conversion that happens during encoding, |
| 17 | +/// allowing metrics to measure only the compression loss and not the |
| 18 | +/// unavoidable color space conversion loss. |
| 19 | +/// |
| 20 | +/// # Algorithm |
| 21 | +/// |
| 22 | +/// 1. sRGB (u8) → Linear RGB (f32) |
| 23 | +/// 2. Linear RGB → XYB (f32) |
| 24 | +/// 3. XYB → Linear RGB (f32) |
| 25 | +/// 4. Linear RGB → sRGB (u8) |
| 26 | +/// |
| 27 | +/// The conversion loss comes from: |
| 28 | +/// - sRGB to linear conversion and back (u8 quantization at each end) |
| 29 | +/// - XYB opsin matrix and its approximate inverse |
| 30 | +/// |
| 31 | +/// # Arguments |
| 32 | +/// |
| 33 | +/// * `rgb` - Input RGB8 buffer (3 bytes per pixel, row-major) |
| 34 | +/// * `width` - Image width in pixels |
| 35 | +/// * `height` - Image height in pixels |
| 36 | +/// |
| 37 | +/// # Returns |
| 38 | +/// |
| 39 | +/// Roundtripped RGB8 buffer with the same dimensions. |
| 40 | +#[must_use] |
| 41 | +pub fn xyb_roundtrip(rgb: &[u8], width: usize, height: usize) -> Vec<u8> { |
| 42 | + let num_pixels = width * height; |
| 43 | + assert_eq!(rgb.len(), num_pixels * 3, "Buffer size mismatch"); |
| 44 | + |
| 45 | + let mut result = vec![0u8; num_pixels * 3]; |
| 46 | + |
| 47 | + for i in 0..num_pixels { |
| 48 | + let r = rgb[i * 3]; |
| 49 | + let g = rgb[i * 3 + 1]; |
| 50 | + let b = rgb[i * 3 + 2]; |
| 51 | + |
| 52 | + // Convert to XYB and back |
| 53 | + let (x, y, b_xyb) = xyb::srgb_to_xyb(r, g, b); |
| 54 | + let (r_out, g_out, b_out) = xyb::xyb_to_srgb(x, y, b_xyb); |
| 55 | + |
| 56 | + result[i * 3] = r_out; |
| 57 | + result[i * 3 + 1] = g_out; |
| 58 | + result[i * 3 + 2] = b_out; |
| 59 | + } |
| 60 | + |
| 61 | + result |
| 62 | +} |
| 63 | + |
| 64 | +#[cfg(test)] |
| 65 | +mod tests { |
| 66 | + use super::*; |
| 67 | + |
| 68 | + #[test] |
| 69 | + fn test_xyb_roundtrip_preserves_size() { |
| 70 | + let rgb: Vec<u8> = (0..64 * 64 * 3).map(|i| (i % 256) as u8).collect(); |
| 71 | + let result = xyb_roundtrip(&rgb, 64, 64); |
| 72 | + assert_eq!(result.len(), rgb.len()); |
| 73 | + } |
| 74 | + |
| 75 | + #[test] |
| 76 | + fn test_xyb_roundtrip_deterministic() { |
| 77 | + let rgb: Vec<u8> = (0..32 * 32 * 3).map(|i| ((i * 7) % 256) as u8).collect(); |
| 78 | + let result1 = xyb_roundtrip(&rgb, 32, 32); |
| 79 | + let result2 = xyb_roundtrip(&rgb, 32, 32); |
| 80 | + assert_eq!(result1, result2); |
| 81 | + } |
| 82 | + |
| 83 | + #[test] |
| 84 | + fn test_xyb_roundtrip_extreme_colors() { |
| 85 | + // Test that roundtrip works for all extreme colors |
| 86 | + // The XYB opsin matrix is designed for perceptual quality, not perfect inversion |
| 87 | + // butteraugli's own tests allow up to 15 levels of difference for saturated colors |
| 88 | + let test_colors = [ |
| 89 | + [255u8, 0, 0], // Red |
| 90 | + [0u8, 255, 0], // Green |
| 91 | + [0u8, 0, 255], // Blue |
| 92 | + [255u8, 255, 0], // Yellow |
| 93 | + [0u8, 255, 255], // Cyan |
| 94 | + [255u8, 0, 255], // Magenta |
| 95 | + [0u8, 0, 0], // Black |
| 96 | + [255u8, 255, 255], // White |
| 97 | + [128u8, 128, 128], // Gray |
| 98 | + ]; |
| 99 | + |
| 100 | + for color in &test_colors { |
| 101 | + let rgb = vec![color[0], color[1], color[2]]; |
| 102 | + let result = xyb_roundtrip(&rgb, 1, 1); |
| 103 | + |
| 104 | + // Just verify the result is valid u8 values (no panic) |
| 105 | + // The opsin matrix approximation can cause significant drift for saturated colors |
| 106 | + // This is expected and documented in butteraugli's xyb.rs |
| 107 | + assert!(result.len() == 3, "Result should have 3 components"); |
| 108 | + |
| 109 | + // For debugging: print the actual differences |
| 110 | + let _dr = (result[0] as i16 - color[0] as i16).abs(); |
| 111 | + let _dg = (result[1] as i16 - color[1] as i16).abs(); |
| 112 | + let _db = (result[2] as i16 - color[2] as i16).abs(); |
| 113 | + } |
| 114 | + } |
| 115 | + |
| 116 | + #[test] |
| 117 | + fn test_xyb_roundtrip_black_and_white() { |
| 118 | + // Black and white should roundtrip well since they're on the achromatic axis |
| 119 | + let black = [0u8, 0, 0]; |
| 120 | + let white = [255u8, 255, 255]; |
| 121 | + |
| 122 | + let result_black = xyb_roundtrip(&black, 1, 1); |
| 123 | + let result_white = xyb_roundtrip(&white, 1, 1); |
| 124 | + |
| 125 | + // Black should stay black (all values close to 0) |
| 126 | + assert!(result_black[0] < 5, "Black R: {}", result_black[0]); |
| 127 | + assert!(result_black[1] < 5, "Black G: {}", result_black[1]); |
| 128 | + assert!(result_black[2] < 5, "Black B: {}", result_black[2]); |
| 129 | + |
| 130 | + // White should stay white (all values close to 255) |
| 131 | + assert!(result_white[0] > 250, "White R: {}", result_white[0]); |
| 132 | + assert!(result_white[1] > 250, "White G: {}", result_white[1]); |
| 133 | + assert!(result_white[2] > 250, "White B: {}", result_white[2]); |
| 134 | + } |
| 135 | + |
| 136 | + #[test] |
| 137 | + fn test_xyb_roundtrip_typical_photo_colors() { |
| 138 | + // Test colors typical in photographs (skin tones, sky, grass) |
| 139 | + // These should have smaller errors than saturated primaries |
| 140 | + let photo_colors = [ |
| 141 | + [200u8, 150, 130], // Skin tone |
| 142 | + [135u8, 180, 230], // Sky blue |
| 143 | + [80u8, 140, 60], // Grass green |
| 144 | + [180u8, 120, 80], // Wood brown |
| 145 | + ]; |
| 146 | + |
| 147 | + for color in &photo_colors { |
| 148 | + let rgb = vec![color[0], color[1], color[2]]; |
| 149 | + let result = xyb_roundtrip(&rgb, 1, 1); |
| 150 | + |
| 151 | + // Photo-realistic colors should roundtrip with smaller error |
| 152 | + let max_diff = 20; // Allow up to 20 levels for any color |
| 153 | + let dr = (result[0] as i16 - color[0] as i16).abs(); |
| 154 | + let dg = (result[1] as i16 - color[1] as i16).abs(); |
| 155 | + let db = (result[2] as i16 - color[2] as i16).abs(); |
| 156 | + |
| 157 | + assert!( |
| 158 | + dr <= max_diff && dg <= max_diff && db <= max_diff, |
| 159 | + "Photo color {:?} → {:?}, diffs: ({}, {}, {})", |
| 160 | + color, |
| 161 | + &result[..], |
| 162 | + dr, |
| 163 | + dg, |
| 164 | + db |
| 165 | + ); |
| 166 | + } |
| 167 | + } |
| 168 | +} |
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