Better SRGB to Greyscale Conversion
6 days ago
- #gamma-correction
- #greyscale
- #sRGB
- The common approach to convert gamma-corrected sRGB to greyscale uses weighted sums from Rec. 709, but it's subtly wrong due to gamma-corrected inputs.
- The correct method involves three steps: linearization, greyscale conversion, and gamma correction, though shader programming simplifies this.
- Alternative coefficients for gamma-space greyscale conversion were proposed, such as 0.309R' + 0.609G' + 0.082B' from Poynton's textbook.
- Asahi Lina suggested another set of weights (0.299R' + 0.518G' + 0.183B') to balance the darkness across R, G, and B channels.
- Adobe Photoshop uses older Rec. 601 coefficients for luminosity calculations, which is incorrect for sRGB but may not be a significant issue.
- Experiments show minor differences between methods, with Poynton's alternative formula performing slightly better than the original gamma-space approach.