08-30-2020, 02:41 PM
Yes, if you can make the two images overlap, then you can put the color image in a layer above the grayscale image and set it to Color mode (Gimp 2.8) or LCH Color or HSL Color (Gimp 2.10).
This works because the perceived sharpness of an image is more related to the luminosity than to the color. This is precisely the "chroma sub-sampling" done in the JPEG format. In JPEG, your image is stored as 3 images, a grayscale image that has the definition of your source image, and two color channel images. These images can be kept at the full definition ("full chroma"), but can also be reduced ( half-width, half-height (chroma halved) or both (chroma quartered). Decoding is then the process above, using a low definition color image to color a high-definition greyscale image.
The chroma subsampling is very important in JPEG, just using the quartered chroma divides the final size by 2 (full chrorma is 1+1+1=3, quartered chroma is 1+.25+.25=1.5
This works because the perceived sharpness of an image is more related to the luminosity than to the color. This is precisely the "chroma sub-sampling" done in the JPEG format. In JPEG, your image is stored as 3 images, a grayscale image that has the definition of your source image, and two color channel images. These images can be kept at the full definition ("full chroma"), but can also be reduced ( half-width, half-height (chroma halved) or both (chroma quartered). Decoding is then the process above, using a low definition color image to color a high-definition greyscale image.
The chroma subsampling is very important in JPEG, just using the quartered chroma divides the final size by 2 (full chrorma is 1+1+1=3, quartered chroma is 1+.25+.25=1.5