Use convert to replace lightness channel with another lightness channel

Question

How can I use ImageMagick to replace 1.png's HSL lightness channel with 2.png's HSL lightness channel? I almost cannot understand the convert syntax.

Answer 1

There are at least two ways to do it...

First, and slowest is using -fx and the command will look like this:

convert 1.png 2.png -colorspace HSL -channel L -fx "v" -colorspace RGB result.png

That will load the two images, 1.png and 2.png, and convert them both to HSL colorspace. It will then produce a new Lighness channel by using the value from image v (which is how ImageMagick refers to the second image in a sequence - the first being u). It then converts the resultant image back to RGB colorspace.

The second method looks like this and will be faster, especially on larger images:

convert 1.png 2.png      \
  -colorspace HSL        \
  -separate              \
  -delete 2-4            \
  -combine               \
  -set colorspace HSL    \
  -colorspace rgb        \
  output.png

Basically, it will load both images, convert both to HSL, and separate into layers. Layer 0 will be 1.png's Hue, layer 2 will be 1.png's Saturation and Layer 3 will be 1.png's Lightness. Layers 3-5 will be 2.png's Hue, Saturation and Lightness. Then we delete delete 1.png's Lightness and 2.png's Hue & Saturation then combine the three channels into a new image, convert it back to RGB and save it as result.png

To test it, I create 2 dummy input images like this:

convert -size 256x256              \
      xc:red xc:lime +append       \
   \( xc:blue xc:black +append \)  \
     -append 1.png

and this

convert -size 512x512 gradient:black-white 2.png

then I apply my first technique, namely

convert 1.png 2.png -colorspace HSL -channel L -fx "v" -colorspace RGB result.png

and I get this

The second technique gives identical results.