Applying a colour gradient only to the non transparent area of an image in Image Magick

Question

I would like to apply a coloured gradient to an image while maintaining the transparency of the original image, so that I can show the image on a map, from this original image to this output image (but without the map)

My understanding is that I would need to extract the alpha channels of the image using +clone -alpha extract then apply the gradient to that then composite the gradient on top of the original image.

Currently I've tried adopting a couple of other stackoverflow answers and have managed to output the extracted alpha channels but cannot get the gradient to work.

This is what I have so far:

convert normal.png -write MPR:orig -alpha extract
\( +clone -define gradient:"rgba(0,126,15,5)"-"rgba(255,0,0,5)" \) \
-compose multiply -composite -write alpha.png \
MPR:orig +swap -compose copyopacity -composite result.png

Answer 1

Perhaps this is what you want in Imagemagick. Note that when using gradient:, you have to specify -size WxH for it. So I use -sparse-color barycentric to form the gradient from a clone of the input.

Input:

convert normal.png \
\( +clone -alpha extract -write MPR:alpha +delete \) \
\( -clone 0 -alpha off -sparse-color barycentric "0,0 rgb(0,126,15)  %w,%h rgb(255,0,0)  %w,0 rgb(255,0,0)" \) \
-compose multiply -composite \
MPR:alpha -alpha off -compose copyopacity -composite result.png

Result:

Line 1: Read the input
Line 2: Copy the input, extract the alpha and save to mpr:, then delete the alpha (not the mpr:)
Line 3: Copy the input and create a gradient for over that image
Line 4: Composite the gradient by multiplication with the input
Line 5: Read back the mpr:alpha and put it into the alpha channel of the input in place of the original alpha channel