Problem changing image from monochrome to colour

[dormant]

ImageMagick version

6.9.12-98

Operating system, version and so on

Debian GNU/Linux trixie/sid

Description

I am batch processing a bunch of image files using mogrify. The images contain black lines, which I want to thicken.

As the pixels are actually grayscale pixels, my processing goes like this:

mogrify -grayscale Rec709Luma *.png
mogrify -black-threshold 75% *.png
mogrify -convolve '3x3:1,1,1,1,1,1,1,1,1' *.png
mogrify -monochrome *.png

And this is the result:

For some of the files, I want to convert black to another colour. I tried this:

mogrify -colorspace RGB *.png
mogrify -opaque black -fill red  *.png

But I get a pure white image:

The image seems to be grayscale, not RGB.

How can I do this last step?

[fmw42]

In place of -colorspace RGB, use PNG24:*.png to force 24bit color RGB. See if that works.

Also if you want to thicken your line, you could simply use -morphology open diamond:X Then try varying X=1,2,3...etc until you get the thickness you want. You can also try disk:X which will be thicker for any give X, which can be float.

[dormant]

Many thanks, this is very helpful. The -morphology option is exactly what I need.