How do I get graphicsmagick to draw a line inside a BMP file? - graphicsmagick

Question

I've installed GraphicsMagick and the gm command responds inside the shell.
I have a BMP file of 400 by 400 white pixels.
I tried this
gm convert -draw 'line 10,10 200,200' blank_canvas.bmp
but all I got was
gm convert unable to open file (10,10)[No such file or directory]
I also tried
gm convert blank_canvas.bmp -draw 'line 10,10 200,200'
and got
gm convert: Non-conforming drawing primitive definition (line) [No such file or directory].
And while we are at it, how do I get it to draw a big number of lines, inputing the data from a text file?

Answer 1

Make a white BMP:
gm convert -size 400x400 xc:white white.bmp
Draw a red line on it, being careful to specify both an input and output image:
gm convert white.bmp -fill red -draw 'line 10,10 200,200' result.bmp
If you don't want to specify an input and an output file because you want the operation done "in-place", use mogrify:
gm mogrify -fill red -draw 'line 10,10 200,200' white.bmp
You can also use mogrify if you have hundreds of images to apply the drawing to, like this:
gm mogrify -fill red -draw 'line 10,10 200,200' *.bmp
Be careful - mogrify is a VERY POWERFUL command that can irreversibly alter lots of images very quickly.
If you want to do lots of lines, all listed in a text file, make it with a format like this and save as draw.txt:
fill red
line 10,10,200,200
fill blue
line 200,10,10,200
fill lime
line 80,120,300,400
And run it like this:
gm convert white.bmp -draw #draw.txt result.bmp
Alternatively, you could use an MVG file like this:
viewbox 0 0 400 400
fill white rectangle 0,0 400,400
fill red line 10,10,200,200
fill blue line 200,10,10,200
fill lime line 80,120,300,400
fill magenta circle 250,250 250,400
And run it like this:
gm convert mvg:lines.mvg result.png
Or, like this if you have a program that generates the lines:
cat lines.mvg | gm convert mvg:- result.png
Keywords: ImageMagick, GraphicsMagick, draw lines, multiple lines, multiple shapes, MVG.