Legacy ImageMagick Discussions Archive

I am trying to label photographs with either white or black letters, depending on the mean brightness of the local background, in order to acchieve the maximum contrast. I already read Anthony's remarks on labelling and reached at the following DOS beatch file:
I hope this is somehow legible to non-Windows users. What I basically do is:
1) Generate an image with the label in white letters
2) Identify its size
3) Cut the according piece from the image that I want to label
4) Identify its mean value (fx gives values 0 -- 1)
5) If this value is less than 0.5 --> white letters, otherwise black letters
6) Do the actual labelling with the correct letter color.

Although the code works, I wonder if there is a more elegant solution to the problem, without creating so many temporal files.

One problem was that -gravity isn't available for Identify. And there seems to be no -identify option for Convert, such that I could try to create an interim result and use it in the same command line. Piping in Windows is limited to simple string values.

Wolfgang

In my autocaption script (and somewhat similar in my autolabel script), I do something similar. But I scale the appropriate section of the image to 1x1 pixel (average it), convert to gray, then get the (percent) gray level value in range 0 to 100. Then test that to determine whether to color with white or black. Basically the following although I have expressed it a bit more efficiently here than what I used in my script. If you can convert this for windows, it might save you one step.

gray=`convert $tmpA2[${sizep}x${sizep}+${xxm}+${yym}] -scale 1x1\! -colorspace gray -format "%[fx:round(100*s.r)]" info:`
[ $gray -lt 50 ] && color="white" || color="black"

In unix, this probably could be done in one complex step as (though untested):

[ `convert $tmpA2[${sizep}x${sizep}+${xxm}+${yym}] -scale 1x1\! -colorspace gray -format "%[fx:round(100*s.r)]" info:` -lt 50 ] && color="white" || color="black"

Fred

I would however not only color the text the opposite of the general background, but also surround the text with a slight darkening or lightening (opposite color of the text) to ensure the text remains visible.

See Annotataing Images
http://www.imagemagick.org/Usage/annotating/

You could overlay that lightening or darking using some type of lighten or darken overlay operator, so that it is only added as nessary
http://www.imagemagick.org/Usage/compose/#lighten

Basically you can do a lot, and typically it only depends on how much effort and how far you want to take it.

Thanks to your hints I arrived at a much shorter solution: The decisive hint was that I can substitute Identify by Convert -format ... info:. This allows me to use options which are only available for Convert, such as -gravity.
Another hint was the usage of the ternary conditional expression, which I now use to flip between white and black in the second Convert command. The value of either 1 or 0 is stored in %j and reused to generate the rgb percentages in the final Convert statement.

The statement
FOR /F %%x in ('statement') DO ...
is DOS's crude way to reuse the result of a command line ...