Quick And Dirty: Signatures Pt. 1 - The Command Line

by Mike Kost


After hours working with Povray and rendering your latest and greatest sphere on a checkered plane image, there's nothing more satisfying than marking your work with your illustrious signature. If you're new to the digital graphics world, this may be easier said than done. This first article in a two part series will demonstrate how to overlay a signature on your new rendering using the ImageMagick command line tools.

Quick Reading

A few light reading links, maestro:


ImageMagick is a software library and associated set of command line tools for manipulating digital images. Basic operations like resizing, rotating, or changing file formats (they claim to support over 90 formats - I'll take the word for it since the list is huge) are as simple as a few taps on the keyboard. Beyond the basics, the command line tools are able to do complex image creation including borders, composite images, .GIF animation, and text rendering.

ImageMagick comes by default with just about every Linux distribution, and is downloadable for Windows.

For this Quick and Dirty, I worked with ImageMagick 5.5.6 and in Linux.

Creating The Signature

Lets dive right in and create a signature. From the command line, invoke the following[1]:
convert -size 300x100 xc:black -gravity southeast -font symbol -pointsize 20 \
  -fill gray33 -draw "text 10,30 'John Smith 2005'" signature.png

And, ta da

Well, if you squint, you can see John Smith's weird signature in the lower right corner. That's great, but what did we just do? To figure that out, lets deconstruct what was just executed
'convert' is the ImageMagick command line tool
-size 300x100
This says to generate an image that 300 pixels wide and 100 pixels tall. This was picked arbitrarily, and just has to contain the entire signature. Smaller is better as long as the signature fits.
This indicates that the background should be filled black
-gravity southeast
This indicates that the signature is intended to go in the lower-right (southeast) corner
-font symbol
This indicates that we'll be using the Symbol font. To get a full list of all the fonts that are available in the system, run 'identify -list type'
-pointsize 20
This indicates the font size. A point roughly translates to a pixel under the default configuration.
-fill gray33
This indicates that we want to use gray33 as the color. This roughly correspond to a #545454 gray. Read on for why this color was chosen
-draw "text 10,30 'John Smith 2005'"
This tells the system to write the signature text (John Smith 2005). Because we're anchoring to the southeast, the coordinates are a bit wonky. As run, the ImageMagick places the upper right corner of the text (i.e. the 5) 10 pixels to the left and 30 pixels up from the lower-right corner. Because it's offset from the upper-right corner of the text, make sure to set the second number  greater than the pointsize, otherwise the text will be chopped off.[2][4]
This indicates the output file.

Applying The Signature

Now that we've got a signature, we've got to do something with it. I've generated my latest and greatest sphere on plane image from sphere_on_plane.pov and rendered it.
Sphere On Plane - No Signature

Now, to combine it with my previously generated signature, I execute the following
composite -compose plus -gravity southeast signature.png sphere_on_plane.png sphere_on_plane.signed.png

Again, what have we just done? Lets take a look
Again, this is the command line tool
-compose plus
This indicates that we want to take the two images and add[3] them together. The way the signature shows up is by adding it to the current image - effectively making the pixels lighter. This is why the signature was filled with gray33 instead of white. If white was used, the signature would end up white. By using gray33 (or some other small gray value), the underlying image shows through only lighter giving a watermark effect.
-gravity southeast
This indicates that the signature should be in the southeast corner. Because the signature is smaller than the rendered image, we need to tell composite where to position it. This command tells indicates to align it to the lower-right corner.
This is the file containing the signature
This is the file containing the rendering to be signed
This is the signed output

And finally, our signed creation
Sphere on a plane image with signature

Concluding Remarks

In the end, this tutorial was more an exercise in ImageMagick instead of Povray. Still, it should provide a good base for tweaking and establishing your own personalized signature.

Additional Reading

Notes and Disclaimers.

[1] - The backslash ("\") on the end of the first line is a continuation marker for the Linux shell. I did this for formatting reasons. This should be removed to get the command down to a single line.

[2] - A more general case for the x,y text coordinates should be "text offset,offset+pointsize 'Stuff'". The offset gives padding from the lower-right corner.

[3] - Although this example uses the 'plus' function, if your image is very light, consider using the 'difference' function. This makes the areas under the signature darker instead of lighter. This is a personal preference as well as artistic choice. In that case, the command line would be
composite -compose minus -gravity southeast signature.png sphere_on_plane.png sphere_on_plane.signed.png

[4] - Since writing the article, I tried it using ImageMagick 6.x. and found out that they corrected what looks like a bug. If you use 'southeast' gravity, the text position is referenced from the lower-right corner instead of the upper-right corner. The result of this is that you do not need to consider the high of the characters when positioning the text.

Published: 06/02/05
Last edited: 09/26/05

Copyright (C) 2005 Mike Kost