Dynamic image watermarking - injecting model attributes into watermarks - ruby-on-rails

Question

I'm currently using this Paperclip::Processor to watermark images with imagemagick's composite command.
It allows me to inject a pre-made single file as a watermark, which lives in my public file.
However, I'm trying to work out if it's possible to modify this to dynamically generate watermarks that include model attributes. For example, the watermark includes the name of a photographer, or the model of the camera it has been taken with.
Getty have recently changed their watermark to do just this - it's very clever, and I'm fascinated to know how they've done it.
Many thanks in advance. I'm not expecting people to know exactly how to do this, but any ideas or general principles would be appreciated.

Answer 1

Yes it can be done with Imagemagick; I have done it with php and a batch script. I have no idea how you would convert this batch script into ruby-on-rails though.
You can get most values contained in the EXIF data and use them in some similar code to this.
:: Do not display the code while running
#ECHO OFF
:: Select the F number from the EXIF data and set the FNumber variable
FOR /F %%x IN ('identify -ping -format "%%[EXIF:FNumber]" %1') DO SET FNumber=%%x
:: Set the FNumber1 variable to the F number value
:: Image Magick returns the F number in the format of 700/100 or 40/10
FOR /F %%x IN ('convert xc: -ping -format "%%[fx:%FNumber%]" info:') DO SET FNumber1=%%x
:: Set the value of the shutter variable to the shutter speed
:: Select the shutter speed from the EXIF data
:: Image Magick returns the shutter speed in the format of 810/100
FOR /F %%x IN ('identify -ping -format "%%[EXIF:ShutterSpeedValue]" %1') DO SET shutter=%%x
:: Format the speed to 8.1 and set the speed variable
FOR /F %%x IN ('convert xc: -ping -format "%%[fx:%shutter%]" info:') DO SET speed=%%x
:: Calculate the speed in seconds using the power of 2
:: and save it in the shutterspeed variable
FOR /F %%x IN ('convert xc: -ping -format "%%[fx:floor((pow(2,%speed%)))]" info:') ^
DO SET shutterspeed=%%x
:: Add the F number and shutter speed to the image
:: and save as exif_OriginalImageName
convert %INPUTFILE% ^
-pointsize 16 -fill black -gravity northwest ^
-annotate +10+5 "Aperture: F%FNumber1% Shutter speed: 1/%shutterspeed% sec" "%~p1EXIF_%~n1.jpg"

Answer 2

Thanks for your help Beerlington. I actually worked out a way of doing this by adding convert_options to the image during the paperclip thumbnailing process. So, on the image model, where a is the image:
has_attached_file :image,
processors: [:thumbnail],
styles: {
wide: {
geometry: "1120x",
convert_options: ->(a) { "-quality 92 -font Arial -pointsize 72 -gravity center gradient: -alpha on -channel rgba -fill 'rgba(255,255,255,0.3)' -opaque 'rgba(20,20,20,1)' -draw \"text 0, 340 #{a.picusername}\" -pointsize 30 -draw \"text 0, 390 'license copy here'\"
"}
}
This writes a.picusername on the image with various placement and styling details. You can find more of these here.
A final note - If you are adding attributes to the image model using something like simpleform, the above processing instructions are applied at the instant the image itself is added to the db... so any model attributes added after the image attachement (literally, in the form inputs below the image attach) will not be recognised, as they don't yet exist.