How do I force the output format?

[rmagick]

I'm using the MagickCore API.

Suppose I know I want the format of an image to be JPEG and I want to write the image to a file named "tmp/test.6543.0". How do I make WriteImage write the image in JPEG format?

Currently I store "JPEG" in the image_info->magick field and then call SetImageInfo, but the trailing ".0" on the filename confuses SetImageInfo into setting the image_info->magick field to "0". If I skip the call to SetImageInfo, leaving image_info->magick set to "JPEG" and image->magick set to the current image format, then WriteImage writes the image in the current format, not JPEG.

What am I doing wrong?

[anthony]

write the image to a filename with a 'format prefix. For example "jpg:tmp/test.6543.0". The prefix is not written, but overides any trailing suffix in the filename.

[rmagick]

Well, I don't really have any control over the output filename. It would be nice to be able to specify the output format via some other channel, like the image_info->magick field.

[anthony]

Just take the output filename, and copy it to a new string with the prefix, before using it. Otherwise insist that the supplier of the filename produces one with the right suffix!!!

I mean you really shouldn't have files with bad suffixes, even temporary files. It is bad programming and data handling.

[rmagick]

Thanks for your reply, Anthony. I don't think I'll have much luck persuading the Ruby folks to change the format of temporary file names, so I suppose I'll have to go the route of adding the format as a prefix to the filename. I want to think about what to do in the case that the program specifies the "JPEG" format and the output filename is "gif:foo.bar".

[anthony]

You should be able to use "jpg:gif:foo.bar" testing...

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   convert rose: jpg:gif:foo.bar

It created the file gif:foo.bar

It is only some special output formats (like histogram: ) that would look for another prefix after its own.. EG: -format '%c' histogram:info:- See http://imagemagick.org/Usage/files/#histogram

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file gif:foo.bar

Shows it is indeed a JPEG image file!!!

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identify gif:foo.bar

identify works too.

Looks like image input first trys the given string as a filename before looking for a format prefix hint! to define the type of file.
Of course you can fix that by using file redirection
In shell this is...

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identify - < gif:foo.bar