SCons - convert all images in a directory

Question

Hi, I'd like to write an SConstruct file that will convert (e.g.) all the JPEG files in a directory into PNGs.

I think I have the Builder alright:

ConvToPNG = Builder(action = 'convert $SOURCE $TARGET',
        suffix = '.png',
        src_suffix = '.jpg')
env['BUILDERS']['ConvToPNG'] = ConvToPNG

But then I'm not sure how to make a list of all the targets that need to be built. I can do it in a Python-y way like this:

pix_conversions = [env.ConvToPNG(jpg_src) for jpg_src in Glob('pix/img_*.jpg')]

And then maybe I'll use an Alias for those:

env.Alias('convert_all', pix_conversions)

Or else make a Node by hand and have it Depends on the conversions.

The thing is, this seems like it's something that Scanners are sorta made for. But I couldn't get that to work well. I tried making a Builder that would work on a directory and call a Scanner that would add a bunch of targets, but it didn't work because the SCons internals expect a Builder to run on a file. (or at least, that's what I gleaned from the error messages).

Is there a more SCons-y way of doing this?

Answer 1

Scanners are for implicit dependencies.

e.g. you want to build foo which depends on foo.c which does a #include of bar.c. Thus, when you tell SCons to build foo with dependency foo.c, the C scanner will automatically pick up implicit dependency bar.c based on the explicit dependency foo.c.

Answer 2

Your steps seems fine, but Alias node you need to pass to the AlwaysBuild function:

env.AlwaysBuild(env.Alias('convert_all', pix_conversions))

So the end result would be:

ConvToPNG = Builder(action = 'convert $SOURCE $TARGET',
        suffix = '.png',
        src_suffix = '.jpg')
env['BUILDERS']['ConvToPNG'] = ConvToPNG
pix_conversions = [env.ConvToPNG(jpg_src) for jpg_src in Glob('pix/img_*.jpg')]
env.AlwaysBuild(env.Alias('convert_all', pix_conversions))

It will work fine without any scanners. SCons will check MD5 hash of your JPEG files and if some file changed since last conversion then builder will be launched again.