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Blue is a music composition environment for use with Csound. Blue interacts with Csound by generating .CSD files, which it then feeds to Csound for compilation. Any version of Csound that can be called by commandline is able to be used with Blue.

One of Blue's main features is the graphical timeline of SoundObjects, of which a special SoundObject-the polyObject-is able to group other SoundObjects. polyObjects feature their own timeline, allowing for timelines within timelines. SoundObjects can be anything that generates notes, be it a block of Csound SCO, a script in python, or a graphical object.

SoundObjects may further have noteProcessors added to them, which may do things like "add .3 to all p4 values" or "of these notes, only generate the first three". NoteProcessors are especially useful when used with Instance SoundObjects from the SoundObject Library.

The SoundObject library allows for working with SoundObjects by making instances of a SoundObject. Instances point to a SoundObject, and when they generate notes, they use the SoundObject they are pointing to to generate the notes for them. Instances however can have their own properties which they will apply to the generated notes, so that they can be of different duration and have different start times, as well as have different noteProcessors used with them. the advante of instances versus manually copying a SoundObject over and over again is that if you want to change the notes for all of these SoundObjects, you'd have to manually go and change all of them, while with instances, you'd only have to change the one SoundObject all of the instances are pointing to. (which also means you could make a song template file, all pointing to empty SoundObjects, build the form of the piece, then change the SoundObject's note material to "fill in the form".)

Other features include the orchestra manager, which allows for importing of instruments from .CSD files, a list to manage instruments, and the ability to selectively generate instruments when compiling .CSD files. instruments can be either a single Csound instrument or may be a GUI instrument.