Is there an easy way to access (and hopefully export) the rawest output from Cycles, namely the rendered image as RGB(A) floats in scene linear color space?
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$\begingroup$ "Scene linear" is not a colour space. Scene referred imaging is linearized, but is not a fully qualified colour space due to the fact that any RGB scene referred system can be linearized; it would require defining the RGB primaries to be a fully fledged colour space $\endgroup$ – troy_s Apr 17 '16 at 18:52
Yes!
You can export the image in OpenEXR format, it will save the information as scene linear. You can choose to save as Half Float (16-bit floating-point} or as Full 32-bit floating-point.
In the basic OpenEXR format you can save RGBA + Z depth information.
Using OpenEXR multilayer you can save all of your layers and render passes on the same file.
To save all of your layers you need a file output node and create the links to the layer pass information you want to save.
To set it up please read:
Save all render passes to a single EXR image
and
How can I render all render passes to files in Cycles?
More info on OpenEXR format here: http://www.openexr.com/