I rendered my cover entry at full size too, it does indeed make post work easier, I used RGB in post so that I could use all the pshop filters, I have the same problem with my cover entry as with the image above. The CMYK TIF file looks great in pshop and pants in everything else :-( Note with the above sample all I've done in this session of each program is load the same CMYK tif file.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.