It means postwork is allowed, but if you can use as little as possible you should. So taking a render of a mountain landscape and then drawing 300 trees on it in Photoshop would be a bit to much, but dodging some SFX or adjusting brightness and contrast is allowed. There's no real rules on what 'minimal' is and it's a matter of taste. If the community decides that there's too much postwork to call 'minimal' they will say they think the entry can do with a little less postwork. But usually every entry can enter the contest. I have used some simple compositing in my (maybe still WIP) entry for instance to avoid artefacts where the volume atmosphere and the genesis effect interesect. I rendered the atmosphere seperate and added it as a new layer with adjusted contrast in PSP. Those are minor adjustments.

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