Export unfinished image render My render is "almost" complete... 



It took almost 5 hours to render the center object, and from now on Blender will render nothing but white pixels of the Emission background all the way to the edges.

According to its estimations, Blender is going to spend 2 more hours rendering all the white background:



I don't need Blender to do this; I could easily bring the current data into an app like Photoshop and flatten to get the background to be white.

But I cannot save the image in its current state:



I cannot take screenshots and combine them either - it would take too long at 100% (Image is over 8000 pixels wide).

I'd like to restart my computer, or start another Blender project, etc... and would like not to have to wait 2 hours.

Can I save the current, unfinished state of my render as an image if the elements I need have already been rendered?
Yes, you can.  You will simply click the X beside the Render % or press Esc to cancel the render.  Then press F3 as usual and it will save the rendered portion to your desired format/location.  The unrendered part will be black if saved as a JPEG and transparent if saved as an Alpha PNG.

Just to prove it ;) here you go:



Here's the canceled render of the default scene saved as a JPEG:





EDIT: As was mentioned, Render Border can be useful in certain circumstances.  The problem is it's always rectangular (so it wouldn't have helped in this situation with a circular object) and it's usefulness is relegated to instances where you use multiple render layers.  Otherwise, you can simply crop the render dimensions as close to the edges of your scene as possible which does the same thing.