How can I compare different renders?

I am rendering an object and am making changes and then rendering again. Is there any way to compare my different renders. By compare I mean just quickly swap between the different renders so I can see the minute changes. My interest in is in this to be quick way to judge my last change.

Saving the individual renders to a file and then using an image viewer would allow me to do this, however it is tedious and time consuming. Is there a way to do this in blender?

• One common way is using render slots.. but there are up to 8 of them and they tend to end up quickly. It's kind of possible to create linked copies of the scenes and use compositor with Backdrop feature to connect render of desired scene to the Viewer node but it's a bit tedious as well. – Mr Zak Jul 5 '17 at 22:44

In the UV editor, you can select different render slots. Select Slot 1, for instance, render... then select Slot 2, render again. Switch back and forth between the slots to compare using J and Alt + J

EDIT: Only adding by request... in addition to using J and Alt + J, Lukas also pointed out in his answer that the number keys at the top of the keyboard (not Numpad) can be used to select the slot to view.

It should be noted that pressing J will auto-cycle the renders. If you only have renders in slots 1, 4, and 8, it will only cycle those slots. So it can be handy.

• You can also press number keys 1-8 to go straight to a specific render slot. – sambler Jul 6 '17 at 7:54
• Can you add the keyboard shortcuts to the answer @bertmoog? – Vader Jul 6 '17 at 10:45
• @sambler - Thanks, but Lukas already pointed out the number keys. I offered the alternative (J) for when you're stepping through several renders and you don't want to keep up with which number key you're pressing. – bertmoog Jul 6 '17 at 14:59

You can use the slot feature in the UV image editor. Render your first image, then in the editor switch to slot 2. Press esc, make your changes and then render again. Now switching between slot 1 and 2 will toggle between your renders.

You can use the top row number keys when in the UV image editor to quickly change slots.

You could of course render them to files and upload them to comparison websites like this one online-image-comparison.com or using Desktop/CLI-Tools.