No preview image showing up when I plug the Viewer node in. I read elsewhere that I needed to render the image first. That worked for a little while but now it doesn't work anymore.
2 Answers
Just toggle one of your nodes off and on again.
Viewer/Composite nodes refresh only when there's a change in the compositor.
You can use M on your keyboard to Mute/Unmute a node.
Or just disconnect and reconnect it.
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For future readers (like myself), also make sure you have actually rendered your layer(s) first. The compositor does not render them for you.
You will know you have a rendered layer when you see the thumbnail of the “Render Layers” node, just as you see in jachym michal’s answer.