Best background for cut outs Often I create a 3D scene in order to save the 2D image and do some post-production in Photoshop. Also in some cases the final result (after post) is an Isolated on White image. What is the best way to do this ? Should I create a kind of Infinity wall (Cyclorama) in Blender and use that as a background ? And making it easier to judge the image and also maybe the easiest for post-processing ?>

Like this image below. I created a plane as background (and the World is black). But cutting out the dices would be a lot easier when I would not have this background. By the way ... I want to keep my shadows (because I need those for the final image).
Likely in your situation, you would want to render the dice as a foreground layer, the floor with shadow as a separate layer, and then turn on transparency in the render settings. That way you will get transparency where the black is now.

In Photoshop, you can add a background colour layer with a gradient and blend it into the floor layer easily. No need to render something simple like that.

Render Layer Settings

Please note that the blocks are on Layer 1 of the viewport and the Floor is on Layer 2. In the render layer panel, we select Scene Layers 1 and 2, and then assign Layer 1 to Render Layer 1 (named Blocks) and Layer 2 to Render Layer 2 (named Floor). Make sure the Combined pass is enabled for both layers.



Enable Transparency



In the UV Editor, select Render Result, then you'll have the option to switch between the 2 rendered layers. You can save out each layer as a PNG file (RGBA).



Output Blocks PNG (has alpha channel, though not obvious on SE page here)



Output Floor PNG



Bonus Tip
If you were finding you were re-rendering the image a lot, you could enable compositing and add File Output nodes to write the PNG's to disk as part of the render process. I can detail that process if need be.