I came up with this a few years ago using Bryce.  Sometimes after a long render, you might find an object floating a half inch above a table, or you really need to add, move, or correct an item. Instead of re-rendering the entire scene again, I place this shield right in front of my camera, blocking out everything except the portion that needed correcting. Looking through the opening, I can stretch and scale the shield until it fits my needs. Just used it for the first time in Carrara to add some additional objects. Cut a five hour re-render down to 30mins.

The shield should be solid black with all material settings at zero. Rendering again at the same size, the black area will go a lot faster than the full scene. Afterwards, in your image editor, copy and past the corrected render over the original and delete the black area.

In the zip is an .obj file, (plus a .png thumbnail image and a .daz file for D/S users.) 9kb in size

Spot Render Shield

I hope this makes sense. The thing works for me and has saved hours of render time.

It was genius then, and it's genius now, and as useful a time-saver as it's ever been! I used to use your idea a lot for compositing with Bryce renders. Thank you for reminding me that this good old trick can work with other apps. :)

 I found out this trick in Carrara when I accidentily place a white cube (for reflection) in the render area, and saw how fast the rendering went. The area in the open space still does have te reflections of the whole scene, so that can pose a problem if you have to render higly reflective objects, then the shield could show up in the render.

But for that we could paste parts of the first render back or do some spiffy stuff in Photoshop or Gimp!

Ver good idea!!

Top tip, thanks!

As far as  the black shield showing up in a reflection, that can be a problem. I once did a render with a bunch of reflective spheres and after using the shield to do a spot render, everything looked fine. I finished postwork, then noticed each sphere had a tiny black square reflecting in it. You could duplicate the shield and make the one facing the scene reflective itself. Also, you might get a shadow from the shield in some cases. I don't even know if you can turn cast shadows off on an object, in Carrara.

Hi Frogdot, yes you can turn the shadow off in Carrara

Actually, if you made the shield such that it only has normals facing the camera... Doesn't Carrara have the option to not render polygon backsides? (Under view options in the picture in the post above.) If setup that way, wouldn't it not show in the reflection either?

Just pondering something...

Just testing... maybe could work