Composite Nodes File Output Question/Suggestion

I’ve been working with compositing nodes and I recently thought of a good method for solving a problem I have. I had never used the “file output” node before. But I thought this would be the perfect time to try it.

The problem: I have an animation of crumbling bricks. I am seeking to add dust after the fact. I am not using particles. I am only adding smoke in post.

The solution: In the compositor, I added two image inputs. They are both from the same animation, just offset by a frame. So at any give point I am comparing frames 1 and 2, 4 and 5, 11 and 12, etc. I subtract one frame from the other and then boost the difference so I have a matte of what changed from one frame to the next. All I have to do now is change the color of the matte to my dust color, blur it, and lay it over the top of my animation. This works alright but I want the dust to linger. It shouldn’t be there one frame and be gone the next. To solve this problem I decided to use the file output. I would start with a single image and modify it for each frame, then save it back - basically using the image on disk as an accumulation buffer. This method would save a lot of time vs. writing imagemagik scripts by hand. So I got everything set up and…

It doesn’t work.

The file output node increments the file it saves to. So even though I typed in “dust.png” as my output file, it saves to “dust017.png.” So, is there a way to force this node to save the same file no matter what? If not, could that be implemented? Just a toggle to save to the same file instead of adding the frame number?

This seems like a useful idea; you explained it well.
Perhaps you’d like to post that feature request here:
http://www.blenderstorm.org/

Thanks egan. I tried the link, but can’t get it to load. I’ll try again later.

For anyone persistent enough to read this far, I found a clumsy work around. I can achieve what I wanted (and it looks quite good). Instead of using the single accumulation buffer, I output all the temporary images into a folder. You can set an image input node to read the previous image from the folder, then modify your image and save it back out as the current image in the folder. This is rather awkward, and my request still stands. But this does work, and can be used to great effect.

Forgive my laziness at testing this myself but I understand the Time node should be able to help you with this – without writing out files… http://wiki.blender.org/index.php/Manual/Compositing_Nodes_Input#Time_node

I don’t see how the time node could accomplish this. Perhaps I’m not thinking enough outside the box. But I think the ability to write out to the same file would be a great addition anyway.

Egan, I’m still unable to get that link working. I’ve tried several times over several days. Is the site down?