I wanted to simulate in Blender that kind of flash photography where you leave the camera shutter open for a relatively slow time and then freeze the movement with a flash (either synch’d on the first or the second curtain, to use SLR terms and tech).
For example, I am referring to this type of photography: https://www.digital-photo-secrets.com/tip/1414/slow-sync-flash-1st-curtain-or-2nd-curtain/, https://neilvn.com/tangents/first-curtain-sync-vs-rear-curtain-sync/, https://www.imaging-resource.com/news/2018/10/08/what-is-the-difference-between-first-and-second-curtain-sync
As far as I know, with Cycles you cannot simulate motion blur, therefore I had to fake it; I also had to fake the flash that freezes the motion. This is what I did:
- I created a simple animation, occurring during a very short period of time, to have several frames where the moving object is every time in a different position. This gave me a few frames to work with.
- Because Cycles is slow, I had to use fast – and therefore noisy – renders with few samples.
- To reduce the noise, I resorted to two techniques: in the parts of the image that wouldn’t change, I reduced noise by simply averaging all the layers in Photoshop, showing a bit of each, so that noise, which is stochastic, would cancel itself. To ensure stochastic noise, I manually changed the “seed”in the renderer for each frame.
- To reduce the noise of the moving parts, I blurred each of them with the Motion Blur algorithm in Photoshop, and then averaged them, after masking, again in Photoshop.
- Finally, I re-rendered the initial (or final) frame, with a small area light added, to simulate the flash. For this frame I increased the quality by increasing the number of samples, because, just as in real life, the frame with the flash has greater importance.
These are two examples of simulated flash photography that I created: