I would love to see “Slow Speed” features in Kdenlive ..
Everytime I get an editor to myself I ask if it's possible to do “Real” changes of FPS in ANY editing software and once I've explained what I want to do (which takes about 10 minetes) they tell me that there SHOULD be a way to do it but the only way would be very long winded and involve about four different programs ..
The only way to explain what I want is to explain how we got here .. by the convergence of video and film technologies ..
With a film camera the frame rate depends pretty much on how fast you turn the handle. So varying the frame-rate is easy .. you turn the handle (or motor) faster or slower. Once you start to get involved with modern film cameras and especially modern film cameras with a motion control rig .. you can do all sorts of funky tricks .. ramps from 1fps to 50fps .. seamlessly and with a compensated aperture or shutter ..
With Motioncontrol you simply control the camera as an extra axis on the rig .. so regardless of the camera speed the images will line up perfectly ..
For example .. you set up a move with a car in the foreground. You set the rig to run at 25fps and thus drive the camera at 25fps. For the first pass you put a blue screen behind the car then you run the pass (say a track) with a man walking past opening the door and getting in ..
Now you pull out the blue screen reset the rig to 1fps (or even less) and thus the camera to 1fps (lets forget the aperture pull for a second) now you shoot a second pass with people crossing behind the car.
When played back the second pass will look like ghosts and the two passes together will look like the driver is real but the people surrounding the car are ghosts ..
When they first started to make video cameras that ran at different speeds the camera manufacturers did not (and to a large extent still can't) make the camera run at anything except a fixed speed. Early camera like the Panasonic Varicam would record continuously at 60 fps and then apply a time code flag to each frame it would use .. The disadvantage of this setup was that the shutter speed was preset at 1/60 sec so if you shot at 1fps then you got a frame with a 1/60th sec exposure as opposed to ½ second and as you can imagine this would have made the shot outlined above look totally different ..
The obvious answer would be to shoot at 25fps while running the rig at 1fps and retime the second pass in post .. it will not look like film .. but it will look pretty close ..
The problem with doing that is that the shutter phase will be incorrect and the shutter angle will be 360degrees which is not necessarily what we what ..
What we need is a “Proper” slow speed converter. For instance ..
We shoot our first pass with a 180 degree shutter the rig is synced to the video camera so top of the “shutter fully closed” appears in the same place each time .. the rig also drives a “sync clock” which is put on the board .. either a stepper motor driven one or an electronic one .. See this video that shows a sync clock .. http://www.youtube.com/watch?v=IJt6UieDo6s (this also shows the action of the rolling shutter .. it should at 1/50 sec be exactly 1/2 of the image ..
Then we shoot a second pass also with a sync clock .. the sync clock will now instead of showing the phase with move over several frames ..
So for instance if the first frame of the 25fps move has the sync clock running from 3 o'clock to 9 o'clock then then for the “extraction frames” we need the 12 frames (or 13 depending) where the clock is pointing from 3 – 9 O'clock. The twelve frames we have chosen are then added together to make one new frame. We then need to totally discard the next 13 (or 12) frames (because on our original 25fps pass the shutter was closed). And then start again ..
Of course once you have made the interface to do that you can adjust the shutter angle .. obviously not infinity but to match the frames required .. so for instance if you want 1fps (assuming a 25fps finishing speed) a 180 degree shutter angle is impossible .. but a 172.8 shutter angle or a 187.2 is very simple .. you could also widen out the shutter angle to say 288 with no problems.
It looks and sounds a very semantic difference .. but I don't think it is ..
I'm aware that this can be a difficult idea to get your head round .. so please ask enough questions until I have explained it properly ..