Interesting point, Kentbulza. I believe 16 fps is the correct number, but it may change due to circumstances. As for the persistance of motion, we talked about it some months ago and here's some of my main points:
It is a belief that many people still refer to, that retinal persistance is the effect causing the experience of motion. It is, however, not the case. A more accurate way of describing it would be mental persistance. Science does not as of yet come with a complete explanation of how the eye percieves motion in film viewing. The main belief, however, is that we percieve motion represented on the screen in the same way that we percieve motion in the real world. Two different effects can cause the persistance of motion; the phi effect and the principle of highest probability. These are two different optical/chemical/nervous processes that somehow interrelate in percieving motion.
The Phi effect
The phi effect results from succesive optical stimuli that are fairly similar triggering a chemical response which is sent to 1) the autonomic nervous system where a rough categorization is made and 2) visual cortex in the brain where (for short) it is being decoded. This is a bottom-up process creating and imediate response linked to our instincts.
The optic nerve consists of two different cell types; one type reacting to permanent stimulation and one type reacting to changing stimuli. Therefore the visual system has two types of temporal responses; slow and rapid.
It is in the slow response cell that you find the retinal persistance.
The rapid response cells reacts to two effects important to viewing motion pictures; flicker caused by a frequency descrepancy between the projection speed and the projection light frequenzy (resolved quite rightly by rotating fans, increasing the effective projected image per second), and visual masking. The latter is the effect of a second light stimuli disturbing the perception of the first. This is effective everytime there is an edit.
The rapid response cells does also react to movement. The phi effect, which causes persistance of movement, is the effect of two lights close to each other flashed one after another giving the impression that the first light has moved to the second position (Wertheimer experiment 1912).
The insertion of black spaces inbetween each frame is neccesary to give the impression of continuity, since the black insertion eliminates retinal persistance by creating a visual masking. If the black space was removed the retinal persistance would cause the human eye to see nothing but blur.
The fans then have two purposes, reducing flicker and visual masking, making it essential in the perception of movement on the movie screen.
The principle of highest probability
The principle of highest probabilty is a top-down process, where shapes and images from the visual cortex are sent to the memory to be categorised in sent to the working memory (or short term memory). Somewhere along this process a choice is being made. This makes the process interlectual, based on inheirent experience. It is described in this image. You cannot in the middle images see the duck and the rabbit at the same time. You will have to choose to see the one or the other:
This is the effect causing the motion in the flip book principle. The visual cortex simply cannot see two things at the same time, so it will have to choose. In this way you do not see the flipping of the book and the motion of the drawn object at the same time, it's either or.
While watching a film the brain constantly shiftes between a two dimensional experience of the screen and a three dimensional experience of the objects represented on the screen. This effect is based on recognition of a range of allready know symbolic representation of objects in the visual cortex in the human brain and not by retinal persistance.
When you look at a flip book you will also see a certain visual masking. These are moments where the rapid sensory cells are in a sense reset. Remember that it takes some time to flip a page. In this time you will not see the drawn object flip on the page, your eye will search for the next recognisable image choosing what to see.
In other words, the phi effect is observable by a dog or tortousie but the principle of highest probability requires higher brain functions which only the human brain's neocortex is capable of.
When watching a movie in the theaters we do not 24 different images per second, but due to the 3 blade fan, each image is projected 3 times. An effective 72 fps. I believe this also has an effect as to how we percieve motion as more real. What I'm saying is that reducing flicker has greater importance than higher framerates. A 100htz TV should in that case be more realistic than 50htz (I'm in Pal-country), since the image is more stable, but motion is the same.
Roger, we have come to agree that there is several factors deciding on a realistic experience:
- 1) The size of the image
2) The speed of the object
Taking the phi effect into consideration, there is a certain length that objects can travel before we no longer see them as moving. We will at that point use the principle af highest probability. But if the object moves even further, then we will interlectually have to fill in the space. these are the three modes of experience from the two factors.
Yes, but the degree to which the brain has to fill in the missing info is the degree to which is appears "real" or "recorded". If you had a wall that was about 35-40 feet away with a small 24 inch window in it and, beyond that window, there was an NTSC interlaced HD set showing a nature setting, it would be hard for the eye to determine if that was real or a recorded image.[...]Hoever, if the frame rate of the HD image were reduced to, say 30fps progressive, it would be immediately obvious that this was a recorded image, even though the lack of depth perception due to distance was exactly the same. The reason is that suddenly the eye becomes aware of the increments between one object position and the next. Now, back away from the wall another 40 feet and it might appear real again because the increments are reduced.
In order to experience something as more or less "real" would demand that the viewer is conciously aware of the framerate and aware of the proces of filling in. It would mean that we were experiencing 24fps and interlectually linking them in the on-line experience. But we don't, we percieve motion through the automatic nervous system.
The eye has no conciousness
Theres has not been much academic theoretical discussions of ride films, but basically they work in a way that tricks our senses into believing we are moving. The framerate might help in this perspective, giving sharper images and less blur in the frame. And the realistic experience might come from a cloeser resemblance to objects in the real world.
This is not the case of videoscreens. They are often smaller and more blurry than film screens, due to interlacing. You will then need a higher framerate in order not to have the movement blurred into a big mash of colours. I believe the "real", "live" or "real-time" experience comes from other surrounding properties.
michael
