Improving video quality
How do I get better looking video is a common question and, as usual, the answer is not one thing but a collection of factors that have to be optimized. The software cannot compensate for bad video. A blurred image is usually the result of incorrect camera settings, bad lighting, or both. Read on and see how you can get the best out of your system. Key points to keep in mind:
- Don’t expect HD TV broadcast quality.
- Look at the different options for getting video from the camera to the computer. HDMI is the future and is here now.
- Lighting and camera setup is critical. As a guideline for indoor video capture:
- Walking cycling: At least 500 LUX and 1/250th of a second.
- Running: At least 1000 LUX and 1/500th of a second.
- Sprinting, tennis, golf: At least 2000 LUX and 1/750th of a second.
- Make sure the focus is set.
- Make sure the camera is level and square on to the action.
- Form fitting clothing on the subject.
Camera to Computer Connection
You must use an AV to USB converter. It will also degrade the signal a bit. We are stuck with this method for now as Mini DV FireWire cameras are no longer available.
High Definition (HD)
One of the issues with HD video is that is that it extremely compressed so when you want to step the video frame by frame the software has to decompress each frame of the video which can lead to slow responses and performance unless you have a very highly spec’d computer. If you are using High Definition (HD) cameras then you have 3 options:
- You can transfer the saved video files from the camera to the computer (via USB) where they will need to be converted for frame by frame1 playback (but it still can be HD).
- Live feeds need to be converted down to Standard Definition (SD) to be passed through the AV to USB converter.
- HDMI output and capture. This is the future method for getting get HD footage from the camcorder to the computer and is available using some siliconcoach software right now. In Siliconcoach Pro is still needs to be converted for frame by frame play back1 but Pro will retain HD dimensions. The HD option needs a very fast and precisely spec’d computer.
If you use fisheye lenses they distort the image a bit and the software that un-distorts the image causes quality loss as well. You cannot use de-fish with HD.
Lighting is a critical and often overlooked part of your setup, especially when capturing indoors. The quality of the light greatly influences the quality of your image.
Light intensity is measured in LUX (see
) using a light meter. These LUX meters are not expensive (about $50 USD) and that is all you need and not the more expensive light meters professional photographers use.
When you measure the LUX you must put the meter
next to the segment of interest and
point it directly at the camera, any deviations will give incorrect results.
Light disperses as is moves away from its source so a light rated at 500 LUX at 1 meter will be less than 200 LUX at 2 meters. When you are purchasing or hiring your lights you need to take into consideration that the lights will be some distance from the subject.
- Walking and cycling: At least 500 LUX measured in the camera’s field of view at the distance the foot is from the camera.
- Running: At least 1000 LUX measured in the camera’s field of view at the distance the runner will be from the camera.
- Sprinting, golf, tennis: At least 2000 LUX measured at the foot/club/racket. (Note this is a lot of light).
Type of Lights
Good lighting does not always mean the most intense lights; a few well-placed lights can work just as well. Intense focused lighting will cost more in power, create harsh contrasts in the video image and will also create a lot of heat in the area.
You need lights that:
- do not flicker as the image will flicker on the video clip as well. Fluorescent lighting can cause a lot of flickering.
- shine as closely as possible to white light. Some lights shine light with a yellow or a blue hue.
Overhead lights are of some use but generally if you are interested in the subject’s lower limb then it will be shadowed by the upper body when the light source is in the roof. Lights just a bit above camera height are better but make sure they don’t shine in the subjects eyes.
Studio lighting is very expense however LED panels are now available and offer an affordable lighting option that does not flicker and usually can be set for indoor or outdoor coloring.
If possible use two light sources; one each side of the camera focused on the subject at about 45 degrees. This will light up both sides of the subject and not just the surface directly facing the light. If you only have one light then try it closer to the camera but you may get bright spots on the front surface of your subject with the sides much darker.
The lights need to create a diffuse lighting pattern that does not create shadows or light burn (where the lights cause parts of the skin to look ultra-white on the video).
If you are in doubt about what you should do you should contact your local photography or video expert but make sure they realize this is for movement analysis where the key element is clarity while the subject is in motion and not color tones etc.
Be careful not to have any lights shining on highly reflective surfaces; it causes very bright spots on the video image and the camera will darken the whole image to try and reduce the light ‘burn’.
If you capture poor video there is very little the software can do to improve it so you are left with an unclear or distorted image that will greatly decrease your accuracy. Modern cameras are very good at automatically adjusting to different light conditions but there are a few of things you can do to improve the image quality.
The first thing to check is that your camera’s and the software’s aspect ratios are the same. The software and many camcorders can be set to normal screen (4:3) or wide screen (16:9), if they are different the image will look distorted. For example people could look too tall or too wide and wheels will look oval. If you see this then go into
Capture Settings and change the
Video Aspect Override to something else and see what your image looks like. If it look ok then you are done, if not repeat the process and try the other
Video Aspect Override setting.
A fast setting means the shutter is only open for a very short time; this means there is less subject movement while the shutter is open and therefore less blurring. If your image is blurred it is not a function of the software but a function of the camera settings and lighting; you cannot correct for blurring in the software.
The disadvantage of a fast shutter is that there is less time for light to get onto the camera sensor and this causes dark images. To compensate for this you need lots of light as described in the Lighting section below.
If you can set the shutter manually you should set it to
- 1/250th of a second for walking or cycling
- 1/500th of a second for running
- 1/750th of a second for sprinting
Some cameras use a setting called Sports Mode instead of a manual shutter setting. Sports Mode gives you the fastest shutter speed it can for the given light, therefore to increase shutter speed you need to increase the light.
See your camera manual for more details on setting the Shutter or the Mode.
Frame rate is the number of images the camera takes per second (fps), the faster the frame rate the more images you will get to describe your motion.
Unfortunately you will not have much control over this when using standard camcorders as they only take either 30/60 fps (NTSC: USA, Canada, Japan) or 25/50 fps (Pal: New Zealand, Australia, Europe). Some cameras (e.g. Casio Exilim) can do high frames rates (over 200 fps) and then you can often import them into motion analysis software.
The focus control is usually able to be set to manual or automatic. Automatic will usually work fine except when someone walks between the camera and the subject and thereby changes the focus for an instant. This may occur if you have your camera mounted some distance from the subject, such as across a walkway. In these circumstances it is better to set it to manual, zoom in and then manually adjust the controls to get a clear image.
See your camera manual for more details on setting the focus.
Demo mode on camcorders usually starts up about 10 - 20 minutes after starting the camera and you will see the screen start to do all sorts of strange things. For example it may start shaking, the image make go black and white, then sepia, then blurry, then shake again and so on. You do not want this happening when trying to video a subject but unfortunately most cameras come with the Demo mode turned on.
See your camera manual for more details on turning Demo mode off.
Our eyes are very good at adapting to different lighting conditions such that when there are many shades of near-white we perceive them as pure white. Cameras however are not so accommodating and different light conditions such as outdoor, fluorescent, halogen, all give slightly different shades of near white. Sometimes the video images look yellower (warmer) or bluer (cooler). Your camera probably has auto adjustment to accommodate for this but you can help it by telling it if the video situation is indoor or outdoor. Some cameras even allow you to calibrate them against a white surface.
See your camera manual for more details on setting the white balance.
Make sure you work out which angle the event/s of interest should be best captured. The plane of movement will need to be perpendicular to the camera (i.e. parallel to the camera lens) if you intend to make measurements from the movie. You also need to decide if the whole event be captured without panning the camera. Panning introduces perspective errors that will make any measurements of speed or distance over a time period inaccurate.
Generally the camera should be at approximately half the height of the movement of interest. If you are looking at the whole body it would be about hip height, although for a tennis or volley ball serve it would be higher as the athlete will have their hand above their head. If you were just looking at the lower limbs, the camera should be about knee height or someone on a bike it would be about top tube height of a medium bike in its indoor trainer.
You do not need to change the camera height for each customer in a bike sitting studio.
The camera must be level and at right angles to the subject. It is critical that the cameras are level in all planes, especially if you are using a fisheye lens as it magnifies any error. When you use the drawing tool to draw a horizontal line it assumes the background is also horizontal. You could quite happily make measurements and draw reference lines in the software, but if the image in the background is not correctly aligned with the camera your conclusions will be wrong.
If you want to take distance or speed measurements off the video you will need an object of a known length (calibration scale) in the same plane as the movement and the same distance from the camera as the movement.
Is the person being videotaped wearing clothing that will obscure their actions or limb end point positions? This may be floppy tops, long baggy shorts and skirts or long hair. You need to have the subject wearing form fitting clothing and have all other distractions removed or secured out of the way.
Marking-up the Subject
If you are going to take distance or angle measurements off the screen you can improve your accuracy by marking-up your subject. This could be black dots drawn on their skin or on tape and then the tape stuck to the subject. Usually the ‘dots’ will represent joint centers.
It is important that the background the camera sees is as neutral as possible; do not use anything reflective such as glass. A plain grey, blue or green background is best, try to avoid black or white as they create a lot of contrast and may make your image too dark or too light. Also try to avoid a lot of ‘visual noise’ in the background (e.g. other people, shelves, lots of pictures, etc.), it will cause confusion and make it harder to find the key points on the body when you are doing your analysis.
Cables can be dangerous and where possible, cables should be secured and covered with a rubber mat or something similar. You will need power cables and long AV/USB/HDMI cables (these connect the camera to the computer).
There is also an excellent series of video clips made by the Biomechanics Teaching Team at the Manchester Metropolitan University that can found for public viewing on YouTube at the following locations:
Lighting and Space
Aperture and Lighting
Focus and Field of View