Article
Noise reduction in ALTRON: line and frame averaging and noise filters
Nothing is free in noise reduction. Every method reduces noise and takes something in return: line rate, fine detail or fast response to motion. Our job is to pick the cost your application can afford.
Keep one basic rule in mind. Random noise falls with the square root of the number of samples. Averaging four samples halves the noise, and averaging a hundred reduces it to a tenth. That is why most of the tools in this article are some form of averaging.
For line and frame averaging, ALTRON has a node called Smoothing. You can use it with both line scan and area scan cameras. It contains three stages, each of which you switch on separately. The order of execution is always fixed: Bundle Average, then Box Car, then Continuous Average. Each stage works on the output of the previous one.
This stage is for line scan cameras only. It averages every N consecutive lines and outputs one line. You choose N between 2 and 10000. The output line rate drops by a factor of N.
Where is it useful? When the camera is much faster than you need. Instead of throwing away the extra lines, you use them to reduce noise. Just keep the geometry in mind. If the output becomes four times slower, the camera's line rate must rise by the same factor. Otherwise the image is compressed in the direction of motion.
This one is also for line scan cameras only, but it works along the line itself. Each pixel is averaged with N neighbors on each side, giving a window 2N+1 pixels wide. N is between 1 and 100. Because the information comes from neighboring pixels rather than from time, it works on a still scene too. The cost is lost detail along the line, and edges become softer.
This stage works for both camera types. Each output line or frame is the average of the last N lines or frames. You choose N between 2 and 1024. The rate stays the same and noise goes down. In return, anything that moves leaves a trail N lines or frames long behind it. It is excellent for still scenes or lab measurements. For moving objects, keep N small.
TNR-Temporal Noise Reduction: This filter is for 8-bit or 16-bit monochrome images from area scan cameras, and is mostly used for thermal images. Each frame is blended with the previous filtered frame. The parameter a is the weight of the new frame, between 0 and 1. A smaller value gives less noise but leaves a longer trail behind moving objects. A value of 1 means no effect.
SNR + DDE: This is a guided filter. It splits the image into two layers: a base layer and a detail layer. It then reduces noise in the detail layer and enhances the detail itself. It works on both line scan and area scan cameras. The parameter k sets the smoothing strength. Df sets the enhancement of fine detail, and Dm sets the enhancement of medium detail. For color images, Luma mode is about three times faster and does not add color to edges.
- Line scan camera with spare line rate: Bundle Average.
- Still scene or lab sample: Continuous Average.
- Noise along the line, fine detail not important: Box Car with a small N.
- Thermal image with slow-moving objects: TNR with a medium a.
- You want to see detail but not grain: SNR + DDE.
- Using software noise reduction instead of correct exposure. If the image is underexposed, fix the exposure and lighting first. No filter can bring back a signal that was never recorded.
- Using Continuous Average with a large N on a moving belt, then wondering why edges are blurred in the direction of motion.
- Forgetting Set in the TNR and SNR + DDE panels. In these two filter panels, changes are not applied until you press Set.
Turn the stages on one at a time and check the line chart or image after each one. When two or three filters are on together, you can no longer tell which one ate the detail. Start from the lowest value and go up only until the noise is acceptable, no further.
To learn more about image noise, see Wikipedia article (English).
To see the cameras these tools work with, visit the ALTRON page. If you're not sure which method suits your production line best, get in touch with us.