High-Speed Passive Autofocus Technique
| Autor: | Chia-Ching Huang, 黃嘉慶 |
|---|---|
| Rok vydání: | 2002 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 90 When applying the active autofocus technique, which has been used pervasively in digital cameras, the rangefinder in this case plays an important role in detecting the distance of the object from the camera and in helping the camera to stay in focus. Instead of using the rangefinder to help with focusing, the passive autofocus technique relies on the digitized image to compute the called sharpness measure and to help change the focal length or the distance of the object when searching for the point where the sharpness measure is maximized and the sharpest possible image is performed. By now, quite a few different calculations of sharpness have been proposed, yet none of them could work perfectly under any circumstances. Failing under low intensity of light, being confused with multiple layers, having little resistance toward reflecting light or simply being slow in calculation are all the problems posed in certain methods. Hence, a great time saver with wider range of application should be developed. Focus Searching Method also has an important part on the way of developing passive autofocus technique. Unfortunately, the existing searching methods have been too slow in finding the best focus point. For example, to improve performance of 3D''s measurement, we can increase the speed of the searching focus. Two sharpness calculations have been suggested in this thesis. One is Discrete Cosine Transform (DCT), which is derivative by using frequency transform; the other is YZU, which is derivative by employing the spatial mask and gradient method. Comparing to the existing calculation, both DCT and YZU can reach a wider range when applying, have a better resolution of the image, and have a greater capability in discerning objects with multi-layers. Yet YZU is more practical and outstanding when compacted with higher speed of calculation. The existing Binary and Fibonacci searching method need to be manipulated by reducing the searching field gradually and driving the lens back and forth around the focal point yet eventually getting closer and closer to it. In this case, High-Speed Dynamic Focus Searching Method is recommended in order to save the time of searching that can use under low intensity of light and multiple layers. A couple of searching methods have been derived from the combination of YZU and High-Speed Dynamic Focus Searching Method. Actually, the experiment has shown the combination to be faster and more accurate in focusing that can also apply in the object with multi-layers. It proved true that the combination of DCT, YZU and High-Speed Dynamic Focus Searching Method proposed in this thesis can complete High-Speed Passive Autofocus Technique. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
| Externí odkaz: |
|