隔行扫描
维基百科，自由的百科全书
隔行扫描是一种将图像显示在扫描式的显示设备上的方法，例如阴极射线管(CRT for Cathode Ray Tube)。 在同样帧率的情况下，这种方法比起逐行扫描引起的视觉闪烁比较小。扫描设备交换扫描偶数行和奇数行。在PAL制式和NTSC制式中，都是先扫描偶数行，即偶数场。
目录 |
[编辑] 描述
非隔行扫描的扫描方法(即逐行扫描)通常从上到下的扫描每帧图像。这个过程消耗的时间比较长，阴极射线的荧光衰减将造成人视觉的闪烁感觉。当带宽受限，以至于不可能快到使用逐行扫描而且没有闪烁效应时，通常采用一种折衷的办法，即每次只传输和显示一半的扫描线，既场。一场只包含偶数行 (即偶场) 或者奇数行 (即奇场) 扫描线。由于视觉暂留效应，人眼不会注意到两场只有一半的扫描行，而会看到完整的一帧。
假设我们使用直接驱动的CRT显示器，那么如果不使用隔行扫描，就需要采用下面的方式之一:
- 将传输带宽加倍，按帧而不是按场传输图像。这能够提高一点图像质量，提供的有效分辨率和闪烁速率是相同的。
- 使用相同的传输带宽，按帧传输分辨率为原来一半的图像。这时候图像细节较少了，闪烁速率仍旧相同。
- 使用相同的传输带宽，按帧传输图像，但是帧率为隔行扫描场率的一半。这时闪烁速率降低一半，眼睛非常容易产生疲劳的感觉。
- 和前一个相同，但是使用一个数字缓存将同一帧显示两次。这时闪烁速率相同，但是运动看起来会不是那么平滑，影响视觉质量。
通常有一种误解是，偶场和奇场是由同一帧分拆得来的。实际上，摄像机采集的方式和隔行扫描显示的方式是完全相同的。当摄像机采集图像时，偶场和奇场不是同时采集的。例如在一个每秒50场的摄像机中，第122行和124行的采集在第123行和125行的采集大约1/50秒之后进行。所以如果把一个偶场和奇场简单的拼合在一起，水平方向的运动会造成两场边界上不能完美的拼合。
In an interlaced system, lines are drawn diagonally such that the right end of each line is two lines lower than the left end. The offset between the two fields is then produced by having both an odd number of overall lines and vertical flyback between the odd and even fields occur halfway through one line. For example, in PAL, the blanking period starts after 292.5 lines of the odd field have been transmitted, and lasts for 20 lines. When scanning begins again at the top of the screen, the scanning beam is still halfway across the picture. Because of the slant, the centre top of the picture is one line above the line begun at the top left corner.
In modern monitors and television sets, interlacing is being slowly superseded as the refresh rate of non-interlaced displays increases beyond the level at which flicker can be detected.
[编辑] 广播电视
The early televisions in the 1920s were unable to refresh at high speeds. Instead they were limited to 30 or 25 frames per second. This could potentially cause flickering on the screen. Interlacing solved this. Each frame is split into an odd and even field. When played back on a television, the tv would project the odd field followed by the even field. When an image was moving, then one of the fields would be offset from the other. The image, however, would look smoother than if it had merely shown a single frame.
For example, the PAL television system is based on displaying 50 fields per second, and as two fields contain the full resolution, it corresponds to 25 full video frames per second. Interlaced video is often used as a tradeoff between bandwidth requirements and smooth reproduction of movement.
Interlacing is used by all the analogue TV broadcast systems in current use (mainly NTSC, PAL and SECAM).
[编辑] 缺点
The major disadvantage of interlacing is the reduction in vertical display resolution. Consider a TV image made of alternating white and black horizontal lines, each line being only one TV line high. Instead of a fine black and white striped image, an interlaced display would show a black and white flashing image at the frame rate of the video. This is the most dramatic example of an interference effect called twitter, caused when an interlaced image attempts to display vertical information at too high a spatial frequency.
Another disadvantage is that an interlaced signal cannot be displayed without artifacts on a non-scanning display device such as an LCD or plasma screen. To compensate for the problems, a computationally expensive deinterlacing process is needed.
[编辑] 计算机文件格式
Interlacing is done on some GIF and PNG images (that have been specifically designed to be viewed remotely) so that the viewer on a slow communications line can see what the image looks like before it is finished loading. GIF is interlaced by sending the lines in the order 0, 8, 16, ..., 4, 12, ..., 2, 6, 10, 14, ..., 1, 3, 5, 7, 9, .... PNG is interlaced using the Adam7 algorithm.
[编辑] 参考
- Progressive scan: the opposite of interlacing; the image is displayed line by line.
- Deinterlacing: converting an interlaced video signal into a non-interlaced one
- Telecine: a method for converting film frame rates to television frame rates using interlacing
- Federal Standard 1037C: defines Interlaced scanning
[编辑] 外部链接
- 100FPS.COM - Video Interlacing/Deinterlacing
