Interlaced scanningEach frame is divided into two fields, each containing a frame of all the odd scan lines or even scan lines, usually first scan odd lines to get the first field, and then scan even rows to get the second field. Whether progressive or interlaced, there are three concepts for video files, transmission, and imaging, and these three concepts are interlinked but not identical. The first appearance is interlaced scanning, and the matching produces interlaced transmission. Interlaced video files are in the digital video era only to appear, the purpose is to be compatible with the original interlaced scanning system (interlaced scanning is still widely used). Usually the display is divided into "interlaced scanning" and "progressive scan" two scanning methods. Progressive scanning relative to interlaced scanning is an advanced scanning mode, it refers to the screen display image to scan, from the upper left corner of the first line to start row by line, the entire image scan completed once. So the image display screen flicker small, the display effect is good. Most advanced displays use progressive scan mode. In the case of interlaced scanning, due to the visual persistence effect, the human eye will see a smooth motion rather than a flashing half-frame half-frame image. But this method causes two images to display the time interval is bigger, causes the image picture screen to blink greatly. Therefore, this kind of scanning method is backward, usually used in the early display product.
Scan DifferencesEach frame image is sequentially scanned by an electron beam in sequential succession, which is called progressive scanning. Each frame of the image through the two-field scan is interlaced, two-field scan, the first field (odd Number field) scan only odd lines, sequentially scan 1, 3, 5 ... Row, and the second field (even field) scans only the even rows, sequentially scanning 2, 4, 6 ... Yes. Interlaced scanning technology in the transmission signal bandwidth is not enough to play a large role, progressive scanning and interlaced display of the main difference in the stability above, interlaced lines flashing more obvious, progressive scan to overcome the shortcomings of interlaced scanning, the picture smooth natural flicker-free.
In the standard display mode of the TV, I represents an interlaced scan, and p indicates progressive scanning.
Say interlaced display is not as progressive as line, this is why? This depends on the specific process of the two patterns at the time of the imaging. We have a 30p video to use as a test. Suppose there is a high-speed camera that shoots a flat-screen TV (the CRT has a scanning process that is slightly more complex to explain, so take a flat-screen TV example). Then we re-watch the TV's work with a slow lens and we see the following. 1080p (Progressive): Display 30 images per second (actually 60 fields, because the first 1/60 seconds and the second 1/60 seconds display the same picture, so the difference is not seen). 1080i (interlaced): Show 60 fields per second, we use "first field", "second Field", "third field" and so on in the first, second, third 1/60 seconds in the event. : The first game: show the scene, the end has not come out, so it is empty the second game: The next show, and play still exist, so we see a complete picture. The third game: the play refreshed into the 2nd frame of the content, and the next did not move the fourth game: the next follow-up refresh, so we see the full 2nd Frame Fifth field: The play into the 3rd frame, and the next is still the content of the 2nd frame Sixth field: next to continue to follow up, we saw the complete 3rd frame ... You can see that the 1080p and 1080i are fundamentally different on the display. Since there is always half a time in the unit time to see the interlaced picture, the 1080i display will be slightly blurred.
Interlaced video encoding (Interlaced-scan coding)
There are two fields per frame of interlaced images, so there are three ways to encode interlaced images:
1. Combine two fields into one frame for encoding;
2, two fields are coded separately;
3. Merge two fields into one frame, but at the macro block level, divide a frame macro block into two field macro blocks for encoding.
The first two encoding methods are called image adaptive frame/field coding (picture-adaptive Frame-field, Picaff or Paff)
The third is called the macro block Adaptive frame/field encoding (Macroblock-adaptive Frame-field, Mbaff).
- Paff: For a moving image, because there is a large scanning interval between the fields, the spatial correlation between two adjacent rows in the frame is smaller than the progressive scan image, so coding the two fields will save the code stream. For a non-moving image, there is a large spatial correlation between two adjacent rows, and it is more efficient to combine two fields into one frame encoding. Therefore, the Paff encoding method can be adapted to the whole image encoding, choosing frame coding or field coding.
- Mbaff: When the image has both motion area and non-moving region, the disadvantage of Paff appears, Paff adaptive granularity is too coarse, can not achieve more sophisticated coding requirements, so mbaff came into being. Mbaff the selection of frame encoding or field encoding is based on macro block, Mbaff two fields into one frame to encode, but each frame macro block (16x16) is divided into a field macro block pair (8x16), for each frame macro block, compare the frame encoding and field encoding generated by the size of the stream, using the most efficient way to save the code stream.
Interlaced and progressive scanning
