Personal tools

Frame

From DigiWiki

(Difference between revisions)
Jump to: navigation, search
 
(4 intermediate revisions not shown)
Line 15: Line 15:
-
== 3:2 Pulldown ==
+
==See Also==
-
NTSC television sets refresh at 60 fields per second (30 frames per second), while film is at 23.976 (24) FPS. 3:2 pulldown allows 24 FPS film to be displayed at 30 FPS by first splitting each frame into 2 fields. The first field contains the odd numbered lines in the frame (top field), the second frame the even numbered lines (bottom field) - when two fields are combined, they reproduce the original frame. Then, certain fields are duplicated to create an additional frame for every 4 frames.
+
* [[Telecine]]
-
 
+
-
For example, these are 4 frames in a 24 FPS film:
+
-
 
+
-
1 | 2 | 3 | 4
+
-
 
+
-
Using the interlaced nature of NTSC, these frames are split into odd (T - top) and even (B - bottom) fields:
+
-
 
+
-
1T 1B | 2T 2B | 3T 3B | 4T 4B
+
-
 
+
-
We now apply duplication of certain fields to create the extra frame (* indicate repeated frames):
+
-
 
+
-
1T 1B | 1T* 2B | 2T 3B | 3T 3B* | 4T 4B
+
-
 
+
-
The image below demonstrates the same process.
+
-
 
+
-
[[Image:32pulldown.gif]]
+
-
 
+
-
As you can probably notice, because certain frames contain an extra field, the picture will
+
[[Category:Glossary]]
[[Category:Glossary]]

Current revision as of 07:11, 1 August 2007

Videos are made out of frames. In simple terms, each frame is a picture

Framerate is a property of video that indicates how many frames the video displays per second. It is denoted in FPS (frames per second). Common video framerates are 23.976/24 (NTSC/Film), 25 (PAL) and 29.97 (NTSC).


Types of Frames:

There are several different types of video frames when video compression is used.

  • I picture (key frames) - frames that do not reference other frames, and so they contain all the information needed to render the current frame. Usually occur at scene changes. Compression rate of about 7:1
  • P picture (predictive) - references earlier I and P frames, storing differences data. Compression rate of about 20:1
  • B picture (bi-predictive) - references past and future frames. Compression rate of about 50:1


See Also