I-Frames and DVD Architect: what are they and why are they important?
The MPEG (Moving Pictures Experts Group) standard defines three types of frames that it uses to compress analog or digital video signal into the MPEG format:
Intra Frames (I-Frames)
Predicted Frames (P-Frames)
Bidirectional Frames (B-Frames)
The job of MPEG is to take analog or digital video signals and convert them to packets of digital data that are compressed to reduce the size of the file. MPEG compression takes advantage of the nature of the human eye and removes redundant information that we just do not see. In order to understand I-Frames we must first understand the idea of GOP. A sequence of different frame types, beginning with an I frames and ending just before the subsequent I frame, is called a Group of Pictures (GOP). Commonly, a GOP is 15 frames long, and has the sequence I_BB_P_BB_P_BB_P_BB_P_BB_. A similar 12-frame sequence is also common. For ideal coding efficiency, the placement of I, P and B pictures in the GOP structure may be determined by the nature of the video stream and the bandwidth constraints on the output stream. For example, a low-motion scene is more efficiently encoded with more B frames. Its GOP structure might look like: IBBBPBBBPBBP. A higher motion scene may be more efficiently encoded with fewer B frames. Its GOP structure might look like: IPPBPBPPPPPP. Encoding time may also be a constraint. This is particularly true in live transmission and in real-time applications with limited computing resources, as a stream containing many B frames can take three times longer to encode than an I frame-only stream
So this is where Intra Frames or I-Frames come into play. I-Frames are coded using information only found in the picture itself so they provide all of the information for the frame. Since these frames contain the only real still image for a frame, chapter markers on a DVD can only be set on I-frames. This is important if you have exact spots in your video that you wish to be able to skip to on the DVD. In previous versions of DVD Architect you were not able to add your chapter markers and then move them to the nearest I-Frame therefore forcing DVD Architect to move the Chapter points on your DVD to the nearest I-Frame. This may have caused your chapter marks may be up to a half-second off. In this version of DVD Architect the I-Frames get created automatically where the chapter markers are placed if you recompress your project in DVD Architect or created your DVD ready file in Vegas. If you place a marker in DVD Architect’s timeline and it does not occur at an I-frame, the tag will appear. To snap the marker to the I-frame, drag the marker along the timeline to one of the I-frame reference points which look like this .
I-frame reference points will appear on the timeline once you begin to drag the marker.
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