Reference frames and Bframes

[lolman]

Can someone explain what are Reference frames and Bframes in short please?

What are the similarities and differences?

In what kind of videos more Reference frames would e helpful?

In what kind of videos more Bframes would e helpful?

[redwudz]

From our Glossary:

Video Encoding:
A frame is one picture but depending on the encoding scheme it may not be a complete picture (I-Frame) but dependent on frames before or after the current frame (P-Frame, B-Frame).

B Frame
One of three picture types used in MPEG video. B pictures are bidirectionally predicted, based on both previous and following pictures. B pictures usually use the least number of bits. B pictures do not propagate coding errors since they are not used as a reference by other pictures.

P Frame
A P-frame is a video frame encoded relative to the past reference frame. A reference frame is a P- or I-frame. The past reference frame is the closest preceding reference frame.

Each macroblock in a P-frame can be encoded either as an I-macroblock or as a P-macroblock. An I-macroblock is encoded just like a macroblock in an I-frame. A P-macroblock is encoded as a 16x16 area of the past reference frame, plus an error term. To specify the 16x16 area of the reference frame, a motion vector is included. A motion vector (0, 0) means that the 16x16 area is in the same position as the macroblock we are encoding. Other motion vectors are relative to that position. Motion vectors may include half-pixel values, in which case pixels are averaged. The error term is encoded using the DCT, quantization, and run-length encoding. A macroblock may also be skipped which is equivalent to a (0, 0) vector and an all-zero error term. The search for good motion vector (the one that gives small error term and good compression) is the heart of any MPEG-1 video encoder and it is the primary reason why encoders are slow. MPEG FAQ

[lolman]

Thanks man

[pandy]

From our Glossary:

B Frame
One of three picture types used in MPEG video. B pictures are bidirectionally predicted, based on both previous and following pictures. B pictures usually use the least number of bits. B pictures do not propagate coding errors since they are not used as a reference by other pictures.

FAQ valid for H.262 but not valid for H.264 and H.265 where B frames can be reference frames.