Zealous,
The notion I, P and B frames only applies to an encoded stream. It determines how the frames must be decoded and the level of compression of each frame. Once you decode the video, each of these frames are converted into either RGB or YUV colorspace. VideoReader currently supports only RGB frames.
If you are asking whether you can read the encoded frames as they are present in the video stream using MATLAB functions such as VideoReader, the answer is no. You can always use low-level file I/O such as fopen, fread and directly parse the bit-stream but that requires knowledge of the file format.
If you are asking whether it is possible to do an operation such as "read me all the P-frames in the video stream", the answer again is not directly. You can do this indirectly using tools such as ffmpeg and MATLAB. I can outline the steps but have no code to offer:
1. Download ffmpeg tools. it has a utility called ffprobe which dumps out info about the file.
2. Using the MATLAB's system command, execute:
ffprobe -show_frames -select_streams v:0 myfile.mpg > C:\Temp\myfileinfo.txt
This will generate a text file with information about each frame. You can do some clever text processing on this file using TEXTSCAN and look for PICT_TYPE=P and identify which frames are P-frames.
3. The number of occurrences of the string "PICT_TYPE=" indicates number of frames.
4. Use these indices to read frames using VIDEOREADER. This will decode the P-frames and return RGB data.
Hope this helps.
Dinesh
