[FFmpeg-user] Decode just I Frames in H264

[Aviv Hurvitz]

I'd like to make a CPU-efficient computer vision application based on a
H264 stream.

I have two ideas and wanted to get feedback on their feasibility:

*Idea 1 - temporal subsampling*
1. Read the stream into a buffer.
2. Decode only the I-frames.
3. Process the I-frames
4. If some conditions apply - decode the entire Group Of Pictures for that
I-Frame.

*Idea 2 - spatial subsampling*
1. Decode ROIs from the H264 stream  / or decode low resolution images from
the H264 stream.
2. Process and if some conditions apply
3. Fully decode the buffered stream.

So my questions are:
1. Can Idea 1 and Idea 2 be done? Can they be done with libav ?
2. Am I likely to decrease CPU consumption in this way.

Thanks much !