If using H.264 compression, in theory, what could a video containing no l-frames look like?
Options:
A.
The first P-frame is visible during the whole video
B.
A black image is visible during the whole video
C.
All moving objects will be visible
D.
All non-moving objects will be visible
Answer:
B
Explanation:
H.264 compression uses I-frames (intra-coded frames) as reference points, with P-frames (predictive frames) and B-frames (bi-directional frames) encoding only the changes from the previous frames. If a video contained no I-frames, there would be no reference frame for decoding, resulting in an inability to display any image. This would theoretically result in a black image being visible during the whole video. Therefore, the correct answer is B. For more detailed information, refer to Axis Communications' technical notes on video compression.
Question 102
When focusing a camera with auto iris, focus will be more accurate if
Options:
A.
Set during lowlight conditions, since the depth of field is shorter with a large iris opening.
B.
Set during bright conditions, since the depth of field is shorter with a small iris opening.
C.
Set during lowlight conditions, since the automatic gain control will increase image contrast.
D.
Set during bright conditions, since image contrast will not be reduced by the automatic gain control.
Answer:
A
Explanation:
Focusing a camera with an auto iris is more accurate when set during lowlight conditions. In low light, the iris opens wider, resulting in a shorter depth of field. This forces the focus to be more precise within that narrower range, ensuring that the focus is set correctly. When lighting conditions improve and the iris closes down, the depth of field increases, maintaining a broader range of focus.
