Video frame capture: I-frame vs P-frames
I'm trying to capture various frames within a x264-encoded video files into png files (in python), and then compare the quality of them:
cap = cv2.VideoCapture(filename);
//get frame_number for saving, and save the frame
cap.set(cv2.CAP_PROP_POS_FRAMES,frame_no);
ret, frame = cap.read()
cv2.imwrite(dir_path+'uniform_frame_'+str(frame_no)+".png", frame);
Now I was wondering if I fetch an I-Frame, does it mean that it has a higher quality than other frames, e.g. P-frames and B-Frames? (inherently, keyframes are not compressed, so they must have higher quality and less noise).
video ffmpeg python video-encoding video-capture
asked Sep 11 at 16:21
Tina J
1718
add a comment |
I'm trying to capture various frames within a x264-encoded video files into png files (in python), and then compare the quality of them:
cap = cv2.VideoCapture(filename);
//get frame_number for saving, and save the frame
cap.set(cv2.CAP_PROP_POS_FRAMES,frame_no);
ret, frame = cap.read()
cv2.imwrite(dir_path+'uniform_frame_'+str(frame_no)+".png", frame);
Now I was wondering if I fetch an I-Frame, does it mean that it has a higher quality than other frames, e.g. P-frames and B-Frames? (inherently, keyframes are not compressed, so they must have higher quality and less noise).
video ffmpeg python video-encoding video-capture
asked Sep 11 at 16:21
Tina J
1718
add a comment |
I'm trying to capture various frames within a x264-encoded video files into png files (in python), and then compare the quality of them:
cap = cv2.VideoCapture(filename);
//get frame_number for saving, and save the frame
cap.set(cv2.CAP_PROP_POS_FRAMES,frame_no);
ret, frame = cap.read()
cv2.imwrite(dir_path+'uniform_frame_'+str(frame_no)+".png", frame);
Now I was wondering if I fetch an I-Frame, does it mean that it has a higher quality than other frames, e.g. P-frames and B-Frames? (inherently, keyframes are not compressed, so they must have higher quality and less noise).
video ffmpeg python video-encoding video-capture
asked Sep 11 at 16:21
Tina J
1718
I'm trying to capture various frames within a x264-encoded video files into png files (in python), and then compare the quality of them:
cap = cv2.VideoCapture(filename);
//get frame_number for saving, and save the frame
cap.set(cv2.CAP_PROP_POS_FRAMES,frame_no);
ret, frame = cap.read()
cv2.imwrite(dir_path+'uniform_frame_'+str(frame_no)+".png", frame);
Now I was wondering if I fetch an I-Frame, does it mean that it has a higher quality than other frames, e.g. P-frames and B-Frames? (inherently, keyframes are not compressed, so they must have higher quality and less noise).
video ffmpeg python video-encoding video-capture
video ffmpeg python video-encoding video-capture
asked Sep 11 at 16:21
Tina J
1718
asked Sep 11 at 16:21
Tina J
1718
asked Sep 11 at 16:21
Tina J
1718
asked Sep 11 at 16:21
Tina J
1718
asked Sep 11 at 16:21
Tina J
1718
1718
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
All frames within an encoded video stream are compressed; I-frame simply indicates that the picture can be reconstructed without reference to other coded pictures.
Now, since I-frames are typically used as references for other pictures, a smart encoder goes easy on the quantization, so they typically are of greater quality. P-frames have more quantization applied and B-frames even more.
answered Sep 11 at 16:33
Gyan
14.4k21644
Thanks. So the saved frame files usually have higher qualities.
– Tina J
Sep 11 at 16:35
@TinaJ Usually, yes, unless while encoding, somebody played with the settings. I-frames are more costly to encode than P- or B-frames, so there is a tradeoff to be made. See: en.wikibooks.org/wiki/MeGUI/x264_Settings#ipratio
– slhck
Sep 13 at 11:20
@slhck btw another quick question: what is the intervals between I-frames? I noticed 1 every FPS in the videos I saw (e.g. 1 per 30 frames). Is it always the case? I thought it is content dependent.
– Tina J
Sep 13 at 16:25
It's variable - user can force a constant interval or let encoder decide with some soft constraints like max and min interval.
– Gyan
Sep 13 at 16:48
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Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
All frames within an encoded video stream are compressed; I-frame simply indicates that the picture can be reconstructed without reference to other coded pictures.
Now, since I-frames are typically used as references for other pictures, a smart encoder goes easy on the quantization, so they typically are of greater quality. P-frames have more quantization applied and B-frames even more.
answered Sep 11 at 16:33
Gyan
14.4k21644
Thanks. So the saved frame files usually have higher qualities.
– Tina J
Sep 11 at 16:35
@TinaJ Usually, yes, unless while encoding, somebody played with the settings. I-frames are more costly to encode than P- or B-frames, so there is a tradeoff to be made. See: en.wikibooks.org/wiki/MeGUI/x264_Settings#ipratio
– slhck
Sep 13 at 11:20
@slhck btw another quick question: what is the intervals between I-frames? I noticed 1 every FPS in the videos I saw (e.g. 1 per 30 frames). Is it always the case? I thought it is content dependent.
– Tina J
Sep 13 at 16:25
It's variable - user can force a constant interval or let encoder decide with some soft constraints like max and min interval.
– Gyan
Sep 13 at 16:48
add a comment |
All frames within an encoded video stream are compressed; I-frame simply indicates that the picture can be reconstructed without reference to other coded pictures.
Now, since I-frames are typically used as references for other pictures, a smart encoder goes easy on the quantization, so they typically are of greater quality. P-frames have more quantization applied and B-frames even more.
answered Sep 11 at 16:33
Gyan
14.4k21644
Thanks. So the saved frame files usually have higher qualities.
– Tina J
Sep 11 at 16:35
@TinaJ Usually, yes, unless while encoding, somebody played with the settings. I-frames are more costly to encode than P- or B-frames, so there is a tradeoff to be made. See: en.wikibooks.org/wiki/MeGUI/x264_Settings#ipratio
– slhck
Sep 13 at 11:20
@slhck btw another quick question: what is the intervals between I-frames? I noticed 1 every FPS in the videos I saw (e.g. 1 per 30 frames). Is it always the case? I thought it is content dependent.
– Tina J
Sep 13 at 16:25
It's variable - user can force a constant interval or let encoder decide with some soft constraints like max and min interval.
– Gyan
Sep 13 at 16:48
add a comment |
All frames within an encoded video stream are compressed; I-frame simply indicates that the picture can be reconstructed without reference to other coded pictures.
Now, since I-frames are typically used as references for other pictures, a smart encoder goes easy on the quantization, so they typically are of greater quality. P-frames have more quantization applied and B-frames even more.
answered Sep 11 at 16:33
Gyan
14.4k21644
All frames within an encoded video stream are compressed; I-frame simply indicates that the picture can be reconstructed without reference to other coded pictures.
Now, since I-frames are typically used as references for other pictures, a smart encoder goes easy on the quantization, so they typically are of greater quality. P-frames have more quantization applied and B-frames even more.
answered Sep 11 at 16:33
Gyan
14.4k21644
answered Sep 11 at 16:33
Gyan
14.4k21644
answered Sep 11 at 16:33
Gyan
14.4k21644
answered Sep 11 at 16:33
Gyan
14.4k21644
14.4k21644
Thanks. So the saved frame files usually have higher qualities.
– Tina J
Sep 11 at 16:35
@TinaJ Usually, yes, unless while encoding, somebody played with the settings. I-frames are more costly to encode than P- or B-frames, so there is a tradeoff to be made. See: en.wikibooks.org/wiki/MeGUI/x264_Settings#ipratio
– slhck
Sep 13 at 11:20
@slhck btw another quick question: what is the intervals between I-frames? I noticed 1 every FPS in the videos I saw (e.g. 1 per 30 frames). Is it always the case? I thought it is content dependent.
– Tina J
Sep 13 at 16:25
It's variable - user can force a constant interval or let encoder decide with some soft constraints like max and min interval.
– Gyan
Sep 13 at 16:48
add a comment |
Thanks. So the saved frame files usually have higher qualities.
– Tina J
Sep 11 at 16:35
@TinaJ Usually, yes, unless while encoding, somebody played with the settings. I-frames are more costly to encode than P- or B-frames, so there is a tradeoff to be made. See: en.wikibooks.org/wiki/MeGUI/x264_Settings#ipratio
– slhck
Sep 13 at 11:20
@slhck btw another quick question: what is the intervals between I-frames? I noticed 1 every FPS in the videos I saw (e.g. 1 per 30 frames). Is it always the case? I thought it is content dependent.
– Tina J
Sep 13 at 16:25
It's variable - user can force a constant interval or let encoder decide with some soft constraints like max and min interval.
– Gyan
Sep 13 at 16:48
Thanks. So the saved frame files usually have higher qualities.
– Tina J
Sep 11 at 16:35
Thanks. So the saved frame files usually have higher qualities.
– Tina J
Sep 11 at 16:35
@TinaJ Usually, yes, unless while encoding, somebody played with the settings. I-frames are more costly to encode than P- or B-frames, so there is a tradeoff to be made. See: en.wikibooks.org/wiki/MeGUI/x264_Settings#ipratio
– slhck
Sep 13 at 11:20
@TinaJ Usually, yes, unless while encoding, somebody played with the settings. I-frames are more costly to encode than P- or B-frames, so there is a tradeoff to be made. See: en.wikibooks.org/wiki/MeGUI/x264_Settings#ipratio
– slhck
Sep 13 at 11:20
@slhck btw another quick question: what is the intervals between I-frames? I noticed 1 every FPS in the videos I saw (e.g. 1 per 30 frames). Is it always the case? I thought it is content dependent.
– Tina J
Sep 13 at 16:25
@slhck btw another quick question: what is the intervals between I-frames? I noticed 1 every FPS in the videos I saw (e.g. 1 per 30 frames). Is it always the case? I thought it is content dependent.
– Tina J
Sep 13 at 16:25
It's variable - user can force a constant interval or let encoder decide with some soft constraints like max and min interval.
– Gyan
Sep 13 at 16:48
It's variable - user can force a constant interval or let encoder decide with some soft constraints like max and min interval.
– Gyan
Sep 13 at 16:48
add a comment |
protected by Community♦ Dec 4 at 10:47
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?
