Method and apparatus for reframing and encoding a video signal
Reframing is used to re-size an image or video content, e.g. for displaying video signals with a given aspect ratio on a display having a different aspect ratio. Window cropping parameters (position and size over time) are constrained in order to optimise the rate/distortion of the encoded output video signal. Initial reframing is improved by computing a saliency map representing a user attention map which considers the video coding context and by providing a macroblock coding efficiency cost map and then taking the saliency map or a combined saliency/coding cost map into account so that the coding cost for said reframed video signal is smaller than the coding cost for other candidate reframing windows' sizes and positions.
1. A method for reframing an original video signal, followed by an encoding of the reframed video signal, the method comprising:
computing a saliency map for a current picture of said original video signal, said saliency map representing a user attention map;
providing a macroblock coding efficiency cost map for said current picture of said original video signal;
selecting a candidate reframing window size and position within said current picture of said original video signal;
using in combination said saliency map and said macroblock coding efficiency cost map for determining in said original video signal a different position and/or size of the reframing window, for which different position and/or size the coding cost for said reframed video signal are smaller than the coding cost for said candidate reframing window size and position;
encoding said corresponding reframed video signal.
2. The method according to claim 1 , wherein a Kalman filter is used to control the variation of the position and the size of the reframing window due to variations of the content of the saliency map over time.
3. The method according to claim 2 , wherein a Kalman filter is used in two steps for each picture, for predicting a current state x k with respect to a previous state x k-1 and a state transition model so as to stabilize the reframing window parameters, and for correcting the prediction using a noisy measure, wherein state x k defines the position of the center of the reframing window and its size,
x
k
=
[
centerColumn
centerLine
windowWidth
windowHeight
]
.
4. The method according to claim 1 , wherein said macroblock coding efficiency cost map is provided and, if the aspect ratio of the candidate reframing window resulting from the current content of said saliency map is different than the aspect ratio of said original video signal, the candidate reframing window is extended correspondingly in horizontal or vertical direction so as to achieve said aspect ratio of said original video signal, wherein the direction of said extension is performed such that for the current candidate reframing window the macroblock coding cost derived from the current macroblock coding efficiency cost map is reduced.
5. The method according to claim 1 , wherein said saliency map is merged with said macro-block cost map, such that an expensive coding cost macro-block decreases the corresponding saliency value in a final saliency map applied.
6. Method The method according to claim 1 , wherein said encoding is a H.264/AVC encoding, wherein said encoding means are H.264/AVC encoding means.
7. The method according to claim 6 , wherein said re-framing is included within the encoding loop but the full picture is encoded, and wherein SEI messages are used to signal to the decoder a reframing window to be used.
8. The method according to claim 1 , wherein said reframing window is aligned to macroblock borders.
9. An apparatus for reframing an original video signal, followed by an encoding of the reframed video signal, said apparatus comprising:
computing means which compute a saliency map for a current picture of said original video signal, and for providing a macroblock coding efficiency cost map for said current picture, said saliency map representing a user attention map;
selecting means which select a candidate reframing window size and position within said current picture of said original video signal, and for determining in said original video signal a different position and/or size of the reframing window, by using in combination said saliency map and said macroblock coding efficiency cost map, for which different position and/or size the coding cost for said reframed video signal are smaller than the coding cost for said candidate reframing window size and position; and
encoding means for said corresponding reframed video signal.
10. The apparatus according to claim 9 , wherein a Kalman filter is used to control the variation of the position and the size of the reframing window due to variations of the content of the saliency map over time.
11. The apparatus according to claim 10 , wherein a Kalman filter is used in two steps for each picture, for predicting a current state x k with respect to a previous state x k−1 and a state transition model so as to stabilize the reframing window parameters, and for correcting the prediction using a noisy measure, wherein state x k defines the position of the center of the reframing window and its size,
x
k
=
[
centerColumn
centerLine
windowWidth
windowHeight
]
.
12. The apparatus according to claim 9 , wherein said macroblock coding efficiency cost map is provided and, if the aspect ratio of the candidate reframing window resulting from the current content of said saliency map is different than the aspect ratio of said original video signal, the candidate reframing window is extended correspondingly in horizontal or vertical direction so as to achieve said aspect ratio of said original video signal, wherein the direction of said extension is performed such that for the current candidate reframing window the macroblock coding cost derived from the current macroblock coding efficiency cost map is reduced.
13. The apparatus according to claim 9 , wherein said saliency map is merged with said macro-block cost map, such that an expensive coding cost macro-block decreases the corresponding saliency value in a final saliency map applied.
14. The apparatus according to claim 9 , wherein said encoding means is an H.264/AVC encoder.
15. The apparatus according to claim 14 , wherein said re-framing is included within the encoding loop but the full picture is encoded, and wherein SEI messages are used to signal to the decoder a reframing window to be used.
16. The apparatus according to claim 9 , wherein said reframing window is aligned to macroblock borders.
17. An encoded digital video signal that was, before the encoding, reframed according to the method of claim 1 .