Zero-delay panoramic video bit rate control method considering temporal distortion propagation
A zero-delay panoramic video bit rate control method that considers distortion temporal propagation aims to optimize target bit allocation and achieve global rate-distortion optimization in coding. It encompasses coding tree unit (CTU)-level bit rate control and temporal global rate-distortion optimization. CTU-level bit rate control involves optimizing target bit allocation and updating bit rate control parameters. Temporal global rate-distortion optimization utilizes the reconstruction error and motion compensation prediction error information of the previous coded frame to estimate the temporal dependence between CTUs in the current coding frame and those in the previous coded frame to adjust the coding parameters of the current CTU. The coding parameters are further fine-tuned according to the area stretching ratio encountered during the projection of the panoramic video from a 3D spherical surface to a 2D plane, taking into consideration the detrimental impact of interpolated redundant pixels on the coding process.
1 . A zero-delay panoramic video bit rate control method considering time-domain distortion propagation, comprising the following steps:
S1: inputting a target bit rate and a sequence to be coded to an encoder;
S2: determining whether a current frame is a first frame, and if so, performing the following operations:
calculating a latitude value corresponding to a coding tree unit (CTU) line and calculating an area stretching ratio of a spherical surface and a projection plane thereof,
initializing parameters of a bit rate control unit,
calculating a number of target bits of the current frame,
calculating a frame-level Lagrange multiplier of the current frame according to the number of the target bits,
calculating a frame-level quantization parameter (QP) according to the frame-level Lagrange multiplier and performing intraframe coding,
repeating step S2 after coding the current frame, and
otherwise, entering step S3;
S3: calculating pixel mean and variance values of a frame;
S4: determining whether a current frame number is greater than twice a Group of Pictures (GOP) size, if the current frame number is greater than twice the GOP size, entering S5, otherwise, entering S6;
S5: calculating a global Lagrange multiplier according to an iterative algorithm, optimizing bit allocation of each CTU in the frame, and entering step S7;
S6: using a bit allocation algorithm of the encoder to allocate bits for each CTU in the frame;
S7: calculating the frame-level Lagrange multiplier and QP;
S8: coding the CTUs in sequence;
S9: determining whether a difference value between pixel variance values of the current frame and a previous frame is greater than a threshold, if so, entering step S10, otherwise, entering step S11;
S10: adjusting a Lagrange multiplier of the current CTU according to the area stretching ratio obtained in step S2, and entering step S13;
S11: determining the current frame number is greater than 3, if so, entering S13, otherwise, entering S10;
S12: adjusting a Lagrange multiplier of each CTU according to a time-domain distortion influence factor of the previous frame and the area stretching ratio obtained in S2;
S13: calculating QPs of the current CTU according to the Lagrange multiplier of the current CTU calculated in S10 or S12, and coding the CTU;
S14: determining whether the current CTU is the last CTU, if so, coding the current frame and entering step S15, otherwise, returning to step S8;
S15: updating bit rate control parameters and a number of actually used bits of the current frame and each CTU;
S16: acquiring a reconstruction error and a motion compensation prediction error of each CTU saved in the current frame to calculate a time-domain distortion influence factor of each CTU in the current frame;
S17: determining whether the current frame is the last frame, if so, entering S18, otherwise, entering S2; and
S18: completing coding of a current panoramic video sequence.
2 . The zero-delay panoramic video bit rate control method considering time-domain distortion propagation according to claim 1 , wherein in step S2, a method for calculating the area stretching ratio is as follows: if a radius of a panoramic video sphere is defined as r, an area of a spherical girdle region with a latitude θ is:
S s (θ)=2π· r 2 ·cos θ·sin dθ
after being projected onto a 2D plane, an area of a stretching region is obtained as follows:
S
E
(
θ
)
=
S
E
(
π
2
)
=
2
π
·
r
2
·
sin
d
θ
then the area stretching ratio is:
k
(
θ
)
=
S
S
(
θ
)
S
E
(
θ
)
=
S
S
(
θ
)
S
E
(
0
0
)
=
cos
θ
.
3 . The zero-delay panoramic video bit rate control method considering time-domain distortion propagation according to claim 2 , wherein step S5 is as follows:
R
=
∑
i
=
1
M
r
i
=
∑
i
=
1
M
(
λ
g
c
i
·
k
i
)
-
1
k
i
+
1
=
∑
i
=
1
M
(
α
i
λ
g
)
β
i
wherein R and r are respectively frame-level and CTU-level target bits, λ g is the global Lagrange multiplier, a i =c i ·k i ,
β
i
=
1
k
i
+
1
,
c
i
=
D
i
R
i
-
λ
i
R
i
D
i
,
k
i
=
λ
i
·
R
i
D
i
,
λ i is a slope of a rate-distortion curve, D i and R i are respectively distortion and coding bit of an i th coding unit, and M is a number of CTUs.
4 . The zero-delay panoramic video bit rate control method considering time-domain distortion propagation according to claim 3 , wherein in step S10, a method for adjusting the Lagrange multiplier is as follows:
λ
n
=
λ
P
k
(
θ
)
wherein λ p is the frame-level Lagrange multiplier, and λ n is an adjusted Lagrange multiplier.
5 . The zero-delay panoramic video bit rate control method considering time-domain distortion propagation according to claim 3 , wherein in step S12, a method for adjusting the Lagrange multiplier is as follows:
λ
n
=
λ
P
k
*
k
(
θ
)
wherein k is the time-domain distortion influence factor.