Method for optimizing two-pass coding
A method for optimizing a second coding is provided. The method includes: Setting a quantization parameter of a start frame of a video sequence according to a range of an input quantization parameter QP 0 of a coder; performing first coding with a simplified method, and calculating a frame-level temporal impact factor k i of a current frame and a block-level temporal impact factor k B,j of all 16×16 pixel blocks in the current frame; restoring reference list information of the coder after the first coding is completed, and then determining whether a scene is switched; and performing the second coding by setting quantization parameters with different strategies according to whether the scene is switched.
1. A method for optimizing a two-pass coding, comprising the following steps:
S 1 : setting a start frame of a video sequence as a frame I with a level set to Level 0, wherein the frame I is used as a separate Group of Pictures (GOP); optimizing the start frame, wherein the start frame is the frame I, setting a quantization parameter QP I of the frame I according to a range of an input quantization parameter QP 0 of a coder:
Q
P
I
=
{
Q
P
0
-
4
,
QP
0
>
34
Q
P
0
-
3
,
29
<
Q
P
0
≤
34
Q
P
0
-
2
,
24
<
QP
0
≤
29
;
Q
P
0
-
1
,
19
<
Q
P
0
≤
24
Q
P
0
,
Q
P
0
≤
19
S 2 : reading in a frame to be coded of the GOP by the coder;
S 3 : a first coding: performing the first coding on a current frame by using a default quantization parameter QP HM,i set by an High efficiency video coding (HEVC), and obtaining a frame-level temporal impact factor k i of the current frame and a block-level temporal impact factor k B,j of 16×16 pixel blocks in the current frame:
k
i
=
D
i
D
i
MCP
;
k
B
,
j
=
D
B
,
j
D
B
,
j
MCP
;
wherein D i and D i MCP are respectively a coding distortion and a motion compensation prediction error of the current frame; and D B,j and D B,j MCP are respectively a coding distortion and a motion compensation prediction error of a jth 16×16 pixel blocks in the current frame;
S 4 : restoring reference list information of the coder after the first coding is completed, wherein the first coding does not output a code stream of the current frame or store a reconstructed image; after the first coding, resetting a linked image list in the coder to a state before the current frame is coded, comprising restoring a reference frame identifier in the linked image list;
S 5 : determining whether a scene is switched, if the scene is switched, performing step S 6 , if the scene is not switched, performing step S 7 ; a determination method is: if p i >4 p and p i >10, a scene switching occurs in the current frame, wherein p i is an average motion compensation prediction absolute error of the current frame, and p is an a mean value of the average motion compensation prediction absolute error of previous six frames;
S 6 : setting a quantization parameter of the current frame as QP i =QP 0 , then performing the two-pass coding, and outputting the code stream and storing the reconstructed image in a default way of the coder, entering step S 8 ;
S 7 : setting the quantization parameter of the current frame as
Q
P
i
=
{
QP
HM
,
i
+
Round
(
10
×
k
i
-
5
)
,
k
i
>
0.75
QP
HM
,
i
,
else
,
wherein QP HM,i is the quantization parameter of the current frame set in an original coder, and Round(⋅) is a rounding operator;
after a frame-level Lagrangian multiplier λ p is calculated through QP i , obtaining a Lagrangian multiplier λ n and a quantization parameter QP n for coding each Coding tree unit (CTU) of the CTUs in the current frame by the following formulas:
w
n
=
1
M
∑
j
=
0
M
(
1
+
k
B
,
j
)
;
w
¯
=
1
N
∑
n
=
1
N
w
n
;
W
n
=
W
n
w
¯
;
λ
n
=
λ
P
W
n
;
Q
P
n
=
4.2005
×
ln
(
λ
n
)
+
1
3
.7122
;
wherein M is a number of the 16×16 pixel blocks comprised in an nth CTU of the CTUs, N is a number of the CTUs in a coded frame, w n , w and W n are intermediate variables;
coding the each CTU in the current frame by using the Lagrangian multiplier λ n and the quantization parameter QP n , and outputting the code stream and storing the reconstructed image in the default way of the coder;
S 8 : determining whether the coded frame is a last frame of the video sequence, if the coded frame is the last frame of the video sequence, then ending the two-pass coding; if the coded frame is not the last frame of the video sequence, continuing to determine whether the coded frame is the last frame in a current GOP, if the coded frame is the last frame in the current GOP, then performing step S 2 , if the coded frame is not the last frame in the current GOP, performing step S 3 .