Optimal angular intra prediction for block-based video coding
View Patent ↗Encoding methods directed to making coding decisions and estimating coding parameters including searching for optimal angular prediction in intra-prediction mode; choosing the best intra block subdivision; and providing motion estimation for tree-structured inter coding. The methods are targeted to HEVC specifications of video compression, however, may be used with other video coding standards.
1. A method of selecting an optimal angular intra prediction mode for a block-based video coding, the method comprising:
calculating following functions for a spatial area S including a texture block beginning with coordinates (x, y):
E
=
(
∑
(
x
,
y
)
∈
S
W
(
x
,
y
)
·
D
X
2
(
x
,
y
)
-
∑
(
x
,
y
)
∈
S
W
(
x
,
y
)
·
D
Y
2
(
x
,
y
)
)
,
F
=
∑
(
x
,
y
)
∈
S
W
(
x
,
y
)
·
D
X
(
x
,
y
)
·
D
Y
(
x
,
y
)
,
A
=
E
2
E
2
+
4
·
F
2
;
wherein
W(x,y) is a set of non-negative weight coefficients defined at (x,y)εS;
D x (x,y) and D y (x,y) are variations of function P(x,y) in the neighborhood of (x, y) point:
D x (x,y)=(P(x+1,y)−P(x−1,y))/2;
D y (x,y)=(P(x,y+1)−P(x,y−1))/2; and
calculating a minimal activity direction for each case j= 1,4 :
1. E≦0 and F<0;
2. E>0 and F<0;
3. E≧0 and F≧0; and
4. E<0 and F≧0.
2. The method of selecting an optimal angular intra prediction mode of claim 1 , wherein the minimal activity direction for each case j= 1,4 is calculated as a vector (α(S,W), β(S,W)):
(
α
(
S
,
W
)
β
(
S
,
W
)
)
=
(
(
1
+
A
)
/
2
(
1
-
A
)
/
2
)
;
(
α
(
S
,
W
)
β
(
S
,
W
)
)
=
(
(
1
-
A
)
/
2
(
1
+
A
)
/
2
)
;
(
α
(
S
,
W
)
β
(
S
,
W
)
)
=
(
-
(
1
-
A
)
/
2
(
1
+
A
)
/
2
)
;
and
(
α
(
S
,
W
)
β
(
S
,
W
)
)
=
(
-
(
1
+
A
)
/
2
(
1
-
A
)
/
2
)
calculating a corresponding minimal activity angle φ(S,W)ε[0;π] as φ(S,W)=arccos(α(S,W));
finding an intra prediction angle γ k closest to φ(S,W); and
choosing an optimal intra prediction mode P k as defined by the minimal activity direction.
3. The method of claim 2 , wherein the four cases j= 1,4 correspond to respective intervals of the angle φ(S,W): [0; π/4], [π/4, π/2], [π/2, 3π/4] and [3π/4, π].
4. A method of calculating tables for minimal activity directions and corresponding angular prediction modes using signs of E and F and the value of the ratio
E
F
or
F
E
according to claim 1 , the method comprising:
considering integers K and L defining accuracy of calculations;
calculating constant integer array AngleToMode[ ] of size K+1 providing correspondence between a prediction angle φε[0; π] and a prediction mode index P:
AngleToMode[round( K ·φ/π)]= P k ;
calculating four tables RatioToAngleDirect[j][ ] of size L+1 for each case j= 1,4 , providing correspondence between the ratio |E/F|ε[0; 1] and the prediction angle φε[0;π]:
RatioToAngleDirect[ j ][round( L·|E/F| )]=round( K ·φ/π), wherein φε[0;π];
calculating four tables RatioToAngleInverse[j][ ] of size L+1 for each case j= 1,4 , providing the correspondence between the ratio |F/E|ε[0;1] and the prediction angle φε[0;π]:
RatioToAngleInverse[ j ][round( L·|F/E| )]=round( K ·φ/π), where φε[0;π];
calculating values E and F;
choosing a case number j= 1,4 according to the signs of E and F:
if |E/F|ε[0;1], calculating a prediction mode P k defined by the minimal activity direction as:
P k =AngleToMode[RatioToAngleDirect[ j ][round( L·|E/F| )]]; and
if |F/E|ε[0;1], calculating the prediction mode P k defined by the minimal activity direction as:
P k =AngleToMode[RatioToAngleInverse[ j ][round( L·|F/E| )]];
wherein round( )stays for rounding a number to a closest integer.
5. The method of claim 4 , wherein integers K and L are equal to 256.