Error Concealment for Video Communication Presentation: Guo Li 04/11/2002 Introduction Various channel/network errors can result in damage to or loss of compressed video information during transmission or storage.
Presentation: Guo Li
Intra frame coding : each block is transformed by using block DCT .
Inter frame coding: a motion vector is found, which specifies its corresponding prediction macroblock in its previous frame.Coding Modes:
Each pixel within a block is interpolated from 2 pixels in its two nearest boundaries outside the damaged macroblock.
whole macroblock is treated as 1 entity, a pixel is interpolated from the 4 macroblock boundaries.
An appropriate smoothness measure:
smoothing constraints should be imposed between every 2 adjacent samples across the border of the block in order to propagate the boundary information into the block.
the measure function involves samples in the one-pixel wide boundary outside the block, referred to as external boundary samples.
f(m,n) = b(m,n) for (m,n) Not in B. (4)
The reconstruction is to find ak for all k such that when combined with (2), they minimize the error in (3) among all those solutions that satisfy the boundary condition defined by (4).
When the Dc coefficient alone is lost, the smoothing constraint need be enforced only between the boundary samples, as illustrated in (a); when additional coefficients are lost, necessary to suppress the variation at each point in the direction to towars its nearest boundary, as shown in (b).
Let frepresent the vector composed of the original sample values f(m,n) in B, and a
The vector composed of the transform coefficients ak, k=0,1…..N-1. then the unitary transform defined in (1) becomes:
Here,T=[t0,t1,…tN-1],in which tk is the kth transform basis vector consisting of
Let ar represent the subvector containing the correctly received coefficients and al the subvector including the estimates of the lost coefficients. Let Tr and Tl be the submatrices composed of the column of T corresponding to the entries of ar and al respectively. Then:
The measure function can now be written as :
depend on the weights for a smoothing constraints in 4 direction
choose the smoothing constraints properly, guarantee non-singular,
The proposed procedure is effective only for the lost DC and some low-frequency coefficients. Substituting aopt from (9) into (6), we obtain the maximally smooth solution of the damaged block: (10)
The matrices C and D can be considered as the interpolation filters for estimating f from the boundary values b and the received coefficients ar
Utilize spatially correlated information more thoroughly by performing interpolation based on a large local neighborhood of surrounding pels and to restore edges that are continuous with those present in the neighborhood. [Ramamurthi and Gersho have demonstrated that edges play an important role in the subjective quality of images]
Sobel mask operators:
Sx=[-1 0 1; -2 0 2; -1 0 1], Sy=[1 2 1; 0 0 0; -1 –2 -1]
Magnitude and angular direction of the gradient at coordinate (i,j) are
G=Sqrt (gx^2+gy^2) e=inv(tan(gy/gx))
The gradient measure is computed for every (i,j) coordinate in the neighborhood surrounding the missing macroblock. Gradient angle value is rounded to the nearest 22.5. Corresponding to one of the 8 directional categories.
Each direction has a counter, a voting machine is used that involves incrementing the selected category counter by the magnitude of the gradient if a line is drawn through the pixel at (i,j) with orientation e passes through the missing block.
Simplest way to estimate the coding mode:
Use intra mode and estimated DCT coefficients of the damaged block.
A better way: interpolate from adjacent blocks.
Thank s for your time!