Detection of logos in a sequence of video frames
A method is provided for detecting static graphical elements in a sequence of video frames that compares a selected frame in the sequence to each of a plurality of previous frames in the sequence to identify a graphical element that can be a logo. For each pair of frames compared, an absolute difference frame is determined by acquiring an absolute difference value between pixel values for corresponding pixels over at least a portion of the frames in the frame pair. A metric associated with each absolute difference frame is generated, which reflects a degree of dissimilarity. At least some of the absolute difference frames weighted in accordance with the metric associated therewith are summed to generate an accumulation difference frame such that pairs of frames that are more dissimilar have a greater weight. A static graphical element is then identified over a region of the accumulation difference frame in which pixel values satisfy specified criteria.
1. One or more computer-readable storage media containing instructions which, when executed by the one or more processors performs a method, comprising:
buffering N frames of a video program, where N≥2;
calculating an absolute difference frame D(n,k) between a first subsequent frame n and each of the buffered frames k by determining an absolute value of a difference between pixel values for corresponding pixels in at least a portion of the first subsequent frame n and each of the buffered frames k;
generating a histogram H(n,k) for each absolute difference frame;
for each histogram H(n,k) calculating a weight w(n,k) based on a distribution in the histogram of the absolute value of the difference between pixel values;
repeating the calculating of the absolute difference frame, the histograms and the weights for at least a second subsequent frame n;
generating an accumulation difference frame A, where
A
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n
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=
A
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n
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1
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+
∑
k
=
1
N
w
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n
,
k
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*
D
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n
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k
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and
identifying a region in the accumulation difference frame as a static graphical logo if an average value of the absolute value of the pixels in the identified region of the accumulation difference frame is below a threshold value.
2. The one or more computer-readable storage media of claim 1 , wherein identifying the region as a static graphical logo includes identifying the region as a static graphical logo if at least one additional criterion is satisfied, and further comprising: generating an accumulation frame F, where
F
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n
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n
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1
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w
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Mn
and Mn is a matrix of pixel values for the current frame n, and generating a convergence frame having pixel values determined from an absolute value of a difference in pixel values between corresponding pixels in the second subsequent frame n and the accumulation frame F(n), wherein the at least one additional criterion is satisfied if an average pixel value in a portion of the convergence frame over the identified region converges to a stable value as additional subsequent frames are added to the accumulation frame.
3. The one or more computer-readable storage media of claim 1 , wherein generating the accumulation difference frame A includes excluding from summation any term that includes a weight below a threshold value.
4. The one or more computer-readable storage media of claim 2 , wherein generating the accumulation frame F includes excluding from summation any term that includes a weight below a threshold value.