Frame or field mode coding method

A device comprising a pre-analysis circuit to perform a coding decision algorithm, linked to a coding circuit for the actual coding of a picture in field or frame mode, wherein the coding circuit receives from the pre-analysis circuit coding information in field or frame mode according to the number of groups of macroblocks detected having activity, in the picture, wherein the device is configured to:

• calculate, for a group of macroblocks in field mode, spatial activities of each constituent macroblock and a temporal activity between the macroblocks of each constituent field,
• calculate an activity index relating to the group that is an increasing function of the temporal activity and a decreasing function of the calculated spatial activities,
• compare the activity index relating to the group, to a predefined threshold, to define it as a group having activity, and
• code the picture in field or frame mode according to a number of macroblock groups detected having activity with respect to a number of groups of macroblocks in the picture, wherein said group is a supermacroblock and wherein the activity index equals to
• R
• =
• FLADInterMB
• 1
• +
• FLADIntraMB
• n
• +
• FLADIntraMB
• n
• -
• 1
• ⁢
• ⁢
• with
• ⁢
• ⁢
• FLADInterMB
• =
• ∑
• j
• =
• 0
• 15
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• =
• 0
• 15
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• 
• FY
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• ⁡
• (
• i
• ,
• j
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• FY
• n
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• 1
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• (
• i
• ,
• j
• )
• 
• ,
• being an inter-field luminance gradient;
• FLADIntraMB
• n
• =
• ∑
• j
• =
• 0
• 7
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• =
• 0
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• FY
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• i
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• 
• ,
• being an intra-field luminance gradient in feild n;
• and
• FYn(i,j) representing the luminance value of the pixel of horizontal coordinate i and vertical coordinate j of the field n of the supemacroblock.