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Bob(clip clip [, float b, float c, int height])

Bob takes a clip and bob-deinterlaces it. This means that it enlarges each field into its own frame by interpolating between the lines. The top fields are nudged up a little bit compared with the bottom fields, so the picture will not actually appear to bob up and down. However, it will appear to "shimmer" in stationary scenes because the interpolation doesn't really reconstruct the other field very accurately.

This filter uses BicubicResize to do its dirty work. You can tweak the values of b and c (see BicubicResize). You can also take the opportunity to change the vertical resolution with the height parameter.

A bob filter doesn't really move the physical position of a field. It just puts it back where it started. If you just SeparateFields() then you have 2 half height frames: line 0 is becomes line 0 of frame 0 and line 1 becomes line 0 of frame 1. Thus line 0 and 1 are now in the same place! Bob now basically resizes each frame by a factor of two but in the first frame uses the original lines for the even lines and in the second frame uses the original lines for the odd lines, exactly as is supposed to be. If you just did a resize vertically by a factor of 2 on each frame after doing a SeparateFields(), then it wouldn't work right because the physical position of a field moves.

Schematic: Suppose the lines 0o, 1o, 2o, 3o, ... are original lines and 0i, 1i, 2i, 3i, ... are the interpolated lines.

start with:

line number frame 0
0) 0o
1) 1o
2) 2o
3) 3o

separate fields:

line number frame 0 frame 1
0) 0o 1o
1) 2o 3o

double size:

line number frame 0 frame 1
0) 0o 1o
1) 1i 2i
2) 2o 3o
3) 3i 4i

but this is wrong, because the physical position of the field changed.

Bob does it right:

line number frame 0 frame 1
0) 0o oi
1) 1i 1o
2) 2o 2i
3) 3i 3o

To strictly preserve the original fields and just fill in the missing lines.

Bob(0.0, 1.0)

Bob(0.0, 1.0) preserves the original fields for RGB and YUY2 and preserves the Luma but not the Chroma for YV12.

The filter coefficients with b=0.0 and c=1.0 give you 0 at x=1.0/2.0 and 1 at x=0. Which with the +/-0.25 shift occurring on the original field locations, you get a very crisp cubic filter with -1/8 5/8 5/8 -1/8 coefficients on the x=0.5/1.5 taps for the other field.

However, since the shift on the chroma planes is only 0.125 for YV12 the taps don't end up on exactly the same distances. More discussion.

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