half and quarter sample meaning

Started by Sharan123 1 month ago8 replieslatest reply 3 weeks ago111 views

Hi everyone,

I am new to the world of image and video processing and have been going through some documents to understand some basics. At multiple places, I see references to "half and quarter sample" processing.

I am unable to understand what half or quarter here means.

Is it mid way or 1/4 between 2 samples?

Can anyone give some good example please?

below one snippet where I see such a reference:

first generating the values of one or two
neighboring samples at half-sample positions using six-tap
filtering, rounding the intermediate results,
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Reply by shafie7April 23, 2019

Sounds like interpolating.  What is the context?

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Reply by weetabixharryApril 23, 2019
Sounds like interpolating.  What is the context?

I second that. In fact, that's more or less word-for-word what I was going to reply.

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Reply by djmaguireApril 23, 2019

It is interpolating.  It is spacial, though, in case that is any source of confusion to the OP.

I'm guessing - or rather Google is - that the snippet came from here:

This looks to be a good explanation of the 6-tap filters used in H.264 and their spacial orientation:

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Reply by Sharan123April 24, 2019

Hello all,

Thanks for the replies. The snippet is from an HEVC tutorial.

Anyway, I do understand what interpolation is in image processing context.

But what I am not able to understand is "half sample" or "quarter sample".

This part is confusing me.

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Reply by djmaguireApril 24, 2019

From the paper whose link I posted:


Fig. 8 a) Half-pel pixel values - (b) Quater-pel pixel values.

The integer-pels are your streamed luma samples.  Once the half-pel area is available, the half-pel values can be interpolated using the defined filter.  After that calculation, the quarter-pel values can be calculated.  So... Integer-pel=luma sample.  Half-pel=half luma sample. Quarter pel=quarter luma sample.

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Reply by Sharan123April 28, 2019

Thanks. Do you know why half pel or quarter pels are needed?

Eventually, display is always in terms of pixels.

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Reply by djmaguireApril 28, 2019

I believe that the primary purpose is for more realistic motion.

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Reply by KnutIngeMay 8, 2019

If you have an object having linear movement from one frame to the next, there is no reason to expect its movement to be an exact integer amount of pixels. Rather, it might move -0.22 pixels in the vertical dimensions and 2.71828 pixels in the horisontal.

If you are tracking such movement in order to capitalize on the quality of rendering in the previous frame (and the bits spent in order to accomplish that), to basically say "do this block like the previous frame, only shifted horizontally/vertically by some amount", it makes sense to allow expressing that movement by whatever accuracy strikes a good balance between:

1) The accuracy (improvement) of predicting a block via motion vectors

2) The bits spent encoding those motion vectors

3) The computational cost of finding that motion vector and resampling/shifting