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Discussion Groups | Image Signal Processing | Size of the macro block in DCT
Technical Discussions related to Image Processing (image coding, compression, digital effects, mpeg, etc)
Size of the macro block in DCT - getsunil_6 - Aug 17 11:28:00 2004
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Hello all, I am having a small doubt regarding DCT calculation. Why we are going for 8 x 8 macroblocks in DCT but not higher or lower than that? Please let me know this. Thanks in advance Sunil. |
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RE: Size of the macro block in DCT - Nalin Pithwa - Aug 21 16:28:00 2004
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Hi, It is most probably due to the following reason: We know that motion compensation is used in MPEG 4 and H.263/H.264. For instance, the energy in the residual is reduced by motion compensating each 16 x 16 macroblock. Motion compensating each 8 x 8 (instead of each 16 x 16 macroblock) reduces the residual energy further. These examples show that smaller motion compensation block sizes can produce better motion compensation results. However, a smaller block size leads to increased complexity (more search operations must be carried out) and an increase in the number of motion vectors that need to be transmitted. Sending each motion vector requires bits to be sent and the extra overhead for vectors may outweigh the benefit of reduced residual energy. Cheers, Nalin Pithwa -----Original Message----- From: getsunil_6 [mailto:] Sent: Tuesday, August 17, 2004 4:59 PM To: Subject: [imagedsp] Size of the macro block in DCT Hello all, I am having a small doubt regarding DCT calculation. Why we are going for 8 x 8 macroblocks in DCT but not higher or lower than that? Please let me know this. Thanks in advance Sunil. _____________________________________ Yahoo! Groups Links |
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RE: Size of the macro block in DCT - Lakshman - Aug 23 13:48:00 2004
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Hi, all As we know, the computational complexity of DCT increases with increase in block size. If you plot MSE Vs Block Size, there is no significant gain in choosing block sizes larger than 8x8. Cheers -Lakshman --- Nalin Pithwa <> wrote: > Hi, > > It is most probably due to the following reason: We > know that motion > compensation is used in MPEG 4 and H.263/H.264. For > instance, the energy in > the residual is reduced by motion compensating each > 16 x 16 macroblock. > Motion compensating each 8 x 8 (instead of each 16 x > 16 macroblock) reduces > the residual energy further. These examples show > that smaller motion > compensation block sizes can produce better motion > compensation results. > However, a smaller block size leads to increased > complexity (more search > operations must be carried out) and an increase in > the number of motion > vectors that need to be transmitted. Sending each > motion vector requires > bits to be sent and the extra overhead for vectors > may outweigh the benefit > of reduced residual energy. > > Cheers, > Nalin Pithwa > > -----Original Message----- > From: getsunil_6 [mailto:] > Sent: Tuesday, August 17, 2004 4:59 PM > To: > Subject: [imagedsp] Size of the macro block in DCT > Hello all, > > I am having a small doubt regarding DCT calculation. > Why we are going > for 8 x 8 macroblocks in DCT but not higher or lower > than that? > Please let me know this. > > Thanks in advance > Sunil. > _____________________________________ > > Yahoo! Groups Links _______________________________ |
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RE: Size of the macro block in DCT - venkatesh viswanathan - Aug 24 15:36:00 2004
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It is a tradeoff between quality and complexity as mentioned below.Ideally it would be
nice to have smaller blocks but that also means a lot of motion vectors to be coded (involves
searching and coding).Hence 16 x 16 would have been an optimal solution considering the
complexity. Lakshman <l...@yahoo.com> wrote: Hi, all |
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Re: Size of the macro block in DCT - Omer Nezih Gerek - Aug 25 13:19:00 2004
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Complexity is not the only reason to select 8x8 as the block
size. DCT mimics the behavior of KLT for AR-1 sequences (with rho near to 1) using sinusoidal
bases. For DCT to work well in compression, the data should better be as close to an AR-1 (rho
near to 1) as possible. Although it depends on the resolution (and other things) of the image
that is considered, a 8x8 block for a VGA sized image is usually suitable for hat purpose. For
sizes larger than 8x8, the AR-1 property gets spoiled. However, obviously, this is not true for
images with several megapixels of resolution...
Omer
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