What causes macro blocks?

[bladestorm]

I have an Xvid i've been trying to convert to either KDVD or BDVD using "The Film Machine" or "DIKO" and I can get the overall quality pretty decent, except that I get annoying macro blocks on black backrounds. What causes this, and can anything be done? The blocks are not present on the source. Another question I have is why is the output always brighter then the source?

Thank You

[guns1inger]

Macro-blocks are generally an indication of not enough bitrate, although this is not always the case. Codecs like Divx and Xvid, as well as Mpeg are temporal encoders. This means that they encode within frames, and across time (frames). When compressing within frames, they work within 16 x 16 blocks, which is why macro-blocks look like, well, blocks. They als compress by throwing away details or colours they don't think you will see. This may mean simplyfying gradients, for example. This would explain your problem, where you seen to have colour clumping together instead of smoothly blending. More bitrate might fix this, but unless you can tell you encoder to push the bitrate up for this section, and down again in other sections, you will have to just give it all a bit more and see what happens.

[Soopafresh]

x264.exe has powerful options to allow you to specify frame ranges that require higher bit rates - eg -smoke scenes, water, etc. Then again, the codec is quite a bit newer than Xvid.

[guns1inger]

High-end mpeg encoders allow for this as well.

[junkmalle]

So does Xvid.

[DRP]

I have found that it is caused more by the resizing method than the quality of the encoder or the bitrate used. I was getting the same macroblocking effect on dark backgrounds using TMPGEnc regardless of bitrate when converting XviDs without visible macroblocks. That was when I was using TMPGEnc to do the resizing for me. I don't know what algorithm TMPGEnc uses to resize but it is probably the fastest (and lowest quality) one they could find.

Since I've changed to using a Lanczos3 algorithm and frameserving the result into TMPGEnc instead, the macroblocks have gone away.

[junkmalle]

I have an Xvid i've been trying to convert to either KDVD or BDVD using "The Film Machine" or "DIKO" and I can get the overall quality pretty decent, except that I get annoying macro blocks on black backrounds. What causes this, and can anything be done? The blocks are not present on the source. Another question I have is why is the output always brighter then the source?

Thank You

Maybe the macroblocks are in your source, you just don't see them because the source is darker.

You MPEG encoder is probably adjusting the black level, hence the lighter output. Or maybe it's just the program you're using to view them -- it might automatically brighten up MPG video because it knows computer monitors have different gamma levels than televisions.

Open your input and output files in VirtualDubMod and view them with that -- it doesn't adjust the video in any way so you see exactly what's in the files.