H.264 quality issues in 0.0.9t

[lhawkins]

I just installed the t version a few days ago as it now supports h.264 in the standard .mp4 wrapper. I was hoping for more options when encoding over Handbrake, and it seemed to work perfectly. The files are playing back in the QT player with no issues. I do have one issue though... image quality.

I did some tests with a DVD of TV episodes. (The Thin Blue Line from the BBC) I had already encoded the first few episodes in Handbrake at "Target Size 300MB" and de-interlace on. These took around 8 hours to encode on my G4/533. I tried encoding the first episode again using FFMPEGX and it took about the same time to encode, maybe a bit faster, but the quality is much worse. I tried the default settings and turned on deinterlacing. I then tried turning on use CABAC and B frames. I also tried turning off constant bit-rate. Nothing seems to matter. The quality produced by FFMPEGX seems more blocky than Handbrake. The blocks are not the 8x8 blocks when the bitrate is too low, but more like the vertical resolution is half what it should be. No matter what settings I choose I can't seem to get as high of quality encode.

What I want to produce is a .mp4 file that is 640x480 (Cropping the sides as that is the format of the original DVD. I did use auto-crop to determine this and it is the exact same frame size that Handbrake produces) I want the files to be around 275-300MB (30 minute episodes) and I want it to look great!

Are there any suggestions for what settings I should be using? I assume that both apps are using the same tools behind the scenes, but I just can't get the same results no matter what I try.

While writing this I extracted some stills and decided to zoom in on a small portion of the screen and the difference may not be as great as this post makes it out to be, but I have to say that the Handbrake output is noticeably better. In the past I have been able to achieve the best quality using FFMPEGX no matter what the codec and only started using Handbrake in the meantime until the H.264 code was working in FFMPEGX.

PS. Don't take this totally as a complaint as I still prefer FFMPEGX for its unlimited options and excellent interface!

[major]

Just lower qmin, or disable constant bitrate and set qmax=qmin. Handbrake and ffmpegX use the same encoder (x264).

[zrr]

Where do I find "qmin" and "constant bitrate"?

Thanks
Zeno

[major]

In the Options tab.

[zrr]

Thank's found it!

Zeno

[zrr]

how do I set qmax=qmin? Set both to 0?

Thanks
Zeno

[major]

No, set both to 22 (for MP4 H.264 encoding), or a slightly lesser value like 21 or 20.

[lhawkins]

I'm at work so testing will be pretty limited until I get home, but will this drastically increase the rendering time? or file size? If not then why isn't this the default if it improves quality.

I guess it might be nice to add a possible feature in a future version that would have a couple of sliders that had options for quality vs file size and quality vs encoding time. If I slid the first one all the way to the Quality end and the other one all the way to the opposite end I would get... never mind... how about a square with a dot in it... the x axis would be quality from low to high and the y axis would be speed from quick to slow. Then you could click in the square wherever you wanted and choose all the variables with one simple interface. If you clicked in the upper right corner it would be the best quality and the slowest encode time. the lower left would be worst quality, quickest encode time. the lower right corner would be highest quality in shortest possible time and the upper left corner would be lowest quality and highest encode time (Of course this corner would be mostly unused except by the most masochistic among us I would attach an image, but as I say, I'm at work and it would be quite difficult from here!

This may or may not be a good idea, I'm just throwing it out there, but it seems with the newer codecs there are so many options and each one effects the others in some way so there has to be a simpler way to adjust the parameters so that the encode is always the highest quality in the lowest amount of time. I guess this might sound overly simplistic, but I truly believe that some of the settings could be simplified in some way.

[major]

Yes, it will increase file size and rendering time. The quality feature is a good idea and it is indeed in the to-do list for future versions.

[kaz219]

Ok, I'm doing some tests with H.264 and ffmpegX, and I have some questions about the results.

When I set "constant bitrate" (with the default values for qmin and qmax, 22 and 51), and I encode a movie, the bitrate goes at once above 1000kbps. I have mostly above 1500kbps. Since the "best bitrate" that I obtain by pressing "Best" in the video tab is maximum 1200, depending of the dimensions of the video, I don't understand.

Furthermore, I thought "constant quality" (constant bitrate deactivated) was using only qmin as reference, and not qmax. At least it's what's written on the ffmpegX website. But here you say the opposite, major. Who should I believe? You or you? ;)

Supposing, I don't care about the encoding time, what is giving me the best quality/size ratio? two pass or constant quality?

And finally, I think it shouldn't be possible to choose both "constant quality" and "two pass", since it doesn't make sense, does it? Just as it is not possible to check both 2-pass and 3-pass for a DivX, e.g.

cheers,
kaz

[major]

Strange about the first point, could you please email me to major4@mac.com some details like process outputs or test clips.

In constant quality, only qmin is used (but to avoid confusion with ffmpeg engine, which sets constant quality by itself when qmin=qmax, I probably said to put always qmin=qmax).

Indeed it does not make sense to use 2-pass and constant quality. Must add some more checks on the checkboxes.

[kaz219]

Hi,

I'm sorry, I still don't understand the point with the constant quality. How does this qmin has an influence on the encoding quality? I mean, if I think of video quality, I automatically think average PSNR over one or many GOP. If the PSNR can vary with a constant qmin, it means that qmin doesn't use it, or not only. What kind of metrics does the qmin use then?

I'm not — far from it — a specialist in video compression, but I thought that PSNR was one of the best quality evaluation tool computers have (I heard of a new one, better).

Maybe I'm wrong whith this, and I would be very glad if you had the time to explain this to me.

regards,