h.264 video...constant bitrate or not?

[johnnjr]

Hi,

When you choose the pre-set in ffmpegx for MP4 h.264 video...the settings suggests a bitrate for the video under the video tab and under the option tab contant bitrate is checked.

Does this mean that it will in fact encode the entire video at the suggested rate in the video tab? If so...if I change that rate will it encode at what I specify?

Thanks,

John

[major]

Yes, if you change the bitrate it will use the value you provide.

[johnnjr]

Yes, if you change the bitrate it will use the value you provide.

So, if I uncheck contant bitrate...and specify a bit rate will it vary the bit rate around what I inpute or what it suggests?

-john

[major]

In variable bitrate, the bitrate is totally ignored.

[johnnjr]

In variable bitrate, the bitrate is totally ignored.

How do you control file size then when using variable bit rate? Is it that ffmpegx uses the best bitrate for the source as it goes along trying to optimize both quality and file size?

-john

[major]

When using variable bitrate, the size is unpredictable, as the encoding is done in constant quality, and the size is free to vary to fit movie complexity. You can however control the compression level (leading to bigger or smaller files) with the quantizer. Lower qmin=qmax is higher quality and bigger files.

[johnnjr]

When using variable bitrate, the size is unpredictable, as the encoding is done in constant quality, and the size is free to vary to fit movie complexity. You can however control the compression level (leading to bigger or smaller files) with the quantizer. Lower qmin=qmax is higher quality and bigger files.

Thanks Major!! Makes sense!!

john

[johnnjr]

When using variable bitrate, the size is unpredictable, as the encoding is done in constant quality, and the size is free to vary to fit movie complexity. You can however control the compression level (leading to bigger or smaller files) with the quantizer. Lower qmin=qmax is higher quality and bigger files.

Sorry, Major...one more question....

Do you mean literally make the value for qmin equal to qmax? If they aren't equal...and you uncheck constant bit rate will it do variable bitrate?

-john

[major]

Yes, and it will use qmin anyway as the constant quantizer.