Question about CRF usage and resolutions

[hajj_3]

regarding the topic created by quark.fusion:

" I want to answer this question as many peoples ask it
The answer is pretty simple
1.) For your source select desired quality in CRF value.
2.) Encode your source at this CRF value — you get bitrate for that resolution for your source. (For another source it can be different)
3.) Resample your source to another resolution and repeat (2). Now you have another bitrate for another resolution for your source for a given quality
4.) Plot chart and extrapolate missing resolutions and bitrates."

I would have asked some questions in that thread by neuron closed it....

Where about do i find these CRF value, is it in x264 profile settings in megui or something, cant seem to find it.

Lets say i want to use 900 bitrate for 640*352 captures and that gives a CRF value of N. What bitrate would i need to be able to get the same N value at 720*400 or 704*384?

Thankyou.

[Guest]

Set a Constant Quality profile in MEGUI.

Then Config/Main/Quality

[J_Darnley]

--crf N

[hajj_3]

cant seem to figure out where to set bitrate of 900 to find out the quality value:( Anyone able to tell me what bitrate would be needed for 720*400 to give same quality value if it was 900 bitrate at 640*352?

sorry for being a noob.

[Avenger007]

When you used --bitrate 900 for the first pass you would have seen a corresponding CRF value (say N) in the log file. Use the same bitrate you get when using CRF N for 720*400.
But better yet, just use CRF N for encoding 720*400, no need to do a 2-pass after.

[TL0]

cant seem to figure out where to set bitrate of 900 to find out the quality value:( Anyone able to tell me what bitrate would be needed for 720*400 to give same quality value if it was 900 bitrate at 640*352?

sorry for being a noob.

You can get an approximation by calculating how many pixels your required resolution has compared to the known resolution and bitrate, assuming bpp required is the same. For 720*400, a bitrate of 1150 should give similar CRF to 640*352.

[Sagekilla]

Realistically, bitrates don't scale linearly. 1080p doesn't require 2.25x bitrate over 720p, but maybe more like 1.6 - 2x. Still, straight up muplication of (720 x 400) / (630 x 352) * bitrate gives a reasonable approximation. Be careful when you're working at particularly low resolutions or large jumps in resolution though. Encoding a 1080p source @ 720x400 for an approximation then scaling up to 1080p might not work well, for example.