CRF value not working when encoding with libaom in AV1

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Summary of the bug: CRF value not working when encoding with libaom in AV1
How to reproduce:

% ffmpeg -i input libaom-av1 -crf 15 -strict experimental output
% ffmpeg -i input libaom-av1 -crf 20 -strict experimental output
% ffmpeg -i input libaom-av1 -crf 25 -strict experimental output
% ffmpeg -i input libaom-av1 -crf 30 -strict experimental output

ffmpeg version: ffmpeg-20181007-0a41a8b-win64-static
built on: Windows 10 64-bit

No matter what CRF value is used, the encoded files will have the same bitrate.

Here is the file I used to test (VP9) and the encoded results (AV1) with identical bitrates:

ffmpeg -i C:\Falcon9Timelapse360p.mp4 -c:v libaom-av1 -crf 15 -strict experimental av1_test_Falcon9Timelapse360pcrf15.mkv
ffmpeg -i C:\Falcon9Timelapse360p.mp4 -c:v libaom-av1 -crf 20 -strict experimental av1_test_Falcon9Timelapse360pcrf20.mkv
ffmpeg -i C:\Falcon9Timelapse360p.mp4 -c:v libaom-av1 -crf 25 -strict experimental av1_test_Falcon9Timelapse360pcrf25.mkv
ffmpeg -i C:\Falcon9Timelapse360p.mp4 -c:v libaom-av1 -crf 30 -strict experimental av1_test_Falcon9Timelapse360pcrf30.mkv

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Source Video (VP9)

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AV1 encoded CRF 15

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AV1 encoded CRF 20

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AV1 encoded CRF 25

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AV1 encoded CRF 30

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Maybe I jumped too quick to my conclusion, there is a little bit difference in bitrate between the different CRF values. I expected the difference to be way bigger since the range should be exponential. There should be roughly a factor two in file size difference between each file.

I'm now rendering additional video's with CRF values of 10, 40 and 50 to better understand the curve.

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AV1 encoded CRF 40

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AV1 encoded CRF 50

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It looks like the encoder hit a ceiling around CRF 20 / 393 KB, where a increase in quality could not be realized even with a higher bitrate, because all the detail of the source video was already maintained. Next time I should use a higher quality source video.

This issue can be closed.