Opposite CRF Behavior with Apple VT HEVC Encoder on macOS (Increasing Quality/Bitrate/Filesize with Higher CRF)

[jacobdavidstern]

Operating System Info

macOS 15

Other OS

No response

OBS Studio Version

31.0.1

OBS Studio Version (Other)

No response

OBS Studio Log URL

OBS Studio Log (Recording Session - CRF 0): https://anonymfile.com/BE61W/2025-02-04-18-17-07tx.txt OBS Studio Log (Recording Session - CRF 10): https://anonymfile.com/adlJE/2025-02-04-18-17-07.txt OBS Studio Log (OBS Launch - Before Recordings): https://anonymfile.com/EjVnQ/2025-02-04-18-17-28.txt

OBS Studio Crash Log URL

No response

Expected Behavior

In macOS OBS Studio the Apple VT HEVC hardware encoder Constant Rate Factor (CRF) setting is behaving opposite of expected. Instead of lower CRF values resulting in higher quality and larger file sizes, and higher CRF values result in lower quality and smaller files,

Current Behavior

I am observing the opposite: as CRF values increase, the video bitrate, file size, and visual quality also increase.

Steps to Reproduce

1. Installed OBS Studio using the official installer from the OBS Project website
2. Created a new, clean profile in OBS Studio
3. Created a basic scene with only one source: macOS Screen Capture
4. Configured output settings for recording:
   · Output Mode: Advanced
   · Type: Standard
   · Recording Path: /Volumes/Storage/OBS
   · Format: MP4
   · Video Encoder: Apple VT HEVC Hardware Encoder
   · Rate Control: CRF
   · Keyframe Interval: 2 seconds
   · Profile: main
   · Use B-Frames: Checked
   · Audio Encoder: CoreAudio AAC
   · Audio Bitrate: 64 kbps (mono)
5. Created five separate 5-minute recordings with CRF values: 0, 10, 16, 24, and 32.
6. Used QuickTime Player to analyze, noting video bitrate, file size, and visual quality.

Anything else we should know?

<style type="text/css"></style>

CRF	File Name	Video Bitrate (Kbps)	File Size (MB)	Visual Quality
0	2025-02-04_crf0.mp4	80.93	6	Very Low, Blurry
10	2025-02-04_crf10.mp4	119.73	6.9	Low, Blurry
16	2025-02-04_crf16.mp4	117.51	7.8	Soft, Smudgy
24	2025-02-04_crf24.mp4	168.24	8.9	Detailed
32	2025-02-04_crf32.mp4	204.81	10.3	Very Detailed

As shown in the table, the video bitrate and file size increase as the CRF value increases, which is the opposite of expected behavior. The visual quality also follows this trend, with higher CRF values incorrectly resulting in sharper, more detailed images.

Video Samples:

• CRF 0 Video: https://anonymfile.com/xJjBe/2025-02-04-crf0.mp4
• CRF 10 Video: https://anonymfile.com/2pbjl/2025-02-04-crf10.mp4
• CRF 16 Video: https://anonymfile.com/n0lDL/2025-02-04-crf16.mp4
• CRF 24 Video: https://anonymfile.com/gzlrW/2025-02-04-crf24.mp4
• CRF 32 Video: https://anonymfile.com/9J7jb/2025-02-04-crf32.mp4

Screenshots:

• CRF 0 Screenshot: https://anonymfile.com/PRz2z/screenshot-crf0.webp
• CRF 10 Screenshot: https://anonymfile.com/7LqBe/screenshot-crf10.webp
• CRF 16 Screenshot: https://anonymfile.com/XNb2j/screenshot-crf16.webp
• CRF 24 Screenshot: https://anonymfile.com/RYD2q/screenshot-crf24.webp
• CRF 32 Screenshot: https://anonymfile.com/qZlX7/screenshot-crf32.webp

[Fenrirthviti]

Apple's encoders seem to do their own thing. What is normally known as "CRF" is just "Quality" and higher numbers are expected to give better quality. Probably something we could communicate better, but this is what Apple wants.

[jacobdavidstern]

The OBS developers have clarified that Apple's encoders treat the "CRF" setting as a general "Quality" level, not as a traditional Constant Rate Factor. Higher numbers indicate higher quality, and the encoder dynamically adjusts the bitrate to achieve that quality. This explains the observed behavior of increasing bitrate/filesize with increasing "CRF" values, which is the opposite of how CRF functions.

This non-standard behavior would be best communicated within the OBS interface, perhaps immediately below where the Recording Format info appears ("Warning: Recordings saved to MP4..."). A clear explanation would prevent confusion for macOS users. The message could read:

"Important Note for macOS Users: Apple's HEVC encoder interprets the 'Quality' setting (labeled as 'CRF' in OBS) differently than standard CRF. Higher values result in higher quality and larger file sizes. The range is 0-100*. A setting near 32* compares to the accepted CRF "virtually lossless" level (16) in many scenarios."

This clarification within OBS would be very helpful for macOS users. (*adjust accordingly)

[Fenrirthviti]

Please refrain from using ChatGPT or similar tools for suggestions in the future, they are not welcome here. We're aware this isn't ideal, and it's been assigned to be looked at when time allows.

[jacobdavidstern]

Thanks for your feedback on my previous suggestions.

I understand and will respect your policy regarding the use of AI tools, and apologize for any misunderstanding. Also, I very much value and appreciate your dedication and commitment to open source development generally, and about this matter am aware you have many priorities and will address concerns accordingly. Thank you!