FFMPEG 4k HDR to 1080p SDR Hardware Tonemap Conversion ( OpenCL)

Question

Having trawled the internet I have finally successfully compiled FFMPEG with OpenCL enabled and this has allowed me to attempt HDR -> SDR Conversion using my NVIDIA P400 ( Stream Limit Patch Applied).

This has been in no small part due to the various detailed guides / explanations from 林正浩. Unfortunately I don't have a HUGE understanding of the commands, and I am more or less copying various different commands to try and achieve what I want!

My issue is Hardware tone mapping seems no quicker than software tone mapping.

The following achieves approx 0.4x speed:

ffmpeg -hwaccel nvdec -init_hw_device opencl=ocl -filter_hw_device ocl -threads 4 -extra_hw_frames 3 -I INPUT.mkv -vf "format=p010,hwupload,tonemap_opencl=t=bt709:r=tv:p=bt709:m=bt709:tonemap=hable:format=p010,hwdownload,format=p010" OUTPUT

A command I had tried previously using software achieved similar 0.6x - 0.4x:

ffmpeg -I INPUT.mkv -vf zscale=t=linear:npl=100,format=gbrpf32le,zscale=p=bt709,tonemap=tonemap=hable:desat=0,zscale=t=bt709:m=bt709:r=tv,format=yuv420p -c:a copy -max_muxing_queue_size 9999 /dev/shm/output.mkv

I know they are fairly difference commands but like I said; cutting and pasting from various examples.

My question is What is the most efficient command for HW Accelerated Tonemapping HDR -> SDR, and ideally downscaling to 1080p from 4k? ( I haven't pressed that far into my learning)

I hope people can see I have looked into it and not just asked for help; I have tried and failed to find an answer!

Thank you in advance

Solution

I managed to find a good compromise in quality and speed; with satisfactory results which for me were : FHD SDR tonemapped output.

For anyone else; this is the command I've decided to use going forward:

ffmpeg -vsync 0 -hwaccel cuda -init_hw_device opencl=ocl -filter_hw_device ocl -extra_hw_frames 3 -threads 16 -c:v hevc_cuvid -resize 1920x1080 -i INPUT -vf "format=p010,hwupload,tonemap_opencl=tonemap=mobius:param=0.01:desat=0:r=tv:p=bt709:t=bt709:m=bt709:format=nv12,hwdownload,format=nv12" -c:a copy -c:s copy -c:v libx264 -max_muxing_queue_size 9999 OUTPUT

This completed a conversion from a 4K UHD Dsic Rip MKV in 67m27s, compared to a file runtime of 2h01m49s so and average conversion rate of 1.8x.

Just as a note, I decided to use software encoding due to the significant quality increase but minimal time increase for encoding .