x265 encoding yuv420p better than yuv444p

[Former user]

I have a jpeg picture, and its color space is yuvj444p. I convert jpeg to y4m by ffmpeg with both yuv420p and yuv444p. Then, I use x265 encdode y4m(yuv420p and yuv444p, two pictures) to hevc bitstream. The result from yuv420p have better file size and ssim with raw picture jpeg.

But I think which compressed from yuv444p file should have better performance.

[lastname firstname]

just follow these commands

‌

ffmpeg -i src.jpg

show src.jpeg (yuvj444p)

ffmpeg -i src.jpg -pix_fmt yuv444p src.y4m

ffmpeg -i src.jpg -pix_fmt yuv420p src2.y4m

‌

x265 --crf 21 src.y4m -o test.265

x265 --crf 21 src2.y4m -o test2.265

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ffmpeg -i test.265 test.png

ffmpeg -i test2.265 test2.png

‌

The file size of test2.265 bigger than test.265, and the ssim/psnr of test2.265 is worse than test.265