SSIM¶

SSIM (Structural Similarity Index Measure) quantifies perceptual image quality by comparing luminance, contrast, and structure between a reference and a distorted frame.

Variants¶

Extractor name	Backend	Algorithm	Feature name	Precision vs CPU
`integer_ssim`	CPU	Integer fixed-point	`ssim`	Reference
`vmaf_fex_integer_ssim_cuda`	CUDA	Real int64 moments + double SSIM	`ssim`	bit-exact (diff=0, places=6)
`vmaf_fex_integer_ssim_hip`	HIP	Real int64 moments + double SSIM	`ssim`	places=6 (target)
`vmaf_fex_integer_ssim_sycl`	SYCL	int64 moments + float32 SSIM	`ssim`	places=4–5 (fp64-free, ADR-0220)
`vmaf_fex_integer_ssim_metal`	Metal	Fixed-point, two-pass separable Gaussian	`ssim`	places=4 (target, ADR-0214)

All GPU variants are auto-selected when --backend cuda/hip/sycl/metal is active and the caller requests --feature ssim. They provide the same "ssim" feature name as the CPU extractor so existing VMAF model JSON files work unchanged.

Note: There is also a float_ssim variant on CUDA (11-tap floating-point Gaussian, distinct algorithm). Prior to ADR-0564, the CUDA and HIP backends silently returned float_ssim scores in the "ssim" field. This bug is now fixed.

`integer_ssim` extractor¶

The extractor uses an integer fixed-point computation compatible with the upstream Netflix reference. It is the extractor invoked when VMAF model JSON files reference "integer_ssim". On GPU backends the same algorithm is implemented in ssim_cuda.c (CUDA), integer_ssim_hip.c (HIP), integer_ssim_sycl.cpp (SYCL), and integer_ssim_metal.mm + integer_ssim.metal (Metal, ADR-0564 cross-backend completion).

Output features¶

Feature name	Description	Condition
`integer_ssim`	SSIM on the luma (Y) plane	Always
`integer_ssim_cb`	SSIM on the Cb (U) chroma plane	`enable_chroma=true` only
`integer_ssim_cr`	SSIM on the Cr (V) chroma plane	`enable_chroma=true` only

Options¶

enable_chroma (bool, default false): emit per-plane _cb and _cr scores in addition to luma. YUV400P sources are always luma-only.

How to run¶

# Luma-only SSIM (default)
core/build/tools/vmaf \
    --reference ref.yuv --distorted dist.yuv \
    --width 1920 --height 1080 --pixel_format 420 --bitdepth 8 \
    --no_prediction --feature integer_ssim --output /dev/stdout

# Per-channel SSIM (luma + Cb + Cr)
core/build/tools/vmaf \
    --reference ref.yuv --distorted dist.yuv \
    --width 1920 --height 1080 --pixel_format 420 --bitdepth 8 \
    --no_prediction --feature 'integer_ssim:enable_chroma=true' --output /dev/stdout