How Does the Target SSIM Slider in the Custom Panel Work?
SSIM (structural similarity) measures how similar a compressed image is to the original, with 1.0 meaning identical. Turning on "auto quality" in the custom panel reveals a target SSIM slider that lets you set that similarity threshold yourself.
What a similarity target solves that a fixed quality number doesn't
A fixed quality number (JPG quality 80, say) doesn't produce consistent actual similarity across different images — the same number can be more than enough for a simple, low-detail image and visibly insufficient for a busy, detailed photo. A target SSIM flips the approach: instead of specifying a quality number directly, you specify how similar the result needs to be, and the tool searches for a quality value that hits that target — computed per image, rather than applying the same number to everything.
How this relates to the "Smart auto" preset
The "Smart auto" preset is exactly this — auto quality turned on by default, targeting 0.95 SSIM. This slider lets you change that number yourself in the custom panel instead of being locked to the preset's 0.95. If Smart auto's result feels slightly under-quality or the file could stand to be smaller, adjusting this slider is the most direct fix, without switching back to manual fixed-value mode and guessing from scratch.
References
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