Online Batch Image Contrast Adjustment Tool

What this tool actually does

Batch contrast adjustment for up to 30 selected images. One shared numeric Adjustment strength (%) setting applies to all files; it defaults to 25 and is bounded from -100 to 100. Positive values increase contrast, negative values reduce it and zero leaves RGB unchanged.

Checkable behavior

For amount q, factor f=259*(q+255)/(255*(259-q)). Every RGB channel c becomes f*(c-128)+128, then is clamped and byte-rounded to 0..255. The pivot is fixed channel value 128 rather than each image's measured average; alpha and dimensions remain unchanged.

Controls, output and limitations

Verified pixels: RGB(100,160,240) at amount 25 becomes RGB(94,167,255), clipping blue, while neutral RGB(128,128,128) stays unchanged even at amount 100. Reversing the amount cannot recover tonal distinctions lost to clipping in an exported copy.

Worked batch use: load several flat-lit catalog photos, choose amount 25 and Generate. All receive the same fixed-pivot curve, but exposure differences mean one file can clip before another. Compare shadows, midtones and highlights in each separate preview.

Each input must have an image MIME type, be at most 25 MB and decode to at most 24,000,000 pixels. The selected batch is capped at 100 MB and 40,000,000 decoded pixels; selecting more than 30 files rejects the whole selection.

Every result is encoded as PNG and named from its source base name plus -contrast.png. Download all starts separate downloads about 180 ms apart; it does not create a ZIP, preserve the original format or offer an output-format selector.

Processing is local in the browser and Generate must be pressed after settings change. This is a global RGB transform, not local contrast, sharpening, adaptive histogram equalization, RAW recovery, a region mask or automatic correction. There is no brightness control on this page; use the separate combined tool when both adjustments are required.