Adjust the Gamma of a PNG
Gamma correction brightens or darkens an image non-linearly — it lifts the midtones and shadows while leaving pure black and pure white exactly where they are. That's what makes it the right tool for a dark photo, and a better one than brightness for almost every case.
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Why gamma beats brightness
Brightness adds a constant to every pixel. Push it up to rescue a dark photo and the highlights hit 255 and clip — detail in the bright areas is destroyed, and the whole image looks washed out and flat because the blacks lift too.
Gamma applies a power curve instead: out = 255 × (in / 255)1/γ. Zero maps to
zero and 255 maps to 255 no matter what gamma you choose, so nothing clips and your black point stays
black. Everything in between is redistributed. A gamma of 1.6 on a dark photo brings the shadows up
dramatically while the sky stays intact — that's a correction brightness cannot make.
Reading the number
1.0 changes nothing. Above 1 brightens the midtones — 1.2 is a gentle lift, 1.6 a strong one, 2.2 is dramatic and starts to look milky. Below 1 darkens them — 0.8 deepens shadows and adds richness, 0.45 is heavy. Anything past about 3.0 in either direction is a stylistic effect rather than a correction, and 8-bit images will start to show banding in the stretched region.
The value 2.2 is not a coincidence — it's the approximate gamma of the sRGB encoding that essentially every PNG and JPEG on the web uses. Correcting by 2.2 or 1/2.2 is how you move between sRGB-encoded values and linear light, which is the transform that matters if you're preparing textures for a renderer that expects linear input.
FAQ
Should I use gamma or brightness for a dark photo?
Gamma, nearly always. It rescues the shadows without touching the highlights. Reach for brightness only when the entire image needs shifting including the black point.
Why do I see banding in the shadows?
A strong gamma lift stretches a handful of dark values across a wide range, and 8 bits can't subdivide further. It's inherent to the format. A little noise disguises it effectively.
Does this change the PNG's gAMA chunk?
No — it changes the pixel values themselves. The gAMA chunk is metadata telling viewers how to interpret the pixels; if you want to inspect or strip it, use PNG chunk viewer and Remove metadata.
Is transparency affected?
No. Only the colour channels go through the curve; the alpha channel is untouched.
Is it applied per channel or to luminance?
Per channel, identically — the standard definition of gamma correction. Colour relationships are preserved, though very saturated colours can shift slightly in apparent hue at extreme values.