Image Histogram Viewer
An image histogram plots how many pixels sit at each brightness level from 0 to 255. This viewer draws the red, green and blue channels over each other and reports mean luminance, so you can see clipping, low contrast and colour casts that are hard to judge by eye on a screen.
Last reviewed by the Radiatus Cloud team
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How to read the graph
The horizontal axis runs from pure black on the left to pure white on the right; the height at each point is the number of pixels with that value. A histogram piled against the left edge means shadows are clipped to black and detail there is gone. Piled against the right edge means blown highlights. A narrow hump in the middle is a flat, low-contrast image, and a comb of gaps between spikes shows an image that has been stretched after heavy compression or an 8-bit edit.
Luminance versus the colour channels
Luminance is computed as 0.299 red plus 0.587 green plus 0.114 blue, the Rec. 601 weighting that matches how the eye perceives brightness, with green contributing most. When the three colour curves sit on top of each other the image is neutral. When one channel is shifted right the image carries a cast of that colour; a blue curve leading the others is the classic look of daylight-balanced film shot in shade.
What the viewer does to your image
The image is scaled to at most 400 pixels wide before counting, which keeps the calculation instant on a phone and does not materially change the shape of the distribution. Mean brightness out of 255 and the original resolution are reported below the graph, and the drawn histogram can be downloaded as a PNG.
Using it to fix exposure
- Exposing to the right, then pulling brightness down in editing, keeps shadow noise low: aim for a curve that approaches but does not touch the right edge.
- A high-key portrait legitimately lives on the right; a night scene legitimately lives on the left. The histogram describes the image, it does not grade it.
- Compare the histogram before and after export: JPEG at low quality and aggressive sharpening both show up as new spikes and gaps.
Related tools
- Image Brightness Adjuster — Increase or decrease image brightness levels.
- Image Contrast Tool — Enhance or reduce image contrast locally.
- Image Color Extractor — Extract dominant colors from any image to create color palettes.
- EXIF Viewer — Extract hidden EXIF data (Camera, GPS, Settings) from images.
Frequently Asked Questions
Is my photo uploaded anywhere?
No. The file is read into a canvas element in your browser and the pixel counts are computed there. Closing the tab discards it.
Why is the luminance curve different from the average of the three colours?
Because the eye is far more sensitive to green than to blue. The Rec. 601 formula weights green at 58.7 percent and blue at only 11.4 percent, so a bright blue sky adds less to perceived brightness than its pixel values suggest.
Does downscaling to 400 pixels change the histogram?
Slightly. Averaging neighbouring pixels smooths extreme single-pixel values, so a tiny specular highlight may disappear from the far right. The overall shape, clipping and casts are preserved.
What does a histogram with gaps like a comb mean?
The tonal range was stretched after the image had already lost information, typically from editing an 8-bit JPEG. The gaps are values no pixel can occupy any more. Editing from RAW or 16-bit files avoids it.
Can I use this for scanned documents?
Yes. A well-scanned page shows two clear spikes, near black for ink and near white for paper. A single broad hump means low contrast, usually from a dirty scanner glass or a too-low threshold setting.
Privacy & Security
All processing happens locally in your browser — nothing is uploaded.
How to Use
Upload an image to see its red, green, blue and brightness histograms instantly.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.