What is Image Histogram?

This image histogram tool analyzes the color distribution of any image, showing separate Red, Green, and Blue channel histograms along with a luminance curve. It also shows mean brightness, contrast ratio, and dynamic range, which helps when checking exposure and color balance.

Luminance is computed with the BT.709 coefficients (0.2126 R + 0.7152 G + 0.0722 B), the same weights modern displays use, so the curve matches what a calibrated monitor would show. For each channel you also get mean, median, standard deviation (a quick contrast indicator), and dynamic range (highest non-zero bin minus lowest). Tonal zone bands mark shadows, midtones, and highlights, hovering reveals the exact count at any value, and you can export the chart as PNG or the full 256-value counts as CSV.

How to use

1. Upload an image (PNG, JPG, WebP, or BMP) to analyze its color distribution.
2. View the RGB channel histograms and luminance curve, with statistics showing mean, median, standard deviation, and dynamic range for each channel.
3. Toggle individual channels on or off, hover the chart to read the exact pixel count at any intensity, switch to a log scale or per-channel normalized view, and download the histogram as a PNG image or the raw counts as a CSV file.

When to use

• Checking whether highlights are clipped before sending photos to a client or print lab.
• Spotting a colour cast on a white-background product shot before it goes live.
• Auditing an image dataset for training, verifying mean brightness is consistent across samples.

Result

A product photographer uploads a white-background e-commerce shot. The histogram shows the blue channel peaking below 240, indicating a slight warm color cast that needs correction in post-processing.

FAQ

How do I read the histogram to check exposure?

The horizontal axis is brightness 0-255 from left to right. A spike pressed against the left edge means crushed shadows, a spike pressed against the right means blown-out highlights. A well-exposed photo usually has data spread across the range with no clipping at either end.

What does dynamic range mean in this context?

It's the distance between the darkest and brightest pixels actually present in the image, on a 0-255 scale. A range of 220 means rich tonal variation; a range of 60 means the image is flat and low-contrast, which is fine for some looks but a warning for product shots.

Why does the luminance curve sometimes look different from the green channel?

Green contributes about 71% of perceived brightness, so they often look similar, but the luminance curve also weights in red (21%) and blue (7%). On a deeply red or blue image the two will diverge noticeably.

What does standard deviation tell me about the image?

It's a rough single-number contrast score. Low std dev means most pixels are clustered around the mean (flat image, foggy day). High std dev means strong contrast (lots of dark and light pixels). Print-ready product shots usually land around 50-70.

How do I detect a colour cast from the histograms?

Compare the mean values of the three channels. On a properly neutral image they should be within a few points of each other. If red's mean is 180 but blue's mean is 145, the image is leaning warm. Adjust white balance in your editor by that delta.

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