How to use it
- Choose a picture.
- Pick Brightness for one gray chart, or Red, green, blue to overlay the three channels.
- Tick Logarithmic scale to see small groups of pixels, and read the table below the chart.
How it works
Every pixel is sorted into one of 256 levels per channel. Brightness is calculated as 0.2126 R + 0.7152 G + 0.0722 B and rounded, so pure red (255, 0, 0) lands on level 54, pure green on 182 and pure blue on 18. The mean is the average level, the median is the first level at which half of the pixels are counted, and the range is the darkest and lightest level that occurs.
Worked example
A photo of a snowy field whose brightness chart bunches up at the right edge and shows 4 % at level 255 has lost detail in the brightest snow. Lowering the exposure before saving keeps that texture.
Tips
- Compare the channels: if one channel is shifted, the picture has a color cast.
- A gap at both ends of the chart means low contrast, which curves or levels can stretch.
- Re-saved JPEG files can show small combs in the chart because of rounding.