A lot of strange views about colour and monochrome images exist. Here are a couple of test images which when viewed without gamma correction will show an approximate linear greyscale in different graphics modes. This images require a minimum display hardware colour depth of 15 bit (5 bits each for red, green and blue). Obviously 24 bit colour mode is needed for pure greys.
| Greyscale | Pure greys | Impure | Notes |
| Linear 24 bit | 256 | N | Smooth on 24 bit display |
| Linear 15 bit | 32 | Y | Slight rainbow effect |
| Linear 16 bit | 32 | Y | Greenish cast |
| Linear 15 bit (dithered) | 32 | N | Dither noise visible |
| Band 15 bit | 32 | N | 32 bands of pure grey |
| Quadratic 24 bit | 256 | Y | 11 bits greyscale |
| JPEG | Gray scale | PNG |
|
|
Linear 24 bit |
|
| Linear 15 bit | ||
| Linear 16 bit | ||
| Dithered 15 bit | ||
| Band 15 bit | ||
| Quadratic 24 bit |
The JPG image is a slightly imperfect rendition of the original testcard. GIF struggles a bit as it can only use 256 colours. JPEG with lowest loss is better than I had expected so I have included it here for those without browsers for PNG images. If you see gaps in the 32 pure grey bands then the image has been gamma corrected in your system.
PNG
The PNG image is a lossless and concise rendition of the original testcard which uses 763 colours to represent shades of grey in the main styles which are available. From a distance the top three should look fairly similar in luminance, and on real images the slight colour errors are much harder to see. The linear grey bar shows the slight colour pattern used to create intervening brightness between the true greys. The quadratic greyscale at the bottom uses the same luminance trick on a 24 bit RGB system to fake an 11bit greyscale dynamic range. The colour error in this instance is virtually indistinguishable from true greys (unless you turn the brightness or contrast up high).
Unfortunately PNG files are a bit fussy about appearing inline on some browsers. Does anyone know an HTML incantation which works properly and is browser independent?
Note also the pronounced optical illusion on the 32 grey bands.
