Cos'è lo spazio di colore?

Uno spazio di colore è una gamma di colori determinata. Spazi di colore noti includono sRGB, AdobeRGB e ProPhotoRGB.

Il sistema visivo umano non è un semplice sensore RGB, tuttavia è possibile approssimare la risposta visiva con un diagramma di cromaticità CIE 1931 a forma di ferro di cavallo. È possibile osservare come nella visione umana siano rilevate molte più ombre di verde che di blu o di rosso. Con uno spazio di colore tricromatico come RGB i colori sul computer vengono rappresentati usando tre valori, i quali limitano alla codifica un triangolo di colori.

Using models such as a CIE 1931 chromaticity diagram is a huge simplification of the human visual system, and real gamuts are expressed as 3D hulls, rather than 2D projections. A 2D projection of a 3D shape can sometimes be misleading, so if you want to see the 3D hull, install gnome-color-manager and then run gcm-viewer.

sRGB, AdobeRGB e ProPhotoRGB rappresentati da triangoli bianchi

First, looking at sRGB, which is the smallest space and can encode the least number of colors. It is an approximation of a 10 year old CRT display, and so most modern monitors can easily display more colors than this. sRGB is a least-common-denominator standard and is used in a large number of applications (including the Internet).

AdobeRGB is frequently used as an editing space. It can encode more colors than sRGB, which means you can change colors in a photograph without worrying too much that the most vivid colors are being clipped or the blacks crushed.

PhoPhoto is the largest space available and is frequently used for document archival. It can encode nearly the whole range of colors detected by the human eye, and even encode colors that the eye cannot detect!

Now, if PhoPhoto is clearly better, why don't we use it for everything? The answer is to do with quantization. If you only have 8 bits (256 levels) to encode each channel, then a larger range is going to have bigger steps between each value.

Bigger steps mean a larger error between the captured color and the stored color, and for some colors this is a big problem. It turns out that key colors, like skin colors are very important, and even small errors will make untrained viewers notice that something in a photograph looks wrong.

Of course, using a 16 bit image is going to leave many more steps and a much smaller quantization error, but this doubles the size of each image file. Most content in existance today is 8bpp, i.e. 8 bits-per-pixel.

La gestione del colore è un processo di conversione da uno spazio di colore a un altro, in cui lo spazio di colore può essere ben definito come sRGB, uno spazio di colore personalizzato come il proprio monitor o stampante.