What is Color Depth?
Color Depth or Colour Depth is the unit measure that denotes the number of bits used to indicate the color of a single screen pixel. It is also called bit depth, and pixel depth. It is the maximum number of colors that pixel can be displayed or it is the number of bits used for each and every color components of a single pixel. Generally while referring a pixel we define it as bits per pixels or bpp. Whereas defining the colour component we define it as bits per component, bits per color, bits per pixel component, bits per sample, bits per color channels, or the bits per channel (more generally bpc).
The pixel depth is a just one aspect of color representation. It express the amount of each primary color can be expressed in precision. But broad range of colors can be represented and expressed in color gamut. The color precision and color gamut accomplish the color encoding to the color space.
What is Color Gamut?
The gamut is the complete subset of colors in the color reproduction in photography and computer graphics. It is a subset of colors which correctly represent it in a given color space within a given circumstance for a particular output device. Generally hue saturation plane is specifying the color gamut to produce wide intensity range of colors. Some colors are said to be out of gamut when it cannot be expressed with in a particular color model.
The color gamut is a range of colors with in a spectrum of colors, subset of colors or within the color space, which can be accurately represented or reproduces by a particular output device. The wideness of the color gamut can vary according to the specification of the display device. In other words color gamut is defined as the range of colors that can be displayed by a particular device.
The Indexed Color:
It is typically a number representing a particular color representing the index into the palette or color map. Palette is a colour lookup table that. It takes selected color table from a color space for reproduction of color through the index. The color index helps to describe the actual colors in the color space with minimum data usage. The colors in the palette are fixed by the hardware and it can be modified by the software some times and they are termed as pseudocolor palettes.
In the old model desk top computers the graphic chips can use different palette per two dimensional bitmap or tile to maximize the number of colors displayed and to minimize the bandwidth usage. They used to store the images in two color format but those two colors can be defined separately for every rectangular 8 by 8 block pixels. Every palette has color index or the numbers of bits per entry. The VGA system can offer an 18 bits (262144) color palette to choose the right color.
But 24 bit (16,000,000) color palettes are much common on the hardware or the file format that are using them. In direct colors, color can be configured with pixel values. Therefore the palettes rarely used to describe the depths greater than 12 bits per pixels. The consumption of memory by palette may higher than the necessary memory that is needed for every pixel in direct color; therefore the palettes are not used in direct colors.
List of Common Color Depth:
1-Bit Color or Binary Image:
Binary image is one that has exactly two colors namely black and white. They are also called bi level images. Most of the very first graphic displays were of this type. It is of two colors black and white which are direct colors. Frequently 1 represent Black and 0 represents white which is the inverse of modern standard. It is often referred as 1-Bit. It means the pixel is stored as single bit that is 1 or 0. Binary image is similar to the image in bit map mode. 1- Bit image can be stored as a bitmap as a packed array of bits. 640×480 images can require 37.5 KiB of storage.
2- Bit Color:
It is of four colors from a selection of fixed palettes. In a 2- bit color palette, the pixel value is denoted as 2 bits resulting in a 4- value palette. The 2 bit color depth could have any of the following value 00, 01, 10, 11.
3- Bit Color:
It can display 8 colors that are combination of full intensity Red, Green and Blue.
Systems with a 3-bit RGB palette use 1 bit for each of the red, green and blue color components. That is, each component is either “on” or “off” with no intermediate states. This results in an 8-color palette ((21)3 = 23 = 8) that has black, white, the three RGB primary colors red, green and blue and their correspondent complementary colors cyan, magenta and yellow.
It can display 16 colors from a fixed palette. It is a VGA standard. Its color depth can allow up to 16 grays or 16 colors assigned to the CLUT (color lookup table). Monochrome 4-bit palette is referred as 16 colors on Macintosh systems and EGA standard. In a four bit color palette each pixel value is represented by 4 bits (that is 16- value Palette). It reduces the visible banding of the level change.
5- Bit Color:
It can display 32 colors from a programmable palette. It was employed in old OCS model computers. Programmable color palette is the ability to select between RAM and ROM Table that support 256 colors.
8-bit color can usually display 256 colors from a fully programmable palette. It is a maximum number of grays in ordinary monochrome system. And the image pixel occupies one memory byte. It needs very low memory space that leads to a high speed, therefore it employed in the early computer graphics development. VGA standard for graphical interface is of 8-bit color. It can employed in an environment with limited bandwidth or memory capacity.
It can usually display 4096 colors from a fully programmable palette. The 12-bit system is able to produce 4096 x 4096 x 4096 = 68,719,476,736 colors in total.
16-Bit Color (High Color):
In 15/16 bits color systems 16 bits are used to store data of each pixel. In these two bytes each RGB component is assigned 5 bits each and the remaining unused bit is used to index or mask the channels. It is of the following formats 15-bit (5:5:5) or 16-bit (5:6:5). Therefore in each byte one bit is allocated for alpha channel. With the help of alpha channel, it is possible to express compositing image operations using an algebraic composition. Now a days the term High Color is used to represent color depths that are greater than 24 bits. There is no need for CLUT, because there are enough available colors per pixel to represent the image correctly. But for the sake of image fidelity some image formats save paletted 15/16 bit images with embedded CLUT.
Most of the common inexpensive LCDS use 18-bit color combinations. These LCDs uses twisted nematic display. Its pixel response time is less compared to others. But it suffers poor color reproduction with limited viewing angles. When viewing angle is not perpendicular than colors will shift. They use dithering or frame rate control to get faster color transition time. It can provide 262,144 color combinations. TFT technology employed in LCDs use 18 data lines to describe.
24-bit color or True Color:
It is called true color, 16M color or 24-bits image. The bits are allocated 8 each for Red, Green and Blue in a 24-bits image. There by it can offer 16.7 million color tones. But the human eye can differentiate up to ten million colors. Therefore it can cover more details than the human eye can perceived. Most of the scanners use around 10-bits for color channel and remaining 8 bits to compensate the noise of the scanner. A 24-bit image needs more memory and processing time to manipulate the 24 bit image.
30-Bit Color or Deep Color:
The 30-bit color consists of 1,073,741,824 colors. 10 bits each is assign to Red, green and Blue. If alpha channel is added then each pixel will of 40 bits. In the earlier models these three 10 bit channels are in a 32 bits data unit. The remaining two bits are unused or used as alpha channel. For representing the colors more memory is required therefore today’s computers use video cards to support 32-bit colors at most higher resolutions. The Deep Color consumes more system resources. Latest graphic cards support 30- Bit RGB.
30 bit color is currently supported by:
- Windows Operating system
- Direct 3D 10
10/12 Bit per color needs more bandwidth when compared with 8bpc. Though HDMI 2.0 HDR TV are capable of 10/12 bpc due to bandwidth limitation higher color depths are not possible with4k@60hz. To overcome the issue one should lower the refresh rate or the display resolution.
12 bits per color can produce 36- bit or 68, 719,476,736 colors in total. If alpha channels are added then there are almost 48 bits per pixel. No human eye can resolve almost 70 billion of colors. Even if you split it into four channels of 9bits per channel, it is obviously it is more than what you need. Though the 36 bit per pixel is technically one of the best option for watching movie or gaming, sadly they do not have the option more than 24 bits per pixel.
16 bits per color can produce 36- bit or 281,474,976,710,656 colors in total. If alpha channels are added then there are almost 64 bits per pixel.
What is Bit depth?
The image bit depth is the amount of color information or data available for each pixel of a raster image. The bit depth and image size are directly proportional. In other words more bits of information per pixel results in more accurate color representation that increase the file size of an image. Higher the bit depth, more the color information stored per pixel in an image. The greater bit depths require representing finer gradation of colors in the spectrum. In addition bit depth is different from bit rate. The RAW photos have 14 bits of information per channels that is the reason that is the reason for the higher image size. An image of pixel size 1200 x 800 may occupy 2.88MB of space.