Palette (computing)

In 1987, a single pixel on a computer screen could be described using just 8 bits of data, yet the hardware was capable of displaying over 16 million colors simultaneously. This contradiction formed the foundation of the palette system, a technique that allowed early computers to display complex images despite having severely limited memory. The solution lay in the color lookup table, or CLUT, a correspondence table that assigned specific colors from a vast color space to simple numerical indices. By referencing these colors via an index rather than describing the full color value for every pixel, systems could drastically reduce the amount of data required for processing, transfer, and storage. This method became the standard for indexed color images, allowing the 256-color palette commonly used in the GIF file format to represent images that were originally created with 24-bit color depth. The efficiency gained by this approach meant that images could be stored and transmitted with a fraction of the bandwidth previously required, making the internet and digital graphics possible on the hardware of the time.

The Master Palette Strategy

When a computer screen needed to display a mosaic of many different image thumbnails simultaneously, the hardware color registers could not hold the unique adaptive palettes for every image at once. To solve this, developers created a unique, common master palette that could display any kind of image with reasonable accuracy. This master palette was constructed to comprise a full RGB color space in miniature, limiting the possible levels that the red, green, and blue components could have. The human eye has different sensitivities to the three primary colors, being more sensitive to green and less to blue, so RGB arrangements took advantage of this by assigning more levels to the green component and fewer to the blue. A master palette built this way could be filled with up to 256 colors, though it was more general to use only 240 or 16 colors to leave room for reserved colors. When loading a mosaic of images, the program simply mapped every original indexed color pixel to its most approximated color in the master palette, writing the result in the video buffer without significant loss of color accuracy.

Adaptive Color Selection

Real life images were represented with better fidelity to the truecolor original by using adaptive palettes, sometimes termed adaptative palettes, in which the colors were selected or quantized through some algorithm directly from the original image. This process involved picking the most frequent colors from the source image to create a custom palette for that specific picture. While this allowed the indexed color image to nearly match the original, it created a heavy dependence between the image pixels and its adaptive palette. Assuming a limited 8-bit depth graphic display, it was necessary to load a given image's adaptive palette into the color hardware registers prior to loading the image surface itself into the frame buffer. To display different images with different adaptive palettes, they had to be loaded one by one, much like a slideshow. This limitation meant that early video games and image viewers often had to pause or flicker while switching between images with incompatible color sets, as the hardware could not hold multiple distinct palettes simultaneously.

Common questions

What is the palette system in computing?

The palette system is a technique that allows early computers to display complex images despite having severely limited memory. It uses a color lookup table or CLUT to assign specific colors from a vast color space to simple numerical indices. This method became the standard for indexed color images and allowed the 256-color palette commonly used in the GIF file format to represent images originally created with 24-bit color depth.

When was the palette system first used in computer graphics?

The palette system foundation was established in 1987 when a single pixel on a computer screen could be described using just 8 bits of data. This contradiction formed the basis for the technique that allowed early computers to display over 16 million colors simultaneously despite limited memory. The solution lay in the color lookup table which assigned specific colors from a vast color space to simple numerical indices.

How does a master palette differ from an adaptive palette?

A master palette is a unique common master palette that can display any kind of image with reasonable accuracy by limiting the possible levels of the red green and blue components. An adaptive palette selects or quantizes colors directly from the original image to create a custom palette for that specific picture. While adaptive palettes allow indexed color images to nearly match the original, they create a heavy dependence between the image pixels and its adaptive palette.

What is the role of the transparent color in a palette entry?

A single palette entry in an indexed color image can be designated as a transparent color to allow simple video overlay operations. The transparent color usually has index number 0 and allows one image to be superimposed over a background. This technique was used for pointers in typical 2-D videogames for characters and bullets and for video titling and other image mixing applications.

How does Microsoft Windows manage the system palette?

Microsoft Windows applications manage the palette of 4-bit or 8-bit indexed color display devices through specialized functions of the Win32 API. The system palette is a copy in RAM of the color display's hardware registers and serves as a unique shared common resource of the system. When an application sets its own logical palette the system tries to match the logical colors with physical ones and applies an internal algorithm to discard the least-used color if the intended color is not present.