Epson S1D13705 technical manual Look-Up Table LUT

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4 Look-Up Table (LUT)

This section is supplemental to the description of the Look-Up Table architecture found in the S1D13705 Hardware Functional Specification. Covered here is a review of the LUT registers, recommendations for the color and gray shade LUT values, and additional programming considerations for the LUT. Refer to the S1D13705 Hardware Functional Specification, document number X27A-A-001-xx for more detail.

The S1D13705 Look-Up Table consists of 256 indexed red/green/blue entries. Each entry is 4 bits wide. Two registers, REG[15h] and REG[17h], control access to the LUT.

Each Look-Up Table entry consists of a red, green, and blue component. Each component consisting of four bits, or sixteen intensity levels. Any Look-Up Table element can be selected from a palette of 4096 (16x16x16) colors.

In color display modes, pixel values are used as an index to an RGB value stored in the Look-Up Table. In monochrome modes, pixel values still index into the LUT, but only the green component is used to determine display intensity.

The selected color depth determines how many index positions are used for image display. For example at one bit-per-pixel (bpp) only index positions 0 and 1 of the Look-Up Table are used. At 4-bpp the first 16 index positions of the Look-Up Table are used and at 8-bpp all 256 Look-Up Table index positions are used.

The Look-Up Table mechanism itself consists of an index register and a data register. The index, or address, register determines which element of the Look-Up Table will be accessed. After setting the index the LUT may be read or written through the data register. The first data element read or written is the red component of the entry. Subsequent read/write operations access the green and then the blue elements of the Look-Up Table.

The S1D13705 LUT architecture is designed to provide a high degree of similarity in operation to a standard VGA RAMDAC. However, there are two considerations which must be kept in mind.

•The S1D13705 Look-Up Table has four bits (16 levels) of intensity per primary color. The standard VGA RAMDAC has six bits (64 levels). This four to one difference must be taken into consideration when converting from a VGA palette to a LUT palette. One suggestion is to divide the VGA intensity level by four to arrive at the LUT intensity.

However, most applications specify the red, green and blue components as eight bit intensities. To determine the appropriate S1D13705 Look-Up Table value we recom- mend using the four most significant bits.

Programming Notes and Examples	S1D13705
Issue Date: 02/01/22	X27A-G-002-03