ViewSonic VG150m-1 Service Manual page 20

On-Screen Display
Mascot V OSD display supports 256 different fonts at size of 12x18, 256 fonts contain 128 fixed fonts that are
stored in internal ROM, other 128 programmable fonts are stored in internal SRAM. OSD also supports Overlay
port interface and color look-up table with 4 color indices from external OSD.
Diagram listed below is the concept of how OSD retrieve and display OSD Font. See FIG3-6.
When an OSD key has been pressed, firmware will detect the change and perform the procedure. Once firmware
starts "drawing" OSD, it will program the OSD code and attribute to the OSD SRAM. Chip will look-up the OSD
code then retrieve the character font data that corresponds to the code and then display the character on LCD panel.
There are 320 memory spaces from 0 to 319 in SRAM. It is sequentially mapped to the OSD frame started from
the upper-right corner and goes horizontally to the upper-left corner, and then move to the 2
at left-bottom corner. OSD frame are divided by 0 9 in row, total is 32x10=320 characters.
Diagram listed below is the concept of how 24bit RGB OSD character is composed, see FIG3-7 OSD Display
Flow Chart. Each OSD character font data that is read out from ROM table will use the 4-bit foreground and 4-bit
background color to determine its color. Use these 4-bit color indices to look-up the palette and select related
15bit RGB foreground and background color. Add a 3bit "0" to each R, G, and B channel then generate 24-bit
RGB color for the OSD character.
ViewSonic Corporation
FIG3-6 On-Screen Display
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row and etc. ending
VG150m/mb