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Stepho-wrs (talk | contribs) →Textmode color: cards -> card's |
reference "The IBM 5153's True CGA Palette" article and move the "rule of thirds" approximation formula off to after the RGBI discussion |
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These four colour bits are then interpreted internally by the monitor, or converted to NTSC colours (see below).
{| class="wikitable" style="float:right;"▼
===With an RGBI monitor===▼
When using a direct-drive monitor, the four color bits are output directly to the DE-9 connector at the back of the card.▼
Within the monitor, the four signals are interpreted to drive the red, green and blue color guns. With respect to the RGBI color model described above, the monitor would
! style="color:black; background:#aa0;" colspan="2"|dark yellow▼
|- style="color:black; background:#aa0;"▼
|| 6▼
|| #AAAA00▼
|}▼
Color 6 is treated
This "RGBI with tweaked brown" palette was retained as the default palette of later PC graphics standards such as [[Enhanced Graphics Adapter|EGA]] and [[Video Graphics Array|VGA]], which can select colors from much larger gamuts, but default to these until reprogrammed.▼
Later video cards/monitors in CGA emulation modes would approximate the colors with the following formula:
''red'' := 2/3×(''colorNumber'' & 4)/4 + 1/3×(''colorNumber'' & 8)/8▼
''green'' := 2/3×(''colorNumber'' & 2)/2 + 1/3×(''colorNumber'' & 8)/8▼
''blue'' := 2/3×(''colorNumber'' & 1)/1 + 1/3×(''colorNumber'' & 8)/8▼
if (color == 6)
green := green * 2/3
which yields the canonical CGA palette<ref name="viler"/>:
! colspan="4" | Full CGA 16-color palette
|- style="color:white;"
Line 230 ⟶ 256:
| colspan="4" |<small>''equivalents, internally CGA is 4-bit RGBI''</small>
|}
▲===With an RGBI monitor===
▲When using a direct-drive monitor, the four color bits are output directly to the DE-9 connector at the back of the card.
▲Within the monitor, the four signals are interpreted to drive the red, green and blue color guns. With respect to the RGBI color model described above, the monitor would use approximately the following formula to process the digital four-bit color number to analog voltages ranging from 0.0 to 1.0:
▲ ''red'' := 2/3×(''colorNumber'' & 4)/4 + 1/3×(''colorNumber'' & 8)/8
▲ ''green'' := 2/3×(''colorNumber'' & 2)/2 + 1/3×(''colorNumber'' & 8)/8
▲ ''blue'' := 2/3×(''colorNumber'' & 1)/1 + 1/3×(''colorNumber'' & 8)/8
▲{| class="wikitable" style="float:left;"
▲! style="color:black; background:#aa0;" colspan="2"|dark yellow
▲|- style="color:black; background:#aa0;"
▲|| 6
▲|| #AAAA00
▲|}
▲Color 6 is treated differently; when using the formula above, color 6 would become ''dark yellow'', as seen to the left, but in order to achieve a more pleasing brown tone, special circuitry in most RGBI monitors, starting with the IBM 5153 color display,<ref>International Business Machines Corporation (1983): IBM Personal Computer XT Technical Reference Manual, pages D-42 to D-43.</ref> makes an exception for color 6 and changes its hue from dark yellow to brown by reducing the analogue green signal's amplitude. The exact amount of reduction differed between monitor models: the original IBM 5153 Personal Computer Color Display reduces the green signal's amplitude by about one third,<ref>{{cite web|url=https://forum.vcfed.org/index.php?threads/representing-ibm-5153-color-output-more-accurately.1234458/ |title=Representing IBM 5153 color output more accurately | Vintage Computer Federation Forums |publisher=Forum.vcfed.org |date= December 2021|accessdate=2022-03-21}}</ref> while the IBM 5154 Enhanced Color Display internally converts all 4-bit RGBI color numbers to 6-bit ECD color numbers,<ref name=":5">{{Cite book|url=http://minuszerodegrees.net/oa/OA%20-%20IBM%20Enhanced%20Color%20Display%20(5154).pdf|title=IBM Personal Computer Hardware Reference Library: IBM Enhanced Color Display |page=4}}</ref> which amounts to halving the green signal's amplitude. The Tandy CM-2,<ref>{{Cite book|url=https://archive.org/details/tandy-service-manual-cm-2-color-monitor-26-3212|title=Tandy CM-2 Color Monitor Service Manual|page=48}}</ref> CM-4<ref>{{Cite book|url=https://www.manualslib.com/products/Tandy-Cm-4-8967293.html|title=Tandy CM-4 Color Monitor Service Manual|page=41}}</ref> and CM-11<ref>{{Cite book|url=https://archive.org/details/Sams_Computerfacts_Radio_Shack_Model_CM11_Monitor_1988_Howard_Sams|title=Sams&Company ComputerFacts Technical Service Data: Magnavox® Model 7BM613074G - Radio Shack® Model CM11 Monitor|year=1988 |page=11}}</ref> monitors provide a potentiometer labelled "BROWN ADJ." to adjust the amount of green signal reduction.
▲This "RGBI with tweaked brown" palette was retained as the default palette of later PC graphics standards such as [[Enhanced Graphics Adapter|EGA]] and [[Video Graphics Array|VGA]], which can select colors from much larger gamuts, but default to these until reprogrammed.
===With a composite color monitor/television set===
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