Re: CRT gun driver levels?

On Fri, 21 Aug 1998 15:55:45 -0700, Clay Cowgill <ClayC@diamondmm.com> wrote:

>> The PROM idea should work great. You won't need to run the beam
>> On/Off line
>> through it since it is handled the same by all platforms.
>>
>I'm not following the "run the beam On/Off line through" part. You
>still need beam on/off for blanking control right? If the lookup PROM
>is big enough you could run "on/off" to an output enable (with
>pullups/downs on the outputs) or to a high address line on the PROM so
>that when "off" is asserted the PROM always outputs "00" or something
>(aka, color guns off) for intensity, no?

Sure, that would work great! But if it's easier to run it to a blanking
transistor (or the chip enable lines as you pointed out) that brought the output
to zero, you would only use half as much PROM space (since half the PROM would
just contain zeros). That's all I meant.
>
>> The other cool thing about a PROM as a intensity-look-up is that you
>> could
>> easily convert the two color games to shades of gray. That would be
>> pretty
>> cool.
>>
>Oh, yeah. That would be neat...
>
>> The formula for RGB to gray scale that takes into account the fact
>> that our eyes
>> are more sensitive to green, etc is:
>>
>> gray_value = R * 0.2989 + G * 0.5870 + B * 0.1140
>>
>> It should be pretty easy to adjust the PROM lookup for color to
>> convert it to
>> gray scales.
>>
>Gotcha... So it takes roughly twice as much brightness of a pure red to
>equal the perceived brightness of a pure green? Guess I remember that
> from comp-graphics school... Been a while since I've worried about
>conevrting stuff to grayscale though! ;-)

Yeah, I got this off comp.compression when someone asked how to save color
images as gray scales. Hmm... (I thought) that looks like something that could
be useful some day, so I snagged the formula. (I've seen it since in some
graphic books.)

-Zonn

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Received on Fri Aug 21 18:31:57 1998