[Open-graphics] bootstrapping the video mode (outputting VGA text)

[Hamish Marson]

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Timothy Miller wrote:
> On 8/8/06, James Richard Tyrer <tyrerj at acm.org> wrote:
>> Timothy Miller wrote:
>>
>>> Are you saying interlacing is not an option here?
>>
>> We could do interlacing, but interlaced text can cause problems.
>>
>> IBM and other early systems that used a TV were not interlaced.
>>
>> OTOH, modern game systems are probably interlaced.
>>
>> If we do use interlacing, we need to send exactly the same frame
>> to both fields.  Otherwise, moving text is impossible to read and
>>  not at all pleasant to look at.
>
> Hmmm.... I had assumed we'd send alternating scanlines.  I suppose
> we could do 640x200.  But I'd really like to see what interlacing
> looks like when you try not use the same data each field.  I
> remember not being so bothered by it when seeing an Amiga.

When doing interlacing, it's generally a problem with high contrast
horizontal lines. Because you get flicker. Vertical lines, you tend
not to notice. In between, it depends on contrast & whether the line
is anti-aliased or not... (Stair step jaggies tend to flicker quite
badly).

I suspect it's worse (visibly) on PAL (50Hz/2 frames of 25Hz) than
NTSC (60Hz/2 frames of 30Hz). That extra 5Hz  would tend to make a bit
of difference, especially if your telly has a long persistence
phosphor (Or you trun down the contrast).

(The Amiga was a real pain in high-res interlaced modes on a video
monitor because of this).

Hamish.
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