| Posted By
Luca
on 2023-03-31
13:34:03
 |  [YAPE] The color purple
@Gaia you probably have an explanation for this.
Look at the two screenshots: they are the same thing in the same moment. Look at the purple stripe: it includes some 1x2 characters of the same exact purple ($A4). Left, we have Digitized Colors palette, and chars and background regularly are merged as expected. Right: the other two palettes, and the chars look like blue!
Why?
 
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Posted By
Csabo
on 2023-03-31
15:43:38
| Re: [YAPE] The color purple
What do you mean by $A4? You can have this for a $FF15 value (since the highest bit will always be set), but for the color RAM, $A4 would be flashing.
Also, simplest explanation: $FF15 and the color RAM have two different colors. You can easily verify this:
MONITOR
>FF15 A4
>53B 24
Type anything, and change the palette settings (which will have no effect - the same background + foreground color means invisible).
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Posted By
Luca
on 2023-03-31
16:47:06
| Re: [YAPE] The color purple
Of course I meant bit4 of $FF07 set, in a situation in which I clearly expect the same colour, and facing this paradoxical event I did exactly the test you've described. But most of all, I pointed out that the code is the same and the only thing which differentiates the two results is the choice of the emulator's palette. I also add that the other emulators give what we all expect, i.e. the situation as for the left sided screenshot.
Briefly: no code problem at all.
EDIT: Csabo wisely asked me to have something in order to reproduce this strange behavior in YAPE, so here it is a short stuff in Basic. Enter a value 00-FF which will be used both for background and text, and get back with Run/Stop to try new values, using YAPE with Measured Hues palette on, then do the same but with one of the two other color palettes.
I saw this with the magical value of 24/A4 ($FF07 4bit set, so that's the same), but it does the same with several other colors.
I am quite sure it's my fault as I'm doing a bad assumption somewhere, something very easy which I'm missing to catch at the moment...
 
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Posted By
Gaia
on 2023-04-01
06:58:32
| Re: [YAPE] The color purple
@Luca: what you're seeing is the Hanover bars artifact, specific to the PAL video standard, which is not emulated with a digitized palette. Since not all TV's are showing this artifact I might as well make it optional...
https://en.wikipedia.org/wiki/Hanover_bars
EDIT: I realized there is a bug in the implementation, too :D thanks for pointing this issue out
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Posted By
Luca
on 2023-04-01
07:13:02
| Re: [YAPE] The color purple
@Gaia oh, thank you, I suspected something of that kind, but I'd never expected a so strong result.
I have one more question: even from the sample images offered on the Wikipedia entry, one can see that the Hanover bars artifact alternates bars of desired color / artifact color. Zooming into the PNG I've shown, I see that the character which I expect to have the same exact purple of the background, actually doesn't show the artifact color alternating the desired color, but two completely different colors! Is it a secondary effect of the Hanover bars?
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Posted By
Gaia
on 2023-04-01
08:13:50
| Re: [YAPE] The color purple
Well, it's complicated, theoretically you have a hue phase difference in odd and even lines, this the PAL device is filtering out by averaging the signal so you shouldn't see bars but slightly different colours. As I wrote, there is also a bug in the implementation so it needs to be fixed (and is also way too strong).
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