Plotting Pixels

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
Post Reply
MrVertigo
Posts: 105
Joined: Fri May 27, 2022 9:06 pm

Plotting Pixels

Post by MrVertigo »

In Chapter 18 of the ZX81 manual it says each character space is made up of 4 pixels. But isn’t each character space made up of 64 pixels (8x8)?

Why doesn’t it just say that plot and unplot allow you to create a block of 16 pixels on the screen in a 4x4 square? Is there a reason for that? Or am I misunderstanding something?
User avatar
1024MAK
Posts: 5058
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Plotting Pixels

Post by 1024MAK »

As far as the plot and unplot commands are concerned, a pixel is as described. That is a block, or quarters of a character.

In reality, each character is made up of a number of bytes, each of which has eight pixels per byte. But, under BASIC (assuming no expansions), the program does not have control of these individual byte and hence the individual pixels.

A plot or unplot command actually just changes the character stored in the display file.

Hi-res (all types) and hardware expansions were developed long after the manual was written.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
MrVertigo
Posts: 105
Joined: Fri May 27, 2022 9:06 pm

Re: Plotting Pixels

Post by MrVertigo »

Thanks! That makes sense. So with plot unplot you’re really just getting one of the graphical characters filling the whole character space, depending on which quarters are plotted and unplotted?
User avatar
1024MAK
Posts: 5058
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Plotting Pixels

Post by 1024MAK »

Yes. Note however that the system does take account of a previous plot, but overwrites other characters.
PLOT program
PLOT program
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
MrVertigo
Posts: 105
Joined: Fri May 27, 2022 9:06 pm

Re: Plotting Pixels

Post by MrVertigo »

Ah, yes- I see!

Is there anyway to plot the “chequerboard” quarters of the graphic characters using basic, or is it just black and off?
Moggy
Posts: 3195
Joined: Wed Jun 18, 2008 2:00 pm

Re: Plotting Pixels

Post by Moggy »

I would say only by using UDGs and altering CHR 2 which is the CHR that PLOT uses and is something I have done on some game conversions.
However this supposes ownership of a UDG board of some description or emulation and the use of a UDG designer one of which was supplied by Andy Rea and the other by Paul Farrow both can be found by using search.
MrVertigo
Posts: 105
Joined: Fri May 27, 2022 9:06 pm

Re: Plotting Pixels

Post by MrVertigo »

Thank you! Interesting to know plot uses Chr 2! It’s a clever innovation.
Moggy
Posts: 3195
Joined: Wed Jun 18, 2008 2:00 pm

Re: Plotting Pixels

Post by Moggy »

There is a slight twist to it now I recall and is one of the reasons I stopped using it. I found that after changing CHR 2 to something different then using PLOT, it would only PLOT the changed CHR some of the time, occasionally reverting back to the original "black dot" and then changing back to my altered UDG when called again. I assume that it was sometimes taking the fixed CHR code from the ROM to PLOT and sometimes my altered CHR 2.

Quite why it did this is beyond me and I soon dropped the idea now I think back to it.
MrVertigo
Posts: 105
Joined: Fri May 27, 2022 9:06 pm

Re: Plotting Pixels

Post by MrVertigo »

Good to know!
Crayon21
Posts: 334
Joined: Sun Nov 04, 2018 2:33 am

Re: Plotting Pixels

Post by Crayon21 »

What if I was using graph paper? Where would the X and Y areas start(when the screen is drawn to be precise)? Assuming I have a perfect grid, where would X and Y start on the ZX81?

I need to know this but otherwise, I'm doing it blind
In Heck, there are two options for perpetual torment:

Eat the Puckerberry and suffer for eternity:
drink nothing but a cocktail of The Last Dab and Mexican Cake blended and served with
habanero slices
:twisted:
Post Reply