Plotting Pixels

[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?

[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

[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?

[1024MAK]

Yes. Note however that the system does take account of a previous plot, but overwrites other characters.

PLOT program

Mark

[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]

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]

Thank you! Interesting to know plot uses Chr 2! It’s a clever innovation.

[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]

Good to know!

[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