Starfield demo

[TBCTBC]

Ok, I didn't think I would be able to finish this fast, but here's that starfield demo with source code I promised.
My first program for Creativision

[Mobsie]

Hi,

very nice, thank you.

[MADrigal]

Thanks a lot. Very nice indeed!!!

I'll try to study the source code asap.

[TBCTBC]

I'm actually not using the same method as I did in Photon. Before I used 128 characters for stars, because I only used sprites for the rest of the game. Here I only use two, because in Photon 2 I'm going to have software sprites, so I needed a different approach. As you see, it also takes a lot of shortcuts to be fast and have a small memory footprint. You can see the pattern of stars repeat. There's still some improvements I could do, like changing colors.

[MADrigal]

I still need to figure out the "logic" of the routines that make characters change to resemble scrolling stars.

[TBCTBC]

I thought I would explain a little and realized I change even the slow star character every frame, even though it only moves every other frame. Maybe I should have implemented it differently.
Anyway, this code:

Code: Select all

  ldx #0		; Edit slow character
ml4:
  lda #0
  cpx slowchar
  bne ml5
  lda #8
ml5:
  sta VDP_Data_Write
  inx
  cpx #8
  bne ml4

fills the character for the slow star with zero except on the row specified by slowchar, where it puts 8 instead (the star).
So, by increasing slowchar the star moves down one pixel.
The routine is exactly the same for the fast star.

Then, this code:

Code: Select all

  ldx fastchar		; Change vertical position of star inside fast character
  inx
  cpx #8
  bne lo3$
  ldy #16
  jsr move_stars	; Move fast character
  ldx #0
lo3$:
  stx fastchar

increases the value of fastchar by one every frame, until it reaches eight, when it resets to zero (so the star will be in the top of the character again).
When it resets to zero, I also call the routine move_stars, which moves the character down on the screen.
I hope that explains it a little.

[TBCTBC]

Here's a new version with some blinking stars.

[MADrigal]

I need to study it a bit. I looked at the source code and it seems quite readable

btw I see no blinking stars there.

[TBCTBC]

Well, they are changing color in this order: grey, dark yellow, light yellow, white and back again.

[MADrigal]

There must be some issue with my emulator... I see always the same colour.