Alternating between two frames when walking

[Daniel Manoiu]

Hello everyone, I'm new here and very new to Atari programming. 

I'm having some issues trying to figure out how to have my player character alternate between two frames of animation when moving. So far, this is the code that I came up with:

 

AreWeChanging:
    lda P0FrameCounter    
    cmp #10
    bne ChangeSpriteBack

ChangeSprite:    
    lda #0
    sta P0FrameCounter
    lda #<P0Sprite2 
    sta P0SpritePtr ; lo byte pointer for P0 Sprite
    lda #>P0Sprite2
    sta P0SpritePtr+1 ; hi byte ptr for P0 Sprite
    jmp End

ChangeSpriteBack:

    lda #<P0Sprite1
    sta P0SpritePtr ; lo byte pointer for P0 Sprite
    lda #>P0Sprite1
    sta P0SpritePtr+1 ; hi byte ptr for P0 Sprite
    

End:

    jmp StartFrame  

Basically, it's just switching to the second sprite of the animation when the P0FrameCounter variable reaches 10 and then switches back. I am incrementing the variable when moving the joystick in any direction. This is not great for 2 reasons: it only changes to the seconnd sprite for one frame of animation then changes to the first one for 10 frames; and, seems to be a bit buggy if I'm testing it in the emulator and pressing two directional buttons, or moving diagonally.

 

What approach should I use to alternate between the two sprites for an equal amount of time (let's say 10 frames per sprite), only when the directional buttons are pressed? Thanks in advance for your help!

 

 

 

[DaveM]

Hi Daniel,

 

I'm relatively new to Atari Programming too.  I started working on my first game, Mr. Yo-Yo back in September, and it's still a work in progress.  You can find a thread for it on this forum.  Anyway, I had a similar issue, and I think this will solve it.  Instead of doing a cmp, use an and.  I'd also use 8 frames instead of 10, so you're only dealing with 1 bit.  So your first part of code, the AreWeChanging section, would look like this:

 

AreWeChanging:

   lda P0FrameCounter

   and #8

   bne ChangeSpriteBack

 

This isolates the #8 bit, so when it's set to 1, it branches.  When it's a 0, it doesn't take the branch.

 

Oh, and I should add that by doing it this way, there's no need to reset the P0FrameCounter to 0 in ChangeSprite.  It can continue to count up, and since you're only checking one bit, you don't need a reset. 

 

Hope that helps!

Dave

Edited February 12, 2020 by DaveM

[+SpiceWare]

2 hours ago, Daniel Manoiu said:

I'm having some issues trying to figure out how to have my player character alternate between two frames of animation when moving.

 

My tutorial covers animating the players in step 14. I use a 4 frame animation, if you want just 2 frames then change the and #3 to and #1.

[+Andrew Davie]

You could use your frame number as an index into a lo/hi table pair to give you a pointer to each frame.

So, place the sprite data adress lo bytes into one table, hi bytes into another, load an index register (say, X) with the frame #

Then, load LO,x --> P0SpritePtr and load HI,x --> P0SpritePtr+1 and bob's your uncle.

Then you don't need lots of conditional code.

[bogax]

perhaps you could keep the sprites in the same page and just flip the lo byte of the pointer

something like

 

AreWeChanging:
    lda P0FrameCounter    
    cmp #10
    bne SkipFlip

FlipSpritePointer:    
    lda #0
    sta P0FrameCounter
    lda P0SpritePtr; lo byte pointer for P0 Sprite 
    eor #<P0Sprite1 ^ <P0Sprite2
    sta P0SpritePtr

SkipFlip:

or if a power of 2 will do for the count let the counter run and AND with a 2**n - 1

and bne

Edited February 13, 2020 by bogax

[Daniel Manoiu]

12 hours ago, DaveM said:

Hi Daniel,

 

I'm relatively new to Atari Programming too.  I started working on my first game, Mr. Yo-Yo back in September, and it's still a work in progress.  You can find a thread for it on this forum.  Anyway, I had a similar issue, and I think this will solve it.  Instead of doing a cmp, use an and.  I'd also use 8 frames instead of 10, so you're only dealing with 1 bit.  So your first part of code, the AreWeChanging section, would look like this:

 

AreWeChanging:

   lda P0FrameCounter

   and #8

   bne ChangeSpriteBack

 

This isolates the #8 bit, so when it's set to 1, it branches.  When it's a 0, it doesn't take the branch.

 

Oh, and I should add that by doing it this way, there's no need to reset the P0FrameCounter to 0 in ChangeSprite.  It can continue to count up, and since you're only checking one bit, you don't need a reset. 

 

Hope that helps!

Dave

@DaveM Thank you! The and #8 worked great for the simple animation I was going for. It did shift some things around with the change of sprite when player collides with enemy, but I think I can figure that out. 
 

@SpiceWare thank you, what a great resource! I got into Atari programming from the Udemy course, but I plan to follow all the steps in your tutorial to learn more. 
 

@Andrew Davie that seems like a way more efficient and less verbose way of doing this. I’ll give it a try. 

@bogax yes, the AND/ bne seems to work  great, I didn’t really commit to the 10 frame switch, it was pretty arbitrary based on what I thought would look decent. But will definitely try the lo byte trick. I have an enemy sprite (P1) that I’d like to flip between two frames of animation at controlled random speeds, that will be a new challenge!