Trying to change the frame of a sprite (and failing!)

[pablo]

Hello, it's good to be here
Sorry if my English isn't perfect.
I'm trying to show a sprite as a dying person at the beginning of the game and then show him as dead after we speak to him at first.



EVENT SPRITETYPE8

IF SCREEN = 8             ; at the right screen
  IF IMAGE 12            ; if he is my dying friend
     IF B > 0            ; but the story has advanced and he is already dead
        LET FRAME = 1    ; show his frame as dead
     ENDIF
  ENDIF
ENDIF

IF COLLISION PLAYER
  IF IMAGE 12
     IF B = 0                        ; the story hasn't advanced yet
        COLOUR 71
        AT 10 3
        PRINT "Oh I'm dying...."    ; he dies
        WAITKEY
        INK 7
        LET B = 1                   ; the story advances
        LET FRAME = 1               ; and we show him dead
        REDRAW
     ENDIF
  ENDIF
ENDIF
Why doesn't this work? The message appears and the story advances, but the image is still the first frame (dying instead of dead). What am I doing wrong?

Thanks in advance!

[pablo]

I've managed to initialize the sprite to "dead" but only when you reenter the screen or you die, with this code:


EVENT INITSPRITE

IF TYPE 8
  IF IMAGE 12
     IF B > 0
        LET FRAME = 1
     ENDIF
  ENDIF
ENDIF
But I can't find the way to do it right after you speak with him (the "LET FRAME = 1" in the COLLISION PLAYER doesn't seem to work)

[pablo]

I did it!


IF COLLISION PLAYER
  IF IMAGE 12
     IF B = 0                        ; the story hasn't advanced yet
        LET FRAME = 1               ; We show him dead
        COLOUR 71
        AT 10 3
        PRINT "Oh I'm dying...."    ; he dies
        WAITKEY
        INK 7
        LET B = 1                   ; the story advances
        REDRAW
     ENDIF
  ENDIF
ENDIF
Somehow, if I change the position of that "LET FRAME = 1" right at the beginning of that code block, it works! It doesn't work if it's right before the "REDRAW". Why is that?

[ramon]

Read this article. may be useful for you: pacmanzxspectrum.blogspot.com/2020/11/main-loop.html

[Jonathan Cauldwell]

Aug 11, 2023 8:24:29 GMT pablo said:

I did it!


IF COLLISION PLAYER
  IF IMAGE 12
     IF B = 0                        ; the story hasn't advanced yet
        LET FRAME = 1               ; We show him dead
        COLOUR 71
        AT 10 3
        PRINT "Oh I'm dying...."    ; he dies
        WAITKEY
        INK 7
        LET B = 1                   ; the story advances
        REDRAW
     ENDIF
  ENDIF
ENDIF
Somehow, if I change the position of that "LET FRAME = 1" right at the beginning of that code block, it works! It doesn't work if it's right before the "REDRAW". Why is that?

The way sprites are stored is to have a copy of the previous sprite information (coordinates, image, frame etc) and the sprite's next set of information. When the sprite is redrawn the old sprite is deleted while the new sprite is simultaneously displayed on screen. It's one of the ways we avoid flicker and tearing. Once this is done the new details are copied to the old details and the events code can change the new details. Obviously, calling REDRAW will display the old sprite details as those will be deleted the next time the sprites are updated.

Off the top of my head I suspect the WAITKEY might be the reason why it works as the engine does some housekeeping (I can't remember exactly what) while it waits for a keypress.