MOB slow down

[Descolado]

I'm trying to slow down a MOB at certain points in its movement (when it reaches the top or the bottom of the screen). I'm using the code below and it works for a few seconds. When FRAME reaches a value of 1500 the Enemy MOB simply STOPS on the screen. Does anyone know why this happens? 

 

I remember reading here on the forum once (I didn't find it anymore) something about "moving half a pixel". I've seen in some posts / examples something like "Speed = $ 20" but I don't understand how it works.

Does anyone have another suggestion to change the speed of this enemy (making it slower at certain times)?

 

DIM speedX1: speedX1 = 1

DIM speedY1: speedY1 = 1

DIM delta: delta = 4

 

Enemy: PROCEDURE
    SPRITE 2, X1 + HIT + VISIBLE, Y1 + ZOOMY2, CS_RED + SPR02

    print at screenpos(5,5), <5>frame
        
    IF (FRAME % delta) THEN        
        X1 = X1 + speedX1 
        Y1 = Y1 + speedY1        
    END IF
        
    IF X1 >= 160 THEN speedX1 = -1
    IF X1 <= 8 THEN speedX1 = 1
    
    IF Y1 >= 80 THEN speedY1 = -1: changed = 1
    IF Y1 <= 8 THEN speedY1 = 1  : changed = 0  
    
    IF changed = 0 THEN delta = 4 ELSE delta = 2
    
END

[+nanochess]

You should use "AND delta" and assign values 3 and 1 to 'delta' variable.

 

The reason for this is because the division operation is done with successive subtraction, and once FRAME reachs 1500 it takes more than one frame of video to do a "big" division.

 

The idiom SPEED = $20 could be interpreted like this: you are assigning the speed 32 to a "thing", let us say for #x and #y (we need 16 bits for big values, also put the statement SIGNED SPEED at the start of your code)

 

Let us say that in each frame you add SPEED to #X and #Y, also let us say that you want to move #x and #y for a pixel each frame. So 32 is equal to one pixel per frame.

 

We need to convert this #X and #Y variables to the real pixel coordinates, so we use this code:

 

     x = #x / 32

     y = #y / 32

 

And then we can use the result values for positioning the sprite on screen.

 

The beauty of this is that now we can modify SPEED to be anywhere from $01 to $40 and it will move in different speeds very smoothly! And also can be a negative value to move in reverse.

 

Edited May 30, 2021 by nanochess

[Descolado]

Thanks, Oscar.

I am using his first book to continue the programming process on Intellivision, but I am very rusty, I have not been working with code for 3 years, it is helping a lot. The second book is already in my plans.