Game speed discussion

[BippyM]

Hi Guys,


Been a while since I have created a new thread, but here we go.


What are the common methods used for determining speed withion games? for example.



If a game runs at 50fps, how do you determine the speed the character moves at? Is it just simply skipping the number of frames drawn to the screen, moving the character more pixels to show the increased speed etc??


Would the process be the same for slowing down? At what point does the speed start looking jerky or disjointed??

[phx]

Originally Posted by BippyM

If a game runs at 50fps, how do you determine the speed the character moves at? Is it just simply skipping the number of frames drawn to the screen, moving the character more pixels to show the increased speed etc??

Yes. The simplest approach is to define the speed in pixels per frame.

If you need more fine-grained speeds, like 1.35 px/frame, you would store the coordinates in fixed point format. For example one word for the full pixels and one word for the 1/65536 fractions.

With 1.35 px/frame you would add 1 in the integer word and 22938 in the fractional word. Or in assembler:

        add.l   #$0001599a,xpos

For drawing the object on screen you only use the integer word.

Would the process be the same for slowing down?

Yes. See above.

At what point does the speed start looking jerky or disjointed??

Hmm. Don't know what you mean. With 50fps it is always as good as it can get. You cannot draw intermediate positions of a fast moving objects. Only 50 new positions per second.

[BippyM]

How can you move in fractions though? Like 1 quarter pixel scrolling or moving half a pixel.. Surely it'll only show full pixel movement?

[mcgeezer]

Originally Posted by BippyM

How can you move in fractions though? Like 1 quarter pixel scrolling or moving half a pixel.. Surely it'll only show full pixel movement?

What you do is simply increase the resolution of the screen canvas.

So if you think of a typical 320x200 screen, if you zoomed out by a multiple of 16 the screen size would be 5120x3200.

This then allows you to move objects in fractions (1/16th).

If you have a sprite that has coordinates of 100x100 and you want to move it an entire 1 pixel to the right you'd do something like this....

(rough example...)

  move .w SPRITE_XPOS,d0        ; 100 xpos
  move .w SPRITE_YPOS,d1        ; 100 ypos
  lsl.w #4,d0        
  lsl.w #4,d1
  add.w #16,d0                   ; Speed 1-16 - 1=slowest, 16=fast (1 pixel).
  lsr.w #4,d0
  lsr.w #4,d1
  move.w d0,SPRITE_XPOS        ; 101 xpos
  move.w d1,SPRITE_YPOS        ; 100 ypos

Geezer

[DanScott]

Originally Posted by mcgeezer

(rough example...)

  move .w SPRITE_XPOS,d0        ; 100 xpos
  move .w SPRITE_YPOS,d1        ; 100 ypos
  lsl.w #4,d0        
  lsl.w #4,d1
  add.w #16,d0                   ; Speed 1-16 - 1=slowest, 16=fast (1 pixel).
  lsr.w #4,d0
  lsr.w #4,d1
  move.w d0,SPRITE_XPOS        ; 101 xpos
  move.w d1,SPRITE_YPOS        ; 100 ypos

Geezer

That's a very rough example, as you would lose any of the fractional part when you shift it right and store it back :P

[mcgeezer]

Originally Posted by DanScott

That's a very rough example, as you would lose any of the fractional part when you shift it right and store it back :P

Yeah, I did say rough example. It was just to try and simplify the logic for the OP. PHX's example is much more efficient but might not be obvious as to what's going on for some.

[zero]

Many games use sub-pixel positioning, the most famous being Mario Bros.