Odd update problem

SDL Development
1

Hello,

I’m having an odd update problem with SDL, wrapped via my library ZXLIB.

As an example ZXLIB program I’m developing a somewhat-Pacman-like game in
which you play a professor who has to avoid the bunsen burners and build
his robot (based on an old Spectrum game). On each game cycle I do the
following with the professor and bunsen burner sprites:

  • blank out the previous position, by restoring the background
  • redraw the sprite image
  • update the sprite rectangles, if each sprite moved

It normally works fine. However if two sprites overlap (e.g. two bunsen
burners next to each other chasing the professor down the same passage),
the restoration of the background doesn’t happen under them and they leave
a trail.

I’ve double checked my code and it isn’t anything obvious. I can only
assume it’s something subtle to do with updating, something I have to be
honest consider a bit of a black box and don’t understand fully. (if you
blit an image to screen memory, why do you have to update the screen? This
was not necessary under simpler setups such as MSDOS or 80s micros
graphics mode).

Thanks,
Nick

2

Simple answer. You of course save the background just before each sprite
is blitted. So if two sprites overlap and you don’t restore the
backgrounds in reverse order of the order you blit the sprites it will
leave the parts of the first two overlaping sprites to the screen.

I think this will explane it better.

Initial image:On Friday 10 January 2003 01:56, Nick Whitelegg wrote:

Hello,

I’m having an odd update problem with SDL, wrapped via my library
ZXLIB.

As an example ZXLIB program I’m developing a somewhat-Pacman-like
game in which you play a professor who has to avoid the bunsen
burners and build his robot (based on an old Spectrum game). On each
game cycle I do the following with the professor and bunsen burner
sprites:

  • blank out the previous position, by restoring the background
  • redraw the sprite image
  • update the sprite rectangles, if each sprite moved

It normally works fine. However if two sprites overlap (e.g. two
bunsen burners next to each other chasing the professor down the same
passage), the restoration of the background doesn’t happen under them
and they leave a trail.

I’ve double checked my code and it isn’t anything obvious. I can only
assume it’s something subtle to do with updating, something I have to
be honest consider a bit of a black box and don’t understand fully.
(if you blit an image to screen memory, why do you have to update the
screen? This was not necessary under simpler setups such as MSDOS or
80s micros graphics mode).

After first sprite

-1-

After second sprite

-2-

After restoring the first sprites background

After restoring the second sprites background

-1-

This is because when you blit the second sprite the background that is
saved contains the first sprite.

So if your code is something like this.

// Clean the monster backgrounds
for(i=0; i<MONSTERS; i++)
MyBlit(monster[i].background, monster[i].oldx, monster[i].oldy);

make it go the list backwards like this

// Clean the monster backgrounds
for(i=MONSTERS-1; i>=0; i–)
MyBlit(monster[i].background, monster[i].oldx, monster[i].oldy);

And everything works right.