SDL::Tutorial::Animation(3)				User Contributed Perl Documentation			       SDL::Tutorial::Animation(3)

NAME

       SDL::Tutorial::Animation

SYNOPSIS

	       # to read this tutorial
	       $ perldoc SDL::Tutorial::Animation

	       # to create a demo animation program based on this tutorial
	       $ perl -MSDL::Tutorial::Animation=sdl_anim.pl -e 1

ANIMATING A RECTANGLE

       Now that you can display a rectangle on the screen, the next step is to animate that rectangle.	As with movies, there's no actual motion.
       Computer animations are just very very fast slideshows.	The hard work is creating nearly identical images in every slide (or frame, in
       graphics terms).

       Okay, it's not that difficult.

       There is one small difficulty to address, however.  Once you blit one surface onto another, the destination is changed permanently.
       There's no concept of layers here unless you write it yourself.	If you fail to take this into account (and just about everyone does at
       first), you'll end up with blurry graphics moving around on the screen.

       There are two approaches to solve this problem, redrawing the screen on every frame and saving and restoring the background for every
       object drawn.

   Redrawing the Screen
       Since you have to draw the screen in the right order once to start with it's pretty easy to make this into a loop and redraw things in the
       right order for every frame.  Given a SDL::App object $app, a SDL::Rect $rect, and a SDL::Color $color, you only have to create a new
       SDL::Rect $bg, representing the whole of the background surface and a new SDL::Color $bg_color, representing the background color.  You can
       write a "draw_frame()" function as follows:

	       sub draw_frame
	       {
		       my ($app, %args) = @_;

		       $app->fill( $args{ bg }, $args{ bg_color } );
		       $app->fill( $args{rect}, $args{rect_color} );
		       $app->update( $args{bg} );
	       }

       Since you can change the "x" and "y" coordinates of a rect with the "x()" and "y()" methods, you can move a rectangle across the screen
       with a loop like this:

	       for my $x (0 .. 640)
	       {
		       $rect->x( $x );
		       draw_frame( $app,
			       bg   => $bg,   bg_color	 => $bg_color,
			       rect => $rect, rect_color => $color,
		       );
	       }

       If $rect's starting y position is 190 and its height and width are 100, the rectangle (er, square) will move across the middle of the
       screen.

       Provided you can keep track of the proper order in which to redraw rectangles and provided you don't need the optimal speed necessary
       (since blitting every object takes more work than just blitting the portions you need), this works quite well.

   Undrawing the Updated Rectangle
       If you need more speed or want to make a different complexity tradeoff, you can take a snapshot of the destination rectangle before you
       blit onto it.  That way, when you need to redraw, you can blit the old snapshot back before blitting to the new position.

       Note:  I have no idea how this will work in the face of alpha blending, which, admittedly, I haven't even mentioned yet.  If you don't know
       what this means, forget it.  If you do know what this means and know why I'm waving my hands here, feel free to explain what should and
       what does happen and why.  :)

       With this technique, the frame-drawing subroutine has to be a little more complicated.  Instead of the background rect, it needs a rect for
       the previous position.  It also needs to do two updates (or must perform some scary math to figure out the rectangle of the correct size to
       "update()".  No thanks!).

	       sub undraw_redraw_rect
	       {
		       my ($app, %args) = @_;

		       $app->fill(   $args{old_rect}, $args{bg_color}	);
		       $app->fill(   $args{rect],     $args{rect_color} );
		       $app->update( $args{old_rect}, $args{rect}	);
	       }

       We'll need to create a new SDL::Rect, $old_rect, that is a duplicate of $rect, at the same position at first.  You should already know how
       to do this.

       As before, the loop to call "undraw_redraw_rect()" would look something like:

	       for my $x (0 .. 640)
	       {
		       $rect->x( $x );

		       undraw_redraw_rect( $app,
			       rect	  => $rect,  old_rect => $old_rect,
			       rect_color => $color, bg_color => $bgcolor,
		       );

		       $old_rect->x( $x );
	       }

       If you run this code, you'll probably notice that it's tremendously faster than the previous version.  It may be too fast, where the
       alternate technique was just fast enough.  There are a couple of good ways to set a fixed animation speed regardless of the speed of the
       processor and graphics hardware (provided they're good enough, which is increasingly often the case), and we'll get to them soon.

SEE ALSO

       SDL::Tutorial::Drawing
	   basic drawing with SDL Perl

       SDL::Tutorial::Images
	   animating images

AUTHOR

       chromatic, <chromatic@wgz.org>

       Written for and maintained by the Perl SDL project, <http://sdl.perl.org/>.

BUGS

       No known bugs.

COPYRIGHT

       Copyright (c) 2003 - 2004, chromatic.  All rights reserved.  This module is distributed under the same terms as Perl itself, in the hope
       that it is useful but certainly under no guarantee.

perl v5.12.1							    2010-07-05					       SDL::Tutorial::Animation(3)