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SDL RC 2.5 decides to play with PDL

PDL provides great number crunching capabilities to Perl and SDL provides game-developer quality real-time bitmapping and sound.
You can use PDL and SDL together to create real-time,
responsive animations and simulations.
In this section we will go through the pleasures and pitfalls of working with both powerhouse libraries.
-- David Mertnes


Creating a SDL Surface piddle


PDL's core type is a piddle.
Once a piddle is provided to PDL it can go do a numerous amounts of things.
Please see the example in 'examples/cookbook/pdl.pl' in github.

Creating a simple piddle


First lets get the right modules.

  use PDL;
  use SDL::Rect;
  use SDL::Video;
  use SDL::Surface;
  use SDL::PixelFormat;

Suppose you want a surface of size (200,400) and 32 bit (RGBA).

my ( $bytes_per_pixel, $width, $height ) = ( 4, 200, 400 );

Define the $width, $height and $bytes_per_pixel. Your $bytes_per_pixel is the number of bits (in this case 32) divided by 8 bits per byte. Therefore for our 32 bpp we have 4 Bpp;

my $piddle  = zeros( byte, $bytes_per_pixel, $width, $height );

Create a normal $piddle with zeros, byte format and the Bpp x width x height dimensions.

my $pointer = $piddle->get_dataref();

Here is where we get the acutal data the piddle is pointing to. We will have SDL create a new surface from this function.

my $surface = SDL::Surface->new_from( $pointer, $width, $height, 32,
        $width * $bytes_per_pixel );

Using the same dimensions we create the surface using SDL::Surface->new_form(). The $width * $Bpp is the scanline (pitch) of the surface in bytes.

warn "Made surface of $width, $height and ". $surface->format->BytesPerPixel;
   return ( $piddle, $surface );

Finally make sure that the surface acutally has the correct dimensions we gave.

NOTE: $surface->format->BytesPerPixel must return 1,2,3,4. !!

Now you can blit and use the surface as needed; and do PDL operations as required.

Operating on the Surface safely


To make sure SDL is in sync with the data. You must call SDL::Video::lock before doing PDL operations on the piddle.

SDL::Video::lock_surface($surface);
 
    $piddle ( :, 0 : rand(400), 0 : rand(200) ) .=   pdl( rand(225), rand(225), rand(255), 255 );

After that you can unlock the surface to blit.

SDL::Video::unlock_surface($surface);

Errors due to BPP at blitting


When blitting the new surface check for the return value to see if there has been a problem.

my $b = SDL::Video::blit_surface(
        $surface,  SDL::Rect->new( 0, 0, $surface->w, $surface->h ),
        $app, SDL::Rect->new(  ( $app->w - $surface->w ) / 2, ( $app->h - $surface->h ) / 2, $app->w, $app->h )
       );
 
    die "Could not blit: " . SDL::get_error() if ( $b == -1 );

If the error message is 'Blit combination not supported' that means that the BPP is incorrect or incosistent with the dimensions. After that a simple update_rect will so your new surface on the screen.


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