SDLx::Sprite rotation artifacts and size limitation

[Andreas Lund]

Hi,

Running Strawberry Perl on Win 7 x64 I have installed SDL and Alien::SDL from
CPAN (selecting option 1) and I'm writing a twin stick shooter game for fun. I
have three questions:

1) When rotating sprites using the ->rotation() method the sprites often get a
line of "artifact noise" along one of the sides, usually the right and/or
bottom edge of the original sprite. The effect is not constant and the
original sprites are all 100% transparent at the edges so I believe the
artifacts are introduced by the rotation algorithm. Or is it?

2) After experimenting with different sprite sizes, it seems that there's a
limit somewhere. When attempting to load a sprite that's too large, I get a
seemingly valid sprite object that just doesn't seem to render anything. Is
there a known size limit to sprites or could this be a driver issue? Or am I
overlooking something?

3) How exactly does the 'alpha_key' property work? I tried setting it to the
ubiquitous bright purple (FF00FF) but this just caused half my sprites to
become completely invisible. When using PNG files with an alpha channel, using
a key color seems pointless to me, which makes the POD example code somewhat
confusing:

# spawning can include almost all of the above:
my $sprite = SDLx::Sprite->new(
image => 'hero.png', # or surface => SDL::Surface
rect => SDL::Rect, # or x => $x, y => $y
clip => SDL::Rect,
alpha_key => SDL::Color, # or [$r, $g, $b]
alpha => 1,
rotation => 45, # degrees
);


If anyone is interested in a sneak peek, here's the code:
http://floyd.atc.no/Programming/Perl/IfItMoves/2012-04-23.zip

Suggestions are welcome :-)

--
Andreas Lund (fl...@atc.no)
Tel: +47 90077162
#include <disclaimer.h>
"Look at you hacker, panting and sweating as you run through my corridors.
How can you challenge a perfect immortal machine?" -- SHODAN, System Shock

[Tobias Leich]

Hi Andreas,

Am 23.04.2012 09:53, schrieb Andreas Lund:
> Hi,
>
> Running Strawberry Perl on Win 7 x64 I have installed SDL and Alien::SDL from
> CPAN (selecting option 1) and I'm writing a twin stick shooter game for fun. I
> have three questions:
>
> 1) When rotating sprites using the ->rotation() method the sprites often get a
> line of "artifact noise" along one of the sides, usually the right and/or
> bottom edge of the original sprite. The effect is not constant and the
> original sprites are all 100% transparent at the edges so I believe the
> artifacts are introduced by the rotation algorithm. Or is it?

Do you have a screenshot? I just can think of problems when
anti-aliasing and transparency come together...

>
> 2) After experimenting with different sprite sizes, it seems that there's a
> limit somewhere. When attempting to load a sprite that's too large, I get a
> seemingly valid sprite object that just doesn't seem to render anything. Is
> there a known size limit to sprites or could this be a driver issue? Or am I
> overlooking something?

See http://sdl.perl.org/SDL-GFX-Rotozoom.html, I believe width+height
must be less than 16384.

>
> 3) How exactly does the 'alpha_key' property work? I tried setting it to the
> ubiquitous bright purple (FF00FF) but this just caused half my sprites to
> become completely invisible. When using PNG files with an alpha channel, using
> a key color seems pointless to me, which makes the POD example code somewhat
> confusing:
>
> # spawning can include almost all of the above:
> my $sprite = SDLx::Sprite->new(
> image => 'hero.png', # or surface => SDL::Surface
> rect => SDL::Rect, # or x => $x, y => $y
> clip => SDL::Rect,
> alpha_key => SDL::Color, # or [$r, $g, $b]
> alpha => 1,
> rotation => 45, # degrees
> );

Ya, alpha and blitting is not the easiest things as someone might think.
If you have a png file with alpha channel, and applying an alpha color
key to it should reset the alpha channel. At least that is what I would
expect.
You cant mix the alpha channel and alpha key.

>
>
> If anyone is interested in a sneak peek, here's the code:
> http://floyd.atc.no/Programming/Perl/IfItMoves/2012-04-23.zip

Will have a look then ;o)

Cheers, FROGGS

[Tobias Leich]

Hi again,

Its quit cool, but I'm not that good when it comes to do two things at
the same time ;o)

I can't see anything strange when rotating the player-sprite.

Cheers, FROGGS.

[Andreas Lund]

Mon, 23 Apr 2012 20:56:21 +0200, Tobias Leich <em...@froggs.de> wrote:
> Hi again,
>
> Its quit cool, but I'm not that good when it comes to do two things at
> the same time ;o)
>
> I can't see anything strange when rotating the player-sprite.

The problem isn't constant but it's very annoying when it does appear. I've
posted an example screenshot here:

http://floyd.atc.no/Programming/Perl/IfItMoves/SDL-Sprite-artifacts.png

Tip: If you have downloaded the code, try setting "$cheatmode = 1" to become
immortal. You can now hit TAB a few times to spawn more enemies.