SDL - Alpha channel issues

RuneLancer · 2004-04-06T23:22:28

Heya! I''ve been playing around with alpha transparency using SDL and I''m quite impressed by what it can do (and with so much ease, too!) However, I''ve run into a little bit of a snag. As you can see from the window, I can only seem to set alpha transparency for the full surface itself, not just a part of it (I''m trying to make the inside of the window 50% translucent). The obvious solution would be to use a PNG or other format with an alpha channel, however this raises an other issue: loading it. SDL only contains methods for BMP loading, and I don''t want to add support for a format I only plan on using in one place (as I understand it, this would mean adding support for a host of decompression algorithms and the likes; I''m not being lazy, I''m just trying to avoid something I really don''t need). All''s I need to do is, basically, set 0x00101010 to 0x80101010 (and support having 0x00FF00FF as my color key, too). What can I do?

For Beginners

Started by RuneLancer April 04, 2004 02:33 PM

50 comments, last by RuneLancer 20 years, 1 month ago

Gaiiden

5,720

April 05, 2004 02:28 AM

That looks really cool Rune. What kind of framerates you get from it?

It's prob too late to do any good but I figure this thread'll be referenced again so here's my PNG load code in case anyone is still unclear about the format surface funcs:

// load the imagem_pImage = IMG_Load(strFileName.c_str());if (!m_pImage){    // uh oh     return;}// convert the image into the current display formatif (m_pImage->format->alpha)    m_pImage = SDL_DisplayFormatAlpha(m_pImage);else    m_pImage = SDL_DisplayFormat(m_pImage);

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RuneLancer

253

Author

April 05, 2004 02:53 AM

The title screen is prerendered, yeah. I might try having something animated though; my original intent was to have some wavy smoke-like effect with light filtering though (y''know what I mean, right?)

As for the framerate, I actually don''t really know. Gimme a second to test something... Few things though.

And I get... 10 FPS roughly. :D Not bad, huh? Eh. I''ll optimize when I have something that works.

graveyard filla

605

April 05, 2004 02:58 AM

once you convert to the display surface i bet you will get around 30 fps or more

FTA, my 2D futuristic action MMORPG

RuneLancer

253

Author

April 05, 2004 03:11 AM

Hrm, are SDL_DisplayFormat surfaces converted, or copied?

SDL_Label = SDL_DisplayFormat(SDL_CreateRGBSurface(SDL_HWSURFACE, W, H, 32, 0xFF, 0xFF00, 0xFF0000, 0x00));

With this, the surface SDL)CreateRGBSurface creates, is it lost or just converted and then thrown into SDL_Label?

I might have a leak otherwise

Edit: Huh... I cut out the display entirely and even then, when it's just my main loop, I get 24 FPS tops...

	while(1)	{		TickCount = SDL_GetTicks();		while(SDL_PollEvent(&SDL_Msg))			switch(SDL_Msg.type)			{				case SDL_QUIT: CleanUp(); return 0;			}		//if(!UpdateFrame()) { CleanUp(); return 0; }		//while(SDL_GetTicks() - TickCount < 15) Sleep(1);#if defined(DEBUG)		nFrame++;#endif	}

Am I doing something wrong? :/

[edited by - RuneLancer on April 5, 2004 4:17:18 AM]

leiavoia

960

April 05, 2004 10:32 AM

try this before your main() :

#include <time.h>#define FRAME_RATE_SAMPLES 100int FrameCount = 0;float FrameRate = 0;static void CalcFPS() {   static clock_t last=0;   clock_t now;   float delta;   if (++FrameCount >= FRAME_RATE_SAMPLES) {      now  = clock();      delta= (now - last) / (float) CLOCKS_PER_SEC;      last = now;      FrameRate = FRAME_RATE_SAMPLES / delta;      FrameCount = 0;   }}

Then at the end of your game loop, call

CalcFPS();

you can then look at the FrameRate variable to know what your current rate is over the last 100 frames (or less if you change it)

When using SDL_GetTicks(), it''s limited to 1000 per second, so it''s accuracy isn''t top notch. Also, you want to test over many frames. If you measure just one frame, you margin of error goes way up and it will look like your FPS is awefull.

------

SDL_DisplayFormat() copies a surface, so you''ll need to FreeSurface() your original one. That''s a mem leak just waiting to happen!

RuneLancer

253

Author

April 05, 2004 11:13 AM

Ah, I thought so; I was pretty certain that surface wasn''t just magically being transformed into a new surface.

No biggie.

I was testing over the course of ~20 seconds, though I''ll give your code a shot. It sounds a little hard to believe that a simple SDL "message pump" would run at 24 FPS...

Gaiiden

5,720

April 05, 2004 01:36 PM

Yes Rune, I get 22fps tops when I run my SDL engine in windowed mode. Haven''t gotten around to seeing how fast it runs in fullscreen mode tho.

Drew Sikora
Executive Producer
GameDev.net

graveyard filla

605

April 05, 2004 02:08 PM

its probably because your re-drawing the entire screen every frame.... why? i dont see any reason to.. just draw it one time in the begining... and thats it... if you have anything animated, just slice the background and draw it over the animated part.

FTA, my 2D futuristic action MMORPG

RuneLancer

253

Author

April 05, 2004 03:26 PM

Ah, this is more like it. I''m hitting 20 FPS now. I''ve let it run a little while and it renders like a charm.

Granted, I''ve gotten to the "prelude" part, which is text/window only. Heh. That would explain the rise.

This leaves me to worry about a few things. When I''ll have a background and two tile layers (maybe I can cut it down to one, depends how good it looks), a couple of sprites, and need of decently fast player input, I might have a problem on my hands. I will have to redraw the background everytime the player moves, for obvious reasons... so my FPS are gonna plummet. Well, I''ll see how it looks when I get there. Premature optimization will lead this project to a standstill in no time at all, which is bad.

I''ll just have to wait and see.

leiavoia

960

April 05, 2004 08:03 PM

I think that all just comes down to your hardware. Standard SDL doesn''t use your graphics card''s acceleration since it''s dealing with pixels not polys (at least it doesn''t use hardware on my system). So i think the framerate (with SDL at least) is limited by your CPU. I''m running an Athlon 2500 and i blit a 1024x768 background image (the whole thing), a few hundred 8x8 PNG particles, and do collision detection on a dozen objects, plus updating potentially thousands of particle objects and still get around ~40 FPS, and that''s a bit low.

SDL - Alpha channel issues

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