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Fullscreen interpolation

Started by biovenger Feb 23, 2003 at 5:29 AM 10 replies 550+ views

Original Post

Feb 23, 2003 at 5:29 AM

I have just a small question. The latest addition to the engine I am coding is an ability to use full-screen interpolation. It is the fastest interpolation; Linear. The thing is, it is horridly slow, so my question is how to speed it up. Can it be done through hardware or is there another way to make it smooth? An example of fullscreen interpolation is the SNES emulator ZSNES. Can it be done fullscreen without any noticable loss in speed? ----------------------------- Final Frontier Trader

sphinx23

Feb 23, 2003 at 5:32 AM

use OpenGL and GL_LINEAR for Linear filtering.
(SNES does this)

biovenger

Author

biovenger

Feb 23, 2003 at 5:53 AM

My engine uses only SDL and I don''t want to mix in any extra libraries.

It is as I thought, OpenGL does linear with hardware. That''s why software is slow...

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Final Frontier Trader

biovenger

Author

biovenger

Feb 23, 2003 at 6:47 AM

The code is following

  
void Linear_Interpolate(Uint8 a, Uint8 b, float x)
{
   return (a + b) * x;
}

void Interpolate()
{
   int amount = WIDTH * HEIGHT;

   // ...get rgbvalues and interpolate with Linear_Interpolate();


   screen->DrawPixel(x, y, color);
}

I''m using SDL and CAN use hardware via Open GL but choose not to.

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Final Frontier Trader

Cornutopia

128

Cornutopia

Feb 23, 2003 at 12:45 PM

If getting/setting a pixel locks then unlocks the video memory each time it will be slow. For speed alone the best option would be to store the source/dest images in sepearate arrays I think and read/write array slots, only when the whole scene is changed blit the full picture.

Mark
Cornutopia Games
http://www.cornutopia.net
Bytten Independent Games Magazine
http://www.bytten.com

Merle

Feb 23, 2003 at 9:22 PM

MarkSheeky: "If getting/setting a pixel locks then unlocks the video memory each time it will be slow."

Yes -- plus drawing directly to the screen, rather than a second buffer, will result in a less than optimal looking game. Do some searches on "double buffering".

biovenger

Author

biovenger

Feb 26, 2003 at 4:26 AM

My display surface is double-buffered. I always draw on a back buffer and my problem isn't hacked animations, but slow movement.

The surface doesn't need to be locked since it isn't a screen surface.

[EDIT]

Here is the exact sourcecode of the interpolation function. (Note that the lock and unlock methods are there for security only, since the surface shouldn't need to be locked)

  
void Interpolation()
{
	// calc len

	int len = WIDTH * HEIGHT;

	// colors

	static Uint32 first, last;

	static Uint8 r, g, b, r2, g2, b2;

	// pixel ptr

	Uint32 *ptr = (Uint32 *)screen->surface->pixels;
	SDL_PixelFormat *frm = screen->surface->format;

	// lock

	screen->Lock();

	// start the interpolation loop

	for(int i = 1; i < len; i++)
	{
		// get first pixel colors

		SDL_GetRGB(ptr[i - 1], frm, &r, &g, &b);

		// get second pixel colors

		SDL_GetRGB(ptr[i + 1], frm, &r2, &g2, &b2);

		// interpolate and draw

		ptr[i] = SDL_MapRGB(frm, Uint8(Math_LinearInterpolate(r, r2, 0.5)),
								Uint8(Math_LinearInterpolate(g, g2, 0.5)),
								Uint8(Math_LinearInterpolate(b, b2, 0.5)));
	}

	// unlock

	screen->Unlock();
}

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Final Frontier Trader

[edited by - biovenger on February 26, 2003 5:27:40 AM]

EEROOKIE

Feb 26, 2003 at 4:07 PM

Are you sure that the linear interpolation is taking the time? Replace your LI function with a function that simply returns a floating point value and see if your timing changes. I expect that the timing issues you have are a result of calling to SDL_GetRGB and SDL_MapRGB for each pixel.

Ready4Dis

Feb 26, 2003 at 4:24 PM

Also, inlining that inerpolate function could help tremendously...

  
__forceinline Uint8 Linear_Interpolate(Uint8 &a, Uint8 &b, float &x)
{
   return (a + b) * x;
}

Btw, I changed the return type from void -> Uint8, otherwise you can''t return a value

. I also pass by refrence, so no extra memory is being used, and no copying of the variables needs to be done.

biovenger

Author

biovenger

Feb 27, 2003 at 11:26 AM

The Math_LinearInterpolate is an inline function. I will check replacing the SDL_GetRGB with direct access to the pixels and checking where the red, green and blue values are.

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Final Frontier Trader

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