src/video/SDL_gamma.c

/*
    SDL - Simple DirectMedia Layer
    Copyright (C) 1997-2006 Sam Lantinga

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

    Sam Lantinga
    slouken@libsdl.org
*/

/* Gamma correction support */

#include "SDL_config.h"

#ifdef HAVE_MATH_H
#include <math.h>	/* Used for calculating gamma ramps */
#else
/* Math routines from uClibc: http://www.uclibc.org */
#include "math_private.h"
#include "e_sqrt.h"
#include "e_pow.h"
#include "e_log.h"
#define pow(x, y)	__ieee754_pow(x, y)
#define log(x)		__ieee754_log(x)
#endif

#include "SDL_sysvideo.h"


static void CalculateGammaRamp(float gamma, Uint16 *ramp)
{
	int i;

	/* 0.0 gamma is all black */
	if ( gamma <= 0.0 ) {
		for ( i=0; i<256; ++i ) {
			ramp[i] = 0;
		}
		return;
	} else
	/* 1.0 gamma is identity */
	if ( gamma == 1.0 ) {
		for ( i=0; i<256; ++i ) {
			ramp[i] = (i << 8) | i;
		}
		return;
	} else
	/* Calculate a real gamma ramp */
	{ int value;
		gamma = 1.0f / gamma;
		for ( i=0; i<256; ++i ) {
			value = (int)(pow((double)i/256.0, gamma)*65535.0+0.5);
			if ( value > 65535 ) {
				value = 65535;
			}
			ramp[i] = (Uint16)value;
		}
	}
}
static void CalculateGammaFromRamp(float *gamma, Uint16 *ramp)
{
	/* The following is adapted from a post by Garrett Bass on OpenGL
	   Gamedev list, March 4, 2000.
	 */
	float sum = 0.0;
	int i, count = 0;

	*gamma = 1.0;
	for ( i = 1; i < 256; ++i ) {
	    if ( (ramp[i] != 0) && (ramp[i] != 65535) ) {
	        double B = (double)i / 256.0;
	        double A = ramp[i] / 65535.0;
	        sum += (float) ( log(A) / log(B) );
	        count++;
	    }
	}
	if ( count && sum ) {
		*gamma = 1.0f / (sum / count);
	}
}

int SDL_SetGamma(float red, float green, float blue)
{
	int succeeded;
	SDL_VideoDevice *video = current_video;
	SDL_VideoDevice *this  = current_video;	

	succeeded = -1;
	/* Prefer using SetGammaRamp(), as it's more flexible */
	{
		Uint16 ramp[3][256];

		CalculateGammaRamp(red, ramp[0]);
		CalculateGammaRamp(green, ramp[1]);
		CalculateGammaRamp(blue, ramp[2]);
		succeeded = SDL_SetGammaRamp(ramp[0], ramp[1], ramp[2]);
	}
	if ( (succeeded < 0) && video->SetGamma ) {
		SDL_ClearError();
		succeeded = video->SetGamma(this, red, green, blue);
	}
	return succeeded;
}

/* Calculating the gamma by integrating the gamma ramps isn't exact,
   so this function isn't officially supported.
*/
int SDL_GetGamma(float *red, float *green, float *blue)
{
	int succeeded;
	SDL_VideoDevice *video = current_video;
	SDL_VideoDevice *this  = current_video;	

	succeeded = -1;
	/* Prefer using GetGammaRamp(), as it's more flexible */
	{
		Uint16 ramp[3][256];

		succeeded = SDL_GetGammaRamp(ramp[0], ramp[1], ramp[2]);
		if ( succeeded >= 0 ) {
			CalculateGammaFromRamp(red, ramp[0]);
			CalculateGammaFromRamp(green, ramp[1]);
			CalculateGammaFromRamp(blue, ramp[2]);
		}
	}
	if ( (succeeded < 0) && video->GetGamma ) {
		SDL_ClearError();
		succeeded = video->GetGamma(this, red, green, blue);
	}
	return succeeded;
}

int SDL_SetGammaRamp(const Uint16 *red, const Uint16 *green, const Uint16 *blue)
{
	int succeeded;
	SDL_VideoDevice *video = current_video;
	SDL_VideoDevice *this  = current_video;	
	SDL_Surface *screen = SDL_PublicSurface;

	/* Verify the screen parameter */
	if ( !screen ) {
		SDL_SetError("No video mode has been set");
		return -1;
	}

	/* Lazily allocate the gamma tables */
	if ( ! video->gamma ) {
		SDL_GetGammaRamp(0, 0, 0);
	}

	/* Fill the gamma table with the new values */
	if ( red ) {
		SDL_memcpy(&video->gamma[0*256], red, 256*sizeof(*video->gamma));
	}
	if ( green ) {
		SDL_memcpy(&video->gamma[1*256], green, 256*sizeof(*video->gamma));
	}
	if ( blue ) {
		SDL_memcpy(&video->gamma[2*256], blue, 256*sizeof(*video->gamma));
	}

	/* Gamma correction always possible on split palettes */
	if ( (screen->flags & SDL_HWPALETTE) == SDL_HWPALETTE ) {
		SDL_Palette *pal = screen->format->palette;

		/* If physical palette has been set independently, use it */
		if(video->physpal)
		        pal = video->physpal;
		      
		SDL_SetPalette(screen, SDL_PHYSPAL,
			       pal->colors, 0, pal->ncolors);
		return 0;
	}

	/* Try to set the gamma ramp in the driver */
	succeeded = -1;
	if ( video->SetGammaRamp ) {
		succeeded = video->SetGammaRamp(this, video->gamma);
	} else {
		SDL_SetError("Gamma ramp manipulation not supported");
	}
	return succeeded;
}

int SDL_GetGammaRamp(Uint16 *red, Uint16 *green, Uint16 *blue)
{
	SDL_VideoDevice *video = current_video;
	SDL_VideoDevice *this  = current_video;	

	/* Lazily allocate the gamma table */
	if ( ! video->gamma ) {
		video->gamma = SDL_malloc(3*256*sizeof(*video->gamma));
		if ( ! video->gamma ) {
			SDL_OutOfMemory();
			return -1;
		}
		if ( video->GetGammaRamp ) {
			/* Get the real hardware gamma */
			video->GetGammaRamp(this, video->gamma);
		} else {
			/* Assume an identity gamma */
			int i;
			for ( i=0; i<256; ++i ) {
				video->gamma[0*256+i] = (i << 8) | i;
				video->gamma[1*256+i] = (i << 8) | i;
				video->gamma[2*256+i] = (i << 8) | i;
			}
		}
	}

	/* Just copy from our internal table */
	if ( red ) {
		SDL_memcpy(red, &video->gamma[0*256], 256*sizeof(*red));
	}
	if ( green ) {
		SDL_memcpy(green, &video->gamma[1*256], 256*sizeof(*green));
	}
	if ( blue ) {
		SDL_memcpy(blue, &video->gamma[2*256], 256*sizeof(*blue));
	}
	return 0;
}