//  SuperTuxKart - a fun racing game with go-kart
//  Copyright (C) 2013 the SuperTuxKart team
//
//  This program is free software; you can redistribute it and/or
//  modify it under the terms of the GNU General Public License
//  as published by the Free Software Foundation; either version 3
//  of the License, or (at your option) any later version.
//
//  This program 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 General Public License for more details.
//
//  You should have received a copy of the GNU General Public License
//  along with this program; if not, write to the Free Software
//  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.


// motion_blur.frag

// The actual boost amount (which linearly scales the blur to be shown).
// should be in the range [0.0, 1.0], though a larger value might make
// the blurring too string. Atm we are using [0, 0.5].
uniform float boost_amount;

// The color buffer to use.
uniform sampler2D color_buffer;

// Center (in texture coordinates) at which the kart is. A small circle
// around this center is not blurred (see mask_radius below)
uniform vec2 center;

// The direction to which the blurring aims at
uniform vec2 direction;

// Radius of mask around the character in which no blurring happens
// so that the kart doesn't get blurred.
uniform float mask_radius;

// Maximum height of texture used
uniform float max_tex_height;

// Number of samples used for blurring
#define NB_SAMPLES 8

void main()
{
	vec2 texcoords = gl_TexCoord[0].st;

	// Sample the color buffer
	vec3 color = texture2D(color_buffer, texcoords).rgb;

	// Compute the blur direction.
	// IMPORTANT: we don't normalize it so that it avoids a glitch around 'center',
	// plus it naturally scales the motion blur in a cool way :)
	vec2 blur_dir = direction - texcoords;

	// Compute the blurring factor:
	// - apply the mask, i.e. no blurring in a small circle around the kart
	float blur_factor = max(0.0, length(texcoords - center) - mask_radius);

	// - avoid blurring the top of the screen
	blur_factor *= (max_tex_height - texcoords.t);

	// - apply the boost amount
	blur_factor *= boost_amount;

	// Scale the blur direction
	blur_dir *= blur_factor;

	// Compute the blur
	vec2 inc_vec = blur_dir / vec2(NB_SAMPLES);
	vec2 blur_texcoords = texcoords + inc_vec;
	for(int i=1 ; i < NB_SAMPLES ; i++)
	{
		color += texture2D(color_buffer, blur_texcoords).rgb;
		blur_texcoords += inc_vec;
	}
	color /= vec3(NB_SAMPLES);
	gl_FragColor = vec4(color, 1.0);

	// Keep this commented line for debugging:
	//gl_FragColor = vec4(blur_factor, blur_factor, blur_factor, 0.0);
}