/**
 ** Supermodel
 ** A Sega Model 3 Arcade Emulator.
 ** Copyright 2011-2012 Bart Trzynadlowski, Nik Henson 
 **
 ** This file is part of Supermodel.
 **
 ** Supermodel 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.
 **
 ** Supermodel 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 Supermodel.  If not, see <http://www.gnu.org/licenses/>.
 **/
 
/*
 * Fragment.glsl
 *
 * Fragment shader for 3D rendering.
 */

#version 120

// Global uniforms
uniform sampler2D	textureMap;		// complete texture map, 2048x2048 texels
uniform vec4	spotEllipse;		// spotlight ellipse position: .x=X position (screen coordinates), .y=Y position, .z=half-width, .w=half-height)
uniform vec2	spotRange;			// spotlight Z range: .x=start (viewspace coordinates), .y=limit
uniform vec3	spotColor;			// spotlight RGB color
uniform vec3	lighting[2];		// lighting state (lighting[0] = sun direction, lighting[1].x,y = diffuse, ambient intensities from 0-1.0)

// Inputs from vertex shader 
varying vec4		fsSubTexture;	// .x=texture X, .y=texture Y, .z=texture width, .w=texture height (all in texels)
varying vec4		fsTexParams;	// .x=texture enable (if 1, else 0), .y=use transparency (if > 0), .z=U wrap mode (1=mirror, 0=repeat), .w=V wrap mode
varying float		fsTexFormat;	// T1RGB5 contour texture (if > 0)
varying float		fsTransLevel;	// translucence level, 0.0 (transparent) to 1.0 (opaque)
varying vec3		fsLightIntensity;	// lighting intensity 
varying float		fsSpecularTerm;	// specular highlight
varying float		fsFogFactor;	// fog factor
varying float		fsViewZ;		// Z distance to fragment from viewpoint at origin

/*
 * WrapTexelCoords():
 *
 * Computes the normalized OpenGL S,T coordinates within the 2048x2048 texture
 * sheet, taking into account wrapping behavior.
 *
 * Computing normalized OpenGL texture coordinates (0 to 1) within the 
 * Real3D texture sheet:
 *
 * If the texture is not mirrored, we simply have to clamp the
 * coordinates to fit within the texture dimensions, add the texture
 * X, Y position to select the appropriate one, and normalize by 2048
 * (the dimensions of the Real3D texture sheet).
 *
 *		= [(u,v)%(w,h)+(x,y)]/(2048,2048)
 *
 * If mirroring is enabled, textures are mirrored every odd multiple of
 * the original texture. To detect whether we are in an odd multiple, 
 * simply divide the coordinate by the texture dimension and check 
 * whether the result is odd. Then, clamp the coordinates as before but
 * subtract from the last texel to mirror them:
 *
 * 		= [M*((w-1,h-1)-(u,v)%(w,h)) + (1-M)*(u,v)%(w,h) + (x,y)]/(2048,2048)
 *		where M is 1.0 if the texture must be mirrored.
 *
 * As an optimization, this function computes TWO texture coordinates
 * simultaneously. The first is texCoord.xy, the second is in .zw. The other
 * parameters must have .xy = .zw.
 */
vec4 WrapTexelCoords(vec4 texCoord, vec4 texOffset, vec4 texSize, vec4 mirrorEnable)
{
	vec4	clampedCoord, mirror, glTexCoord;
	
	clampedCoord = mod(texCoord,texSize);						// clamp coordinates to within texture size
	mirror = mirrorEnable * mod(floor(texCoord/texSize),2.0);	// whether this texel needs to be mirrored

	glTexCoord = (	mirror*(texSize-clampedCoord) +
					(vec4(1.0,1.0,1.0,1.0)-mirror)*clampedCoord +
					texOffset
				 ) / 2048.0;
	return glTexCoord;
}

/*
 * main():
 *
 * Fragment shader entry point.
 */

void main(void)
{	
	vec4	uv_top, uv_bot, c[4];
	vec2	r;
	vec4	fragColor;
	vec2	ellipse;
	vec3	lightIntensity;
	float	insideSpot;
	int		x;
	
	// Get polygon color for untextured polygons (textured polygons will overwrite)
	if (fsTexParams.x < 0.5)
		fragColor = gl_Color;		
	else
	// Textured polygons: set fragment color to texel value
	{			
		/*
		 * Bilinear Filtering
		 *
		 * In order to get this working on ATI, the number of operations is
		 * reduced by putting everything into vec4s. uv_top holds the UV 
		 * coordinates for the top two texels (.xy=left, .zw=right) and uv_bot
		 * is for the lower two.
		 */

		// Compute fractional blending factor, r, and lower left corner of texel 0
		uv_bot.xy = gl_TexCoord[0].st-vec2(0.5,0.5);	// move into the lower left blending texel 
		r = uv_bot.xy-floor(uv_bot.xy);					// fractional part
		uv_bot.xy = floor(uv_bot.xy);					// integral part
		
		// Compute texel coordinates
		uv_bot.xy += vec2(0.5,0.5);	// offset to center of pixel (should not be needed but it fixes a lot of glitches, esp. on Nvidia)
		uv_bot.zw = uv_bot.xy + vec2(1.0,0.0);			// compute coordinates of the other three neighbors
		uv_top = uv_bot + vec4(0.0,1.0,0.0,1.0);

		// Compute the properly wrapped texel coordinates
		uv_top = WrapTexelCoords(uv_top,vec4(fsSubTexture.xy,fsSubTexture.xy),vec4(fsSubTexture.zw,fsSubTexture.zw), vec4(fsTexParams.zw,fsTexParams.zw));
		uv_bot = WrapTexelCoords(uv_bot,vec4(fsSubTexture.xy,fsSubTexture.xy),vec4(fsSubTexture.zw,fsSubTexture.zw), vec4(fsTexParams.zw,fsTexParams.zw));

		// Fetch the texels
		c[0]=texture2D(textureMap,uv_bot.xy);	// bottom-left (base texel)
		c[1]=texture2D(textureMap,uv_bot.zw);	// bottom-right
		c[2]=texture2D(textureMap,uv_top.xy);	// top-left
		c[3]=texture2D(textureMap,uv_top.zw);	// top-right		

		// Interpolate texels and blend result with material color to determine final (unlit) fragment color
		// fragColor = (c[0]*(1.0-r.s)*(1.0-r.t) + c[1]*r.s*(1.0-r.t) + c[2]*(1.0-r.s)*r.t + c[3]*r.s*r.t);
		// Faster method:
		c[0] += (c[1]-c[0])*r.s;			// 2 alu
		c[2] += (c[3]-c[2])*r.s;			// 2 alu
		fragColor = c[0]+(c[2]-c[0])*r.t;	//2 alu
	
		/*
		 * T1RGB5:
		 *
		 * The transparency bit determines whether to discard pixels (if set).
		 * What is unknown is how this bit behaves when interpolated. OpenGL
		 * processes it as an alpha value, so it might concievably be blended
		 * with neighbors. Here, an arbitrary threshold is chosen.
		 *
		 * To-do: blending could probably enabled and this would work even
		 * better with a hard threshold.
		 *
		 * Countour processing also seems to be enabled for RGBA4 textures.
		 * When the alpha value is 0.0 (or close), pixels are discarded 
		 * entirely.
		 */
		if (fsTexParams.y > 0.5)	// contour processing enabled
		{
			if (fragColor.a < 0.01)	// discard anything with alpha == 0
				discard;
		}
		
		// If contour texture and not discarded, force alpha to 1.0 because will later be modified by polygon translucency
		if (fsTexFormat < 0.5)		// contour (T1RGB5) texture map
			fragColor.a = 1.0;
	}

	// Compute spotlight and apply lighting
	ellipse = (gl_FragCoord.xy-spotEllipse.xy)/spotEllipse.zw;
	insideSpot = dot(ellipse,ellipse);
	if ((insideSpot <= 1.0) &&  (fsViewZ>=spotRange.x) && (fsViewZ<spotRange.y))
		lightIntensity = fsLightIntensity+(1.0-insideSpot)*spotColor;
	else
		lightIntensity = fsLightIntensity;
	fragColor.rgb *= lightIntensity;
	fragColor.rgb += vec3(fsSpecularTerm,fsSpecularTerm,fsSpecularTerm);
	
	// Translucency (modulates existing alpha channel for RGBA4 texels)
	fragColor.a *= fsTransLevel;

	// Apply fog under the control of fog factor setting from polygon header
	fragColor.rgb = mix(gl_Fog.color.rgb, fragColor.rgb, fsFogFactor);

	// Store final color
	gl_FragColor = fragColor;
}