/*
Copyright (c) 2019 The Khronos Group Inc.
Use of this source code is governed by an MIT-style license that can be
found in the LICENSE.txt file.
*/


#ifdef GL_ES
#extension GL_OES_standard_derivatives : enable
precision mediump float;
#endif

// setting a boundary for cases where screen sizes may exceed the precision
// of the arithmetic used.
#define SAFETY_BOUND 500.0

// Macro to scale/bias the range of output.  If input is [-1.0, 1.0], maps to [0.5, 1.0]..  
// Accounts for precision errors magnified by derivative operation.
#define REDUCE_RANGE(A) ((A) + 3.0) / 4.0

// This fragment shader computes an image representation of the derivative of
// sine.  The derivative of sine is cosine.  This shader's output is compared to
// the reference shader that computes an image representation of cosine
// directly.  

uniform float viewportwidth;
uniform float viewportheight;

varying vec2 vertXY;

void main (void)
{
	const float M_PI = 3.14159265358979323846;
	float sine;
	float cosine;

#ifdef GL_OES_standard_derivatives
	sine = sin(fract(gl_FragCoord.x / 128.0) * (2.0 * M_PI));
	cosine = REDUCE_RANGE((128.0 / (2.0 * M_PI)) * dFdx(sine));
#else
	cosine = 0.5;
#endif

	if( gl_FragCoord.x < SAFETY_BOUND )
	{
                gl_FragColor = vec4(cosine, cosine, cosine, 1.0);
	}
	else discard;
}