[vert.glsl]
#version 120

attribute vec3 in_vertex;
attribute vec4 in_color;
attribute vec4 in_texcoord;

uniform mat4  in_model_view;
uniform mat3  in_normal_mat;
uniform mat4  in_proj;
uniform float in_sprite_orientation;

varying vec4 pass_texcoord;
varying vec4 pass_color;

void main(void)
{
    vec4 vertex = in_model_view * vec4(in_vertex - vec3(0.0,0.5,0.0), 1.0);

    vec3 v_offset = vec3(1.0 * in_texcoord.z, -1.0 * in_texcoord.w, 0.0);

    float sinX = sin(in_sprite_orientation);
    float cosX = cos(in_sprite_orientation);
    float sinY = sinX;
    mat2 rotationMatrix = mat2(cosX, -sinX, sinY, cosX);

    v_offset.xy = rotationMatrix * v_offset.xy;

    //Billboard calculations
    vertex.xyz += v_offset;

    //pass datas
    pass_texcoord = in_texcoord;
    pass_color = in_color;

    gl_Position = in_proj * vertex;
}

[frag.glsl]
#version 120

#if defined GL_ES
precision highp float;
#endif

uniform sampler2D   in_texture;
uniform float       in_sprite_flip;

varying vec4 pass_texcoord;
varying vec4 pass_color;

const float cos_45      = 0.70710678118;
const float PI = 3.14159265358979323846264;

vec3 rgb2hsv(vec3 c)
{
    vec4 K = vec4(0.0, -1.0 / 3.0, 2.0 / 3.0, -1.0);
    vec4 p = c.g < c.b ? vec4(c.bg, K.wz) : vec4(c.gb, K.xy);
    vec4 q = c.r < p.x ? vec4(p.xyw, c.r) : vec4(c.r, p.yzx);

    float d = q.x - min(q.w, q.y);
    float e = 1.0e-10;
    return vec3(abs(q.z + (q.w - q.y) / (6.0 * d + e)), d / (q.x + e), q.x);
}

vec3 hsv2rgb(vec3 c)
{
    vec4 K = vec4(1.0, 2.0 / 3.0, 1.0 / 3.0, 3.0);
    vec3 p = abs(fract(c.xxx + K.xyz) * 6.0 - K.www);
    return c.z * mix(K.xxx, clamp(p - K.xxx, 0.0, 1.0), c.y);
}

void main(void)
{
    vec4 color = texture2D(in_texture, pass_texcoord.xy -
                                 vec2(pass_texcoord.z * in_sprite_flip, 0.0));
    if (color.a < 0.01)
        discard;
    vec3 hsv = rgb2hsv(color.rgb);
    hsv.x = fract(hsv.x + rgb2hsv(pass_color.rgb).x);
    gl_FragColor = vec4(hsv2rgb(hsv), color.a);
}