[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;

void main(void)
{
    vec4 color = texture2D(in_texture, pass_texcoord.xy -
                                         vec2(pass_texcoord.z * in_sprite_flip, 0.0)) *
                                         pass_color;
    if (color.a < 0.01)
        discard;
    gl_FragColor = color * pass_color;
}