function init(self)
    msg.post(".", "acquire_input_focus")
    
    local scale = 0.75
    local spacingx = 160 * scale + 10
    local spacingy = 190 * scale + 10
    local startx = 40 + spacingx*0.5
    local starty = 40 + spacingy*0.5

    local maxy = 3
    local maxx = 4

    self.urls = {}
    for y = 0, maxy do
        for x = 0, maxx do
            local p = vmath.vector3(startx + x*spacingx, starty + y*spacingy, 0.5)
            local id = factory.create("#factory", p, nil, nil, vmath.vector3(0.8, 0.8, 1))
            local url = msg.url(nil, id, "sprite")
            table.insert(self.urls, url)
            
            go.set(url, "mycolor", vmath.vector4(x/maxx, y/maxy, 0, 1))
        end
    end

    self.updated = false
end

function update(self, dt)
    self.updated = true
end

function on_input(self, action_id, action)
    if action_id == hash("touch") and action.pressed and self.updated then
        for _, url in ipairs(self.urls) do
            go.animate(url, "mycolor", go.PLAYBACK_ONCE_PINGPONG, vmath.vector4(0, 0, 1, 1), go.EASING_LINEAR, .2)
        end
    end
end

uniform highp mat4 view_proj;

// positions are in world space
attribute highp vec4 position;
attribute mediump vec2 texcoord0;
attribute mediump vec4 mycolor;

varying mediump vec2 var_texcoord0;
varying mediump vec4 var_mycolor;

void main()
{
    gl_Position = view_proj * vec4(position.xyz, 1.0);
    var_texcoord0 = texcoord0;
    var_mycolor = mycolor;
}

varying mediump vec2 var_texcoord0;
varying mediump vec4 var_mycolor;

uniform lowp sampler2D texture_sampler;
uniform lowp vec4 tint;

void main()
{
    // Pre-multiply alpha since all runtime textures already are
    lowp vec4 tint_pm = vec4(tint.xyz * tint.w, tint.w);
    gl_FragColor = texture2D(texture_sampler, var_texcoord0.xy) * tint_pm * var_mycolor;
}