lighttroupe/luz

#
# Hit testing is used for finding the object under the mouse cursor.  It works by rendering each object with a unique color, then reading back the pixel under the cursor.
#
module DrawingHitTesting
    HIT_TEST_INCREMENT = 1        # (only useful to change this value when testing visually)

    # Enable and initialize hit-testing mode
    def with_hit_testing
        $engine.with_env(:hit_test, true) {
            $hit_test_id = 0
            $hit_test_options = {}

            with_offscreen_buffer(:small) { |buffer|
                buffer.using {
                    with_env(:output_height, buffer.height) {        # hit testing code needs to know how big the output buffer is
                        with_env(:output_width, buffer.width) {
                            yield
                        }
                    }
                }
            }
        }
    end

    def next_hit_test_id
        $hit_test_id += HIT_TEST_INCREMENT
    end

    def with_unique_hit_test_color_for_object(object, user_data_integer=0)
        hit_test_id = next_hit_test_id
        $hit_test_options[[hit_test_id, user_data_integer]] = object
        saved = GL.GetColorArray
        GL.Color4ub(hit_test_id, user_data_integer, 0, 255)
        yield
        GL.Color(*saved)
    end

    # returns [hit_test_id, handle_id] or [0, nil]
    def hit_test_object_at_luz_coordinates(x, y)        # coordinates with 0-centered unit square
        x_index, y_index = (x + 0.5) * ($env[:output_width]-1), ((y + 0.5)) * ($env[:output_height]-1)
        hit_test_object_at(x_index, y_index)
    end

private

    def hit_test_object_at(x, y)        # pixel coordinates
        color = glReadPixels(x, y, 1, 1, GL_RGB, GL_UNSIGNED_BYTE).unpack("CCC")
        object = $hit_test_options[[color[0], color[1]]]
        [object, color[1]]
    end
end