The above colour spaces have drawbacks when used in colour segmentation: RGB factors the luminance information into all three axes, HSI is a polar representation, so that hue is undefined when saturation is zero, and error propagation is unstable for small saturation values, and YIQ and YUV use more-or-less arbitrary scalings. Therefore, Tina implements a colour space developed in-house specifically for colour segmentation, known as IJK. The I axis is the luminance axis of the RGB colour space. The J axis lies in the plane orthogonal to the I axis, and is in the direction of red in the original RGB colour space. The K axis is orthogonal to both the I and J axes, and so lies in the plane orthogonal to the I axis in the direction of blue-green. The advantages of this colour space are that the luminance information is separated out onto the I axes, all three axes are orthogonal, and the conversion to RGB (and so the propagation of errors from RGB values) is particularly simple.