1. A computer-implemented method for rendering a straight line image object on a display device, comprising: receiving at a computer device line vector image data identifying a straight line to be rendered within an image on a display device; determining, using a computer device, a desired width for the straight line based on the line vector image data, the desired width of the straight line being determined in the direction normal to the straight line; determining, using a computer device, an extended width of the straight line to be used in anti-aliasing the straight line when the straight line is rendered in the image on the display device, the extended width of the straight line being determined in the direction of the normal to the straight line and extending the width of the straight line equally on either side of the straight line; determining one or more preliminary scaling vectors, the values of which are linearly related to the distance from the center of the straight line in the direction of the normal to the straight line over at least a portion of the extended width of the straight line; determining a scaling vector to be applied to pixel positions along the normal to the straight line within the extended width of the straight line, wherein the scaling vector is derived from the one or more preliminary scaling vectors such that the scaling vector has a value that differs from the value of each of the one or more preliminary scaling vectors for at least one distance from the center of the straight line; using the scaling vector to determine a pixel value for each of the pixel positions along the normal to the straight line within the extended width of the straight line; and rendering, using the computer device, the straight line in the image on the display device using the determined pixel values at each of the pixel positions along the normal to the straight line within the extended width of the straight line.