A method of controlling a colour image forming apparatus that prints a colour image using a plurality of colour channels, wherein the plurality of colour channels comprise C, M, Y and K channels, the method comprising: determining whether original image data is in a colour image region; detecting boundary region information on the plurality of colour channels when it is determined that the original image data is in the colour image region; and selecting a colour channel to be extended using the detected boundary region information, characterised by extending the selected channel in accordance with following conditions: the K channel is not extended; the C, M and Y channels are extended when it is determined that there is no value of the K channel in a pixel position that forms a boundary when the C, M and Y channels are extended; both of the C channel and the M channel are extended when the C channel and the M channel are adjacent to each other so that the edge of the C channel and the edge of the M channel are in opposite directions; either the C channel or the M channel is extended when the C channel or the M channel is adjacent to the K channel so that the edge of the C channel and the edge of the M channel are in opposite directions; and only the Y channel is not extended when the Y channel is adjacent to the C, M, and K channels. determining whether original image data is in a colour image region; detecting boundary region information on the plurality of colour channels when it is determined that the original image data is in the colour image region; and selecting a colour channel to be extended using the detected boundary region information, characterised by extending the selected channel in accordance with following conditions: the K channel is not extended; the C, M and Y channels are extended when it is determined that there is no value of the K channel in a pixel position that forms a boundary when the C, M and Y channels are extended; both of the C channel and the M channel are extended when the C channel and the M channel are adjacent to each other so that the edge of the C channel and the edge of the M channel are in opposite directions; either the C channel or the M channel is extended when the C channel or the M channel is adjacent to the K channel so that the edge of the C channel and the edge of the M channel are in opposite directions; and only the Y channel is not extended when the Y channel is adjacent to the C, M, and K channels. the K channel is not extended; the C, M and Y channels are extended when it is determined that there is no value of the K channel in a pixel position that forms a boundary when the C, M and Y channels are extended; both of the C channel and the M channel are extended when the C channel and the M channel are adjacent to each other so that the edge of the C channel and the edge of the M channel are in opposite directions; either the C channel or the M channel is extended when the C channel or the M channel is adjacent to the K channel so that the edge of the C channel and the edge of the M channel are in opposite directions; and only the Y channel is not extended when the Y channel is adjacent to the C, M, and K channels.