1. Apparatus for blending a first image comprising a first plurality of pixels with a second image comprising a second plurality of pixels, each of the first plurality of pixels comprising a displayable pixel value and an alpha control bit which controls the blending of the displayable pixel value with a corresponding displayable pixel value from the second image, the apparatus comprising: a memory for storing the first image; a line buffer for storing a line of pixels of the first image read from the memory; a multiplexer having a first input for receiving a line of pixels from the memory, a second input for receiving a line of pixels from the line buffer, and an output which alternates between the first and second inputs in accordance with a clock signal; an alpha generation circuit, responsive to each alpha control bit in the first image which is output from the multiplexer, for outputting a stored alpha value .alpha. comprising a plurality of bits; an alpha multiplier for multiplying the alpha value output by the alpha generation circuit with one of the displayable pixel values in the first image; and a chroma comparison circuit which compares each displayable pixel value in the first image with a stored chroma value and, responsive to a determination that the displayable pixel value matches the stored chroma value, prevents the stored alpha value from being multiplied with the displayable pixel value of the pixel in the first image.