US 6,023,302 AGranted

Blending of video images in a home communications terminal

AssigneePowerTV, Inc.

InventorsAlexander G. MacInnis, Jiann-Tsuen Chen

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Abstract

A graphics blending feature for a terminal such as a home communication terminal (HCT) allows an overlay image to be selectively blended with a background image through the use of a chroma key function and one or more alpha control bits. The chroma key function is used to determine whether the overlay will be completely transparent or not, and the one or more alpha control bits are used to look up a larger alpha value used to blend the overlay and background images. By using a small number of alpha control bits to retrieve a larger alpha value for blending, memory requirements per pixel are reduced. The chroma key function may be implemented by comparing each overlay pixel value to a chroma value and, responsive to a match, making the overlay pixel transparent (i.e., no blending occurs). A dithering function is included in various embodiments to smooth out the resulting image.

Claims13

Dependent claims

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.

4 more elements

Dependencies are inferred from the claim text.