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Anti-aliasing raster operations utilizing sub-pixel crossing information to control pixel shading

  • US 4,908,780 A
  • Filed: 10/14/1988
  • Issued: 03/13/1990
  • Est. Priority Date: 10/14/1988
  • Status: Expired due to Term
First Claim
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1. An apparatus including a central processing unit for generating control signals including background color control signals and foreground color control signals, said apparatus for performing Boolean raster operations on source and destination data for storage in a frame buffer memory for a plurality of planes, said source data being selected from one of a font register and a pattern register, said destination data being selected from said frame buffer, wherein said destination data stored in said frame buffer is organized as pixels of information to be displayed, and each of said pixels is logically divided into a plurality of sub-pixels, said apparatus comprising:

  • (a) source data select means coupled to said font register and said pattern register for selecting source data;

    (b) anti-aliasing mask logic coupled to said source data select means and said central processing unit for generating for each of said pixels to be displayed a fraction between 0 and 1 representing the ratio of the number of sub-pixels crossed by an image segment going through the pixel to the total number of sub-pixels within the pixel to the total number of sub-pixels with the pixel corresponding to said sub-pixels;

    (c) filter means coupled to said mask logic means for encoding the output generated by said mask logic means, said encoded output corresponding to one of a plurality of shades of gray for each of said pixels to be displayed;

    (d) multiplexer means coupled to said central processing unit and said anti-aliasing mask logic means for selecting a Boolean raster operation to be performed for each of said plurality of planes using said foreground color control signals and said background color control signals;

    (e) logic means coupled to said multiplexer means and said central processing unit for generating SSEL0 and SSEL1 control signals used by said anti-aliasing mask logic means, a saturation control signal and a +/- control signal;

    (f) adder/subracter means coupled to said source data select menas, said frame buffer and said logic means for adding and subtracting the sub-pixel values for each row of sub-pixel information in each pixel;

    (g) saturation logic means coupled to said adder/subtracter means for saturating the values output by said adder/subtracter means to values between 0 and 128.

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