Color image data processing apparatus comprising monochrome pixel detector

A color image data processing apparatus converting three input data of red, green and blue of a specified pixel into four output data of cyan, magenta, yellow and black, comprising:

• generation means in response to one input data preselected among said three input data for generating threshold data for representing upper and lower limits of respective ranges in which two input data other than said preselected one input data are to be present when said specified Fixel is a monochrome pixel of white, black or half-tone black;
• judgement means for comparing said other two input data with said threshold data generated by said generation means, and judging that said specified pixel is said monochrome pixel when both of said other two input data are respectively fallen in said respective ranges from said upper limit to said lower limit;
• and
• conversion means in response to a judgement result of said judgement means for converting said three input data of red, green and blue of said specified pixel into four output data of cyan, magenta, yellow and black.

A color image data processing apparatus for converting color image data of red, green and blue of a specified pixel into color image data of cyan, magenta, yellow and black, comprising:

• judgement means in response to said color image data of red, green and blue for judging whether or not said specified pixel is a monochrome pixel of white, black or half-tone black;
• and
• conversion means in response to a judgement result of said judgement means for determining an under color removal amount and a black adding amount and converting said color image data of red, green and blue of said specified pixel into color image data of cyan, magenta, yellow and black with said determined under color removal amount and said determined black adding amount,
• wherein said judgement means generates under color removal coefficient data .alpha. and black adding coefficient data .beta. in response to said judgement result of said judgement means and outputs them to said conversion means, and
• said conversion means detects minimum data DMIN among said color image data of red, green and blue, generates said color image data of cyan, magenta and yellow by respectively subtracting a product of said under color removal coefficient data .alpha. and said detected minimum data DMIN from said color image data of red, green and blue, and generates said color image data of black by multiplying said black adding coefficient data .beta. by said detected minimum data DMIN.

A method for processing three color image data of three primary colors for representing densities of the three primary colors of a specified pixel, including the following steps of:

• generating, in response to one color image data preselected among said three color image data, threshold data for representing upper and lower limits of respective ranges in which two color image data other than said preselected one color image data are to be present when said specified pixel is a monochrome pixel of white, black or half-tone black;
• comparing said other two color image data with said generated threshold data;
• and
• judging that said specified pixel is said monochrome pixel when both of said other two color image data are respectively fallen in said respective ranges from said upper limit to said lower limit.