Color image picture forming process and apparatus which improves the quality of the black portions of the picture

A color image picture forming process comprising the steps of:

• forming dots of colorants of four colors of cyan, magenta, yellow and black;
• a first expression step of expressing black in the picture using the cyan, magenta and yellow colorants;
• a second expression step of expressing black in the picture using the black colorant;
• selecting said first expression step when a black component of color image picture data is smaller than a predetermined value;
• and
• selecting said first and second expression steps alternately or at random when the black component is larger than the predetermined value.

A color image picture forming apparatus comprising:

• dot forming means for forming dots of colorants of four colors consisting of three primary colors and black;
• detecting means for obtaining a black component from color image picture data;
• first processing means for processing the color image picture data to express black in the picture by combining dots of the three primary colors;
• second processing means for processing the color image picture data to express black in the picture using black dots;
• and
• selecting means for selecting either one or both of said first and second processing means in accordance with the magnitude of the black component, wherein said selecting means selects both said first and second processing means in a mutually exclusive alternating manner when the black component is in a predetermined range.

A color image picture forming apparatus comprising:

• dot forming means for forming dots of colorants of four colors consisting of three primary colors and black;
• detecting means for obtaining a black component from color image picture data;
• first processing means for processing the color image picture data to express black in the picture by combining dots of the three primary colors;
• second processing means for processing the color image picture data to express black in the picture using black dots;
• and
• selecting means for selecting either one or both of said first and second processing means in accordance with the magnitude of the black component, wherein said selecting means selects both said first and second processing means in a mutually exclusive random manner when the black component is in a predetermined range.

A color image picture forming process comprising the steps of:

• forming dots of colorants consisting of three primary colors and black;
• a first expression step of expressing black in the picture by a combination of dots of the three primary colors;
• a second expression step of expressing black in the picture using black dots;
• and
• selecting either one or both of said first and second expression steps in accordance with the magnitude of a black component of color image picture data, wherein said first and second expression steps are both selected in a mutually exclusive alternating manner when the black component is in a predetermined range.

A color image picture forming process comprising the steps of:

• forming dots of colorants consisting of three primary colors and black;
• a first expression step of expressing black in the picture by a combination of dots of the three primary colors;
• a second expression step of expressing black in the picture using black dots;
• and
• selecting either one or both of said first and second expression steps in accordance with the magnitude of a black component of color image picture data, wherein said first and second expression steps are both selected in a mutually exclusive random manner when the black component is in a predetermined range.

A color image picture forming process comprising the steps of:

• forming dots of colorants of three primary colors and black;
• detecting a black component from color image picture data;
• combining the black component with a noise signal when the black component is larger than a first predetermined value, wherein the first predetermined value is 50% or more of the highest optical density expressible using the black colorant;
• expressing black in the picture using the three primary color colorants when the black component is smaller than a second predetermined value lower than the first predetermined value;
• and
• expressing black in the picture using the black colorant when the black component is larger than the second predetermined value.

A color image picture forming apparatus comprising:

• dot forming means for forming dots of colorants of three primary colors and black;
• combining means for combining a dither signal with a black component signal obtained from color image picture data;
• means for inhibiting the operation of said combining means when the level of the black component signal is smaller than a first predetermined value, wherein the first predetermined value is 50% or more of the highest optical density expressible using the black colorant;
• first processing means for processing the color image data to express black in the picture by combining dots of the three primary colors when the black component is smaller than a second predetermined value lower than the first predetermined value;
• and
• second processing means for processing the color image data to express black in the picture by using black dots when the black component is larger than the second predetermined value.