A host computer converts an entered gray scale image into a raster image expressing multiple tonal ranges, using an intermediate spot pattern determined in advance. Here, the intermediate spot pattern is a spot pattern composed of pixels which can take only binary value densities with information indicating tonal ranges embedded in the manner of arrangement of the pixels in the spot pattern. Therefore, this raster image logically expresses a large number of tonal ranges which cannot be actually expressed with pixels in binary value densities. A printer receives this raster image from the host computer and converts the intermediate spot pattern in this raster image into an output spot pattern which can assume tertiary values or more in density, thereby printing out a half-tone image actually expressing multiple tonal ranges.
To convert gray-scale data of each pixel of image data into pulse-width data for each pixel, a RAM is provided. The RAM has nine pixel memory regions corresponding to nine pixels a to i for forming, for example, a 3.times.3 pixel matrix on the output image. In each of the pixel memory regions, there are stored pulse-width data items corresponding to values which are permitted for gray-scale data to have. Pulse-width data items in the pixel memory regions are slightly different from one another among pixel memory regions. Gray-scale data of each pixel of image data and address word composed of gray-scale data of each pixel of image data and coordinates of each pixel in the 3.times.3 matrix is supplied to the RAM. Thus, pulse-width data corresponding to gray-scale data is read from the pixel memory region so that a laser drive pulse having a pulse width corresponding to the read pulse-width data is generated.
An image transformation apparatus for performing a transformation processing for outputting a color image by an image output device such as a color printer. The image transformation apparatus has an image receiving section for receiving an image represented by digital data, an image sort discrimination section for discriminating whether the image received by the image receiving section is a halftone dot image or a multiple gradation image, an image transformation section for applying a conversion processing for the multiple gradation image to the halftone dot image when it is decided by the image sort discrimination section that the image received by the image receiving section is the multiple gradation image, and a driver section for outputting an image discriminated as the halftone dot image by the image sort discrimination section and an image converted into the halftone dot image by the image transformation section to an image output device.
