1. A printer comprising at least a processor and memory, and further comprising: a receiver, configured to receive print data described by a page description language from a host computer; a generator, configured to generate rendering commands of objects by analyzing the received print data; a determiner, configured to determine whether a rendering command instructs a rendering of a black object or not; a rendering section, configured to render bitmap images of the objects and generate attribute information for respective pixels in accordance with the rendering commands, wherein the attribute information indicates an image type of a pixel, with a bit width of the attribute information less than that of each bitmap image; and an image processor, configured to generate bitmap images for printing having a CMYK format based on the attribute information, wherein said rendering section stores in the memory the rendered bitmap images having a RGB format and the attribute information indicating no black image to be rendered as instructed by the rendering commands, and stores in the memory no bitmap image but only the attribute information indicating a black image to be rendered as instructed by the rendering commands, and wherein said image processor generates the bitmap images for printing from the bitmap images stored in the memory if the corresponding attribute information indicates no black image, and generates the bitmap images for printing representing the black image from the attribute information stored in the memory if the attribute information indicates the black image.