A computer program for executing on a computer system a computer process for generating 1-bit image data from multiple-bit image data, the 1-bit image data comprising "on" and "off" pixel values, each pixel value of the 1-bit image data corresponding to a pixel of an output medium, which pixel an output device would attempt to mark when printing the 1-bit image data if the pixel value were "on", the 1-bit image data producing an image constituted by dots, each dot corresponding to a plurality of pixel values of the 1-bit image data, which pixel values correspond to a respective block of M*N horizontally and/or vertically adjacent pixels of an output medium, at least one of M and N being greater than one, wherein M defines the width in pixels of a full sized dot and N defines the height in pixels of a full sized dot, the process comprising the steps of: receiving multiple-bit image data comprising multiple-bit pixel values; deriving said 1-bit image data from the multiple bit pixel values, by applying a screening algorithm to the multiple-bit pixel values to generate 1-bit pixel values, which are set to "on" and "off", wherein "on" 1-bit pixel values correspond to dots of the image; where densities of dots are greater than a first threshold density converting each "on" 1-bit pixel value to a reduced size dot, wherein where M or N is equal to one, a pixel value corresponding to a first or last pixel of a row of horizontally adjacent pixels, or to a first or last pixel of a column of vertically adjacent pixels of the corresponding M*N block of pixels is "off', and wherein, where both M and N are greater than one, the pixel values corresponding to the pixels of at least one of the first and last rows of horizontally adjacent pixels of the corresponding block are "off' and the pixel values corresponding to the pixels of at least one of the first and last columns of vertically adjacent pixels of the corresponding M*N block of pixels are "off", the reduced size dots of the image thus being obtained by not marking the pixels of at least one of the first and last rows of horizontally adjacent pixels and not marking the pixels of the at least one of the first and last columns of vertically adjacent pixels of the respective M*N block. receiving multiple-bit image data comprising multiple-bit pixel values; deriving said 1-bit image data from the multiple bit pixel values, by applying a screening algorithm to the multiple-bit pixel values to generate 1-bit pixel values, which are set to "on" and "off", wherein "on" 1-bit pixel values correspond to dots of the image; where densities of dots are greater than a first threshold density converting each "on" 1-bit pixel value to a reduced size dot, wherein where M or N is equal to one, a pixel value corresponding to a first or last pixel of a row of horizontally adjacent pixels, or to a first or last pixel of a column of vertically adjacent pixels of the corresponding M*N block of pixels is "off', and wherein, where both M and N are greater than one, the pixel values corresponding to the pixels of at least one of the first and last rows of horizontally adjacent pixels of the corresponding block are "off' and the pixel values corresponding to the pixels of at least one of the first and last columns of vertically adjacent pixels of the corresponding M*N block of pixels are "off", the reduced size dots of the image thus being obtained by not marking the pixels of at least one of the first and last rows of horizontally adjacent pixels and not marking the pixels of the at least one of the first and last columns of vertically adjacent pixels of the respective M*N block.