Photos: Looking at a halftoned image using a magnifying glass
Halftoning for Appearance Printing
Many reproduction devices, e.g., printers, have a limited number of inks which makes it difficult to reproduce shades of different colors. All they can do is to get inks from the printer cartridge and deposit spots of ink on a piece of paper. If you take a newspaper’s picture and use a magnifying glass and zoom in, you will see that the image consists of dots put together. But these small dots are not visible to the human eye from a normal viewing distance, so we see the image as a continuous tone image.
But, how to turn an image into spots of ink? Continuous tone images should go through a process called halftoning before being printed. Digital halftoning is a technique to reproduce different shades of colors using only a limited number of inks. Halftoning leverages the fact that the human eye performs as a low pass filter, hence, if the dots are small enough and the viewing distance is sufficiently large, the halftoned image appears pleasant to the eye. Halftoning algorithms provide methods to optimally place dots in a way that the human eye is not able to distinguish between the original image and the halftoned one.
Studies have shown that halftoning has a constructive impact on the visual quality in printing. Fereshteh Abedini’s research objective within the ApPEARS project is to develop novel methods for halftoning to better mimic a specific surface structure for realistic appearance printing. She has been investigating aspects of adapting and applying 2D halftoning algorithms to halftone three-dimensional surfaces. In collaboration with her supervisors at the Linköping University in Sweden, she has now extended a high-quality 2D algorithm for 3D halftoning. She recently presented this work at the Printing for Fabrication conference 2020 and published a research paper titled “3D Halftoning based on Iterative Method Controlling Dot Placement”. You can find link to the paper below.
Fereshteh Abedini is also interested in studying the effect of different halftone patterns on the perceived appearance of 3D printed shapes. Work carried out so far shows that using different halftones based on the 3D geometrical structure of the surface and/or the viewing angle in combination with the structure of the texture being mapped on the surface can alter the appearance of 3D surfaces. According to their findings, applying different halftones based on the geometrical characteristics of the 3D surface could emphasize or diminish the perceived 3D geometrical structures of a shape. If you are interested in having more information about it, please find the details of their work in: “3D Surface Structures and 3D Halftoning”, also presented at the Printing for Fabrication conference 2020. Link is provided below.