Printing appearance effects beyond colour - such as gloss - is an active research topic in the scope of multi-layer printing (2.5D or 3D printing). Such techniques may enable a perceptually more accurate reproduction of optical material properties and are required to avoid appearance related artefacts sometimes observed in regular colour printing - such as bronzing and differential gloss. In addition to technical challenges of printing such effects, a perceptual space that describes the related visual attributes is crucial; particularly to define perceptually meaningful tolerances and for appearance gamut mapping. In this paper, we focus on spatially-varying gloss created by varnish-halftones. This enables us to print specular gloss effects covering a large portion of the NCS gloss scale from full matte to high gloss. We then conduct a psychophysical experiment to find the relationship between measured specular gloss and a perceptually uniform gloss scale. Our results show that this relationship can be well described by a power function according to Stevens Power Law.
An important aspect for print quality assessment is the perceived gloss level across the printout. There exists a strong
relationship between the surface roughness of a printout and the amount of specular reflection which is perceived as
gloss variations. Different print parameters influence the surface roughness of the printouts such as the paper substrate,
the type of inks and the print method. The lack of control over the print’s surface roughness may result in artefacts such
as bronzing and differential gloss.
Employing a 2.5D or relief printing system, we are able to control the printout roughness by manipulating the way the
ink is deposited in a layer-by-layer basis. By changing the deposition time in between two layers of white ink and the
order on which the pixels are printed, we achieve different gloss levels from a matte to a glossy appearance that can be
controlled locally. Understanding the relationship between different printing parameters and the resulting gloss level
allows us: to solve differential gloss artefacts (to obtain a print with a full gloss or matte finish) and to use the local gloss
variations to create reflection effects in the printouts. Applications related to security printing have also been explored.
Our results showed a reduced level of gloss toward a matte appearance as the ink deposition time between the layers was
increased, allowing more time for the ink to dry between passes. We measured the gloss levels using a gloss meter and a
psychophysical experiment was conducted to validate our measurements and observations.