We present a study on the resolution limits and resolution factors of terahertz (THz) ptychography. Simulations of a binary amplitude object show that ptychography shares the same intrinsic resolution factors with digital holography, i.e. it is diffraction-limited. Reconstructions of amplitude and phase objects obtained from holographic and ptychographic experiments are comparable. A lateral resolution of around one wavelength λ is achieved on an amplitude object, while a depth resolution of around λ/5 is reported on a weakly diffracting phase object. THz ptychography is expected to complement THz holography for imaging biological samples and THz transparent specimens.
The measurement of image quality requires the judgement by the human visual system. This paper describes
a psycho-visual test technique that uses the internet as a test platform to identify image quality in a more
time-effective manner, comparing the visual response data with the results from the same test in a lab-based
environment and estimate the usefulness of the internet as a platform for scaling studies.
We treat image-to-device gamut mapping as a multi-criteria optimization problem. Our approach leads to a parameterized mathematical optimization problem that allows to constrain the degree to which objectives like contrast preservation, hue preservation, saturation preservation and the continuity of the mapping can be violated while maximizing the device gamut exploitation. We demonstrate the feasibility of our approach on several benchmark image- and device gamuts.
The design of a gamut mapping algorithm (GMA) is always a compromise between preserving different competing aspects such as color, contrast, and lightness. A natural requirement of a GMA is that the algorithmic treatment of this competition has to avoid any additional artifacts such as discontinuities or loss of contrast. In this work, several common gamut mapping algorithms are studied from this aspect, resulting in the observation that problems with geometric discontinuities are widespread. For the assessment of the phenomena induced by local mapping properties, an algorithmic test is developed and applied. This new test supports both the quality check of existing as well as the development of new GMAs.
We present a new method for the computation of both, image and device gamut boundaries. The method has been designed to bypass the quality vs. time trade off that one usually faces when computing gamut boundaries. This trade off is between the geometric accuracy of the boundary and the time it takes to compute it. Our method is geometrically accurate in the sense that the computed gamut boundary tightly encloses the color points that make up the gamut. At the same time it is fast compared to other methods. Thus it can be used in an image-dependent gamut mapping approach. The underlying concept of the presented method is a data structure that we call discrete flow complex which is derived from the discrete distance function to the color points. We have implemented the method and tested it with a suite of test images. Our experimental results show that the method is in fact fast and geometrically accurate. In the future we plan to use the gamut boundaries computed by our method for fast, high-quality, image-dependent gamut mapping in three dimensions.
The design of a gamut mapping algorithm (GMA) is always a compromise between preserving different competing aspects such as color, contrast and lightness. A natural requirement to a GMA is that the algorithmic treatment of this competition has to avoid any additional artefacts such as discontinuities or loss of contrast. In this paper several common gamut mapping algorithms are studied from this aspect, resulting in the observation that problems with geometric discontinuities are widespread. For the assessment of the phenomena induced by local mapping properties, an algorithmic test was developed and applied. This new test supports both, the quality check of existing as well as the development of new GMAs. Finally we present a first new algorithm designed to have a good behavior concerning continuity and contrast conservation which also performs well in classical psychophysical tests.