Browsing by Author "Gastal, Eduardo S. L."

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    Prefilters for Sharp Image Display
    (The Eurographics Association and John Wiley & Sons Ltd., 2020) Rocha, Luís Cláudio Gouveia; Oliveira, Manuel M.; Gastal, Eduardo S. L.; Panozzo, Daniele and Assarsson, Ulf
    In this paper we use a simplified model of the human visual system to explain why humans tend do prefer ''sharpened'' digital images. From this model we then derive a family of image prefilters specifically adapted to viewing conditions and user preference, allowing for the trade-off between ringing and aliasing while maximizing image sharpness. We discuss how our filters can be applied in a variety of situations ranging from Monte Carlo rendering to image downscaling, and we show how they consistently give sharper results while having an efficient implementation and ease of use (there are no free parameters that require manual tuning). We demonstrate the effectiveness of our simple sharp prefilters through a user study that indicates a clear preference to our approach compared to the state-of-the-art.
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    Real-Time Frequency Adjustment of Images and Videos
    (The Eurographics Association and John Wiley & Sons Ltd., 2021) Germano, Rafael L.; Oliveira, Manuel M.; Gastal, Eduardo S. L.; Mitra, Niloy and Viola, Ivan
    We present a technique for real-time adjustment of spatial frequencies in images and videos. Our method allows for both decreasing and increasing of frequencies, and is orthogonal to image resizing. Thus, it can be used to automatically adjust spatial frequencies to preserve the appearance of structured patterns during image downscaling and upscaling. By pre-computing the image's space-frequency decomposition and its unwrapped phases, these operations can be performed in real time, thanks to our novel mathematical perspective on frequency manipulation of digital images: interpreting the problem through the theory of instantaneous frequencies and phase unwrapping. To make this possible, we introduce an algorithm for the simultaneous phase unwrapping of several unordered frequency components, which also deals with the frequency-sign ambiguity of real signals. As such, our method provides theoretical and practical improvements to the concept of spectral remapping, enabling real-time performance and improved color handling. We demonstrate its effectiveness on a large number of images subject to frequency adjustment. By providing real-time control over the spatial frequencies associated with structured patterns, our technique expands the range of creative and technical possibilities for image and video processing.