Martin S. Maltz

Refinement of printer transformations using weighted regression

Printer characterization and color correction are often complex transformations, and are derived with numerous measurements or printer models. There are many sources of errors in these transforms, including inaccuracies in lookup table approximation, errors in the printer model, noise in the data, and spatial and temporal non-uniformities in the printer. A method is proposed to increase the accuracy of an existing printer transform with a relatively small number of refinement measurements. A weighted linear least-squares regression technique is used to improve the fit of the printer response to the refinement data. The hypothesis is that a locally linear transform can adequately capture the difference between the true printer transform and its approximation. In contrast to existing approaches that only refine the individual C, M, Y, K responses, the proposed method attempts to account for cross-colorant interactions by using mixed colors in the refinement set. Furthermore, the refinement data is not restricted to lying on a regular grid, and can be freely chosen based on any a priori knowledge about the printer. The approach is tested for two related transforms: the characterization transform which maps CMYK to L*a*b*; and its inverse, the color correction transform that maps L*a*b* to CMYK. Results show an improvement in transform accuracy with a relatively small number of measurements.

Modeling vibration-induced halftone banding in a xerographic laser printer

In a raster scanning printer, a laser beam is scanned across a photoreceptor in a direction perpendicular to the photoreceptor motion. When there is vibratory motion of the photoreceptor or wobble in the polygon mirror, the raster lines on the photoreceptor will not be evenly spaced. We analyze the positioning error and show that fractional raster spacing error is equal to photoreceptor fractional velocity error. These raster position errors can result in various print defects, of which halftone banding is the dominant defect. The dependences of halftone banding are examined using a first-order geometry-based printing model, an exposure model, and a more sophisticated laser imaging model coupled with a xerography model. The system model is used to calculate print reflectance modulation due to vibrations in both charged-area and discharged-area development modes using insulative or conductive development. System parameters examined are halftone frequency, raster frequency, average reflectance, vibration frequency, and multiple-beam interlace spacing.

Tetrahedralization of point sets using expanding spheres

The expanding sphere algorithm computes an alpha shape tetrahedralization of a point set. Starting with a seed tetrahedron, the circumscribing sphere is squeezed through each face until it either touches another point or exceeds a preset radius. If no point is found, that face of the tetrahedron is part of the surface of an object. If a point is found, a new tetrahedron is constructed. This process is iterated until all the faces of the tetrahedra have been processed and no more connected points can be found. If there are points left over, the process is iterated, creating additional objects. The algorithm generates a list of objects, with an alpha shape tetrahedralization and a surface triangulation for each. Any points that cannot be made part of a valid tetrahedron are also returned in the extra points list. The algorithm is efficient for uniformly distributed point sets, with a running time that is linear in the number of points for such sets. Since the operations are local, it is also robust.

Flash/no-flash fusion for mobile document image binarization

We propose a novel algorithm for improving the quality of binarized document images captured with a mobile device under low light conditions. In such scenarios, images captured without a flash often result in blur artifacts and poor signal-to-noise ratio, while images taken with the flash may produce information loss due to specular reflection in a localized region termed a “flash spot”. Our algorithm automatically triggers the capture of a pair of images, one with and one without flash, in rapid succession. The flash spot region (FSR) is first detected. The two images are then accurately aligned within the FSR using a multiscale alignment technique. Finally the images are binarized and fused in the vicinity of the FSR using an intelligent technique that minimizes fusion boundary artifacts. The result is a binary image that is largely identical to the binarized flash image, except within the FSR where content from the no-flash image is selectively incorporated. To our knowledge this is the first attempt to employ flash/no-flash fusion to improve binarization of mobile document images. Results show superior qualitative and quantitative performance of the proposed algorithm when compared with standard binarization applied to either the flash or no-flash image.

Tensor decomposition for color printer model lookup table

Multidimensional lookup tables (LUTs) are often used to describe the response of physical systems to multiple inputs. However these tables are also tensors, and in this paper we will use tensor decomposition to greatly reduce the number of parameters needed to generate an accurate approximation to the tensor, and discuss how to determine these parameters from a small number of known tensor elements. We will use this approach to generate printer models, which are CMY or CMYK to L*a*b* LUTs where each element is an L*a*b* value for one CMYK formulation. The approach generates accurate results with a reasonable number of L*a*b* measurements, and can be used when nothing else is known about the system. It also runs much faster than the physics based models that are sometimes available for these systems.

Jab to RGB multidimensional lookup tables

The CIECAM color appearance space (Jab) is widely used in color science. The relationship between XYZ and Jab is defined by a set of equations. For ease of computation and architectural flexibility, it is desirable to implement these relationships in multi-dimensional lookup table form. Unfortunately, for some colors just outside the sRgb gamut, these equations give complex numbers. Furthermore, to include colors that printers can produce, a transformation between Jab and a wider gamut RGB is necessary. Such a table would contain problematical colors. To solve this problem we accurately reproduce the CIECAM results within a gamut of reasonable colors. Nodes just outside the gamut are used when interpolating to find colors just inside the gamut so they are also affected by the accuracy constraint. All the nodes outside the gamut are also subjected to a smoothness constraint. Both RGB to Jab and Jab to RGB tables have been produced for a wide gamut RGB, and give quite accurate results.

Creating variable data infrared signals for security applications

Four color printing is the common way to render a color image to paper. This four color printing has certain side effects, among them the metameric representation of colors. This means that a single visual color can be generated through multiple different four color combinations. This is normally considered a problem, however, the problem description can be inverted and information can be embedded in a printed color image that is perceptually invisible under normal illumination, but revealed to an infrared imaging system. This means that certain security aspects can now produced in an essentially cost-neutral way.