Jack M. Holm

Comparison of primary and complementary color filters for CCD-based digital photography

This paper presents a comparison between primary (RGB) and complementary (CYMG) CCD color filters arrays, as applied to digital photography. Our analysis is based upon the measured spectral characteristics of the primary and complementary color versions of the Matsushita MN3776 CCD. The important role of the color correction matrix on the quality of the image is considered both in terms of noise and color saturation. Our calculations show that there is a tradeoff between color saturation and ISO speed, when complementary filters are used. Complementary color filters only gain an ISO speed advantage when the color saturation is low. When the color correction matrix is chosen to make the ISO speeds of the two filter systems equivalent, the well capacity of the complementary CCD must be significantly higher because of the higher overall transmission of its color filters. Our comparison includes ISO speed calculations and plots of the color gamut for primary and complementary color filters with various color correction matrices. We conclude that primary color filters are superior for digital photography.

Some considerations in the development of color rendering and gamut mapping algorithms

Newly developed standard terminology enables improved communication about color processing objectives and research goals. By clarifying specific color processing tasks, one observes that there are de-facto standard practices for many processing steps, that can be used as a baseline recommendation for implementers and future development work. The more explicit descriptions of development goals can also serve to focus research on the desired objectives. This increased clarity leads to work being both more effective at meeting design objectives, and also more apparently relevant to commercial applications. Differences in objectives and requirements for the development of color rendering and gamut mapping algorithms are discussed and contrasted. In some cases, these differences can explain the reasons for different approaches, enabling broader consensus and understanding. Differences between color appearance and reproduction models are also discussed, along with the impact of these differences on their use for imaging applications. The above concepts are related to important color reproduction considerations, such as viewer preferences and media capabilities. If the more explicit terminology is widely adopted, it could accelerate the advance of digital color understanding across both product manufacturers and users, and enable significantly more effective research, development, and use.

Challenges and progress in digital photography standards

The ISO TC42/WG18-20-22-23 and ANSI/I3A IT10 Technical Committees have now been developing digital photography standards for over a decade. This work has led to the publication of standards on digital imaging terminology, digital camera ISO speed measurements, resolution measurements, OECF (linearity) measurements, image formats and metadata, and picture transfer protocol (PTP). More recently, standards on color encoding specifications and color architectures, a JPEG 2000 profile for digital cameras, camera noise and dynamic range measurements, digital camera specification reporting, and scanner resolution have been finalized. Work in progress includes image quality subjective testing methods, digital camera color characterization, and scanner dynamic range measurements. This paper will review past and current technical challenges, and the state of the solutions provided. In most cases, development includes a significant and innovative research component, which is discussed in relation to fundamental imaging issues. These standards are viewed from a broad digital photography perspective, and placed in context with other work in this area. In addition to providing a forum for the development of standards, technical committees are an important avenue for interaction between companies, user groups, and the government. Such avenues can have a great impact on emerging technologies.

A method to improve the invertibility of ICC profiles that use lookup tables

Traditionally, the AToB0 tag of an ICC printer profile, containing the perceptual transform from device specific to PCS Lab or XYZ values, has been generated directly from measurement data without considering the color re-rendering included in the PCS to device-specific transform found in the corresponding BToA0 tag. In this case, the AToB0 color conversion will often not be the inverse of the color conversion in the BToA0 tag. However, with ICC version 4, the AToBn and BToAn transforms are in general supposed to be inverses of each other, to the extent possible. This feature supports the re-purposing of color data within an ICC color management workflow. This inversion is a challenge for profile generation due to issues with either directly inverting 3-D interpolations or inverting every step applied in generating the BToA0 tag. Directly inverting a 3-D LUT may not be feasible because the forward mapping is usually not a one-to-one mapping. Mathematically, inverting every operation for generating a BToA0 tag may also be extremely difficult if not impossible. Consequently, a closed-loop method has been developed which iteratively adjusts AToBn tags to improve the invertibility of ICC profile transforms. The test results are very encouraging.