Image analysis – Color image processing
Reexamination Certificate
2003-02-21
2003-09-30
Ahmed, Samir (Department: 2623)
Image analysis
Color image processing
C348S582000
Reexamination Certificate
active
06628823
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to the processing of digital images to produce desired tone and color reproduction characteristics. Specifically, this invention makes use of capture and output device information, in conjunction with opto-electronic conversion function (OECF) and preferred reproduction models, to determine image-specific processing based on statistics derived from the image data. The processing determined may be applied automatically or with user input.
BACKGROUND OF THE INVENTION
Digital cameras and scanners are used to capture image data from a large variety of scenes and originals. A number of automatic approaches are employed to process this data for reproduction; but when reproduction quality is critical, most images are processed manually by experts. Expertly processed images are also overwhelmingly preferred, even by inexperienced viewers, when comparisons are made.
Manual processing is time consuming and must be done by individuals with significant expertise, partly because the controls found in currently available software packages make it difficult to achieve desired tone and color reproduction. Simple controls tend to vary the reproduction in ways that miss the optimum, and complex controls offer too many degrees of freedom. If a way could be found to produce results similar to those produced by experts, either automatically or with simple and intuitive manual adjustments, digital photography would become a much more attractive alternative to conventional photography.
The practice of conventional photography suggests that improvements in this direction are possible. Currently, conventional photographs tend to be superior to automatically processed digital photographs in tone and color reproduction, with photographs processed at professional laboratories being far superior. Yet the flexibility of digital systems is greater than that of conventional systems. Digital photographs have the potential to be better than conventional photographs, and expertly processed digital photographs are currently at least as good. Digital processing approaches that mimic the relatively fixed behavior of conventional photographic systems should be straightforward to develop. Insight into digital processing approaches can also be obtained by examining what experts do manually. If this is done, it is found that most of the decisions made are based on evaluations of the image with respect to the scene or original and with the desired reproduction goal in mind. It should be possible to develop software algorithms that can perform these evaluations and process images accordingly.
Three major factors have hindered progress in this area. The first is that expert manual processing is almost always image-dependent and is based on understood tone and color reproduction objectives; but the development of most digital tone and color reproduction processing has focused on schemes which do not consider the image data, or consider it without regard for established pictorial considerations. The second is that the exact meaning of the image data, with respect to the scene or original, must be known. To date, the approaches used have ignored many non-linearities, such as those introduced by optical flare and other image capture effects, and have concentrated on techniques based almost exclusively on colorimetry. Colorimetry is strictly applicable only when the capture spectral sensitivities are color matching functions, or when the colorants used in the original are known and limited to a number which is not greater than the number of spectral capture channels. With digital cameras in particular, this is frequently not the case. Other difficulties in determining scene physical characteristics have resulted from a lack of standard, accurate measurement approaches. When basic flaws are present in a measurement approach, such as the omission of flare considerations and the fact that the spectral characteristics of the detector preclude calorimetric information from being obtained, attempts to calculate scene values inevitably produce erroneous results. These errors reduce accuracy expectations and mask other error sources, seriously degrading the correlation between captured data and scene characteristics.
The final factors which have hindered progress are the slow recognition of the need for preferred reproduction as an alternative goal to facsimile reproduction, that preferred reproduction is dependent on the scene or original, and that the output and viewing condition characteristics. As mentioned previously, most digital tone and color reproduction processing development has focused on schemes which do not consider the image data. Also, color management approaches based on colorimetry attempt to produce reproductions with colorimetric values similar to those of the original. While colorimetric measures can consider some viewing condition effects, others are not considered and the effects of the scene characteristics and media type on preferred reproduction are ignored.
It is clear that the factors limiting tone and color reproduction quality in digital images stem from an incomplete and sometimes inappropriate global strategy. The inventor has attempted to deal with these problems in two ways: through participation and leadership in the development of national and international standards and with the inventions presented here. The following list specifies gaps in the strategy and the attempts to fill them in:
1. Inaccurate and non-standard device measurements.
Image capture and output devices are measured in a variety of ways, with various measurement and device effects being ignored. Specifically:
a) Flare and other non-linearities in both capture devices and measuring instruments are frequently not considered, or are measured for a particular condition, and the resulting values are erroneously assumed to be applicable to other conditions.
b) Test targets captured typically have considerably lower luminance ratios than pictorial scenes, so the extremes of the capture device range are truncated or left uncharacterized.
c) Attempts are made to correlate image data to calorimetric quantities in scenes by capturing data of test targets with devices whose channel spectral sensitivities are not color matching functions. Correlations established in this fashion will depend on the test target used and may not apply to image data from other subjects.
d) Measurement precision is frequently specified in linear space, resulting in perceptually large errors for darker image areas. These areas are also most affected by flare, making dark area measurements particularly inaccurate.
e) Measurement geometries and lighting may be inconsistent or inappropriate to the device use conditions.
All of these effects compound to produce device characterization measurements which can be quite inaccurate. A common perception of these inaccuracies is that it is practically impossible to obtain stable measurements and, therefore, measurement accuracy need not be too high. Frequently, assumptions are made about the nature of image data, because it is felt that the assumption will be as close to the real value as a measurement.
However, in conventional photography, standardized density measurement techniques have evolved over decades. These techniques routinely produce repeatable measurements with several orders of magnitude higher accuracy than those obtained for digital systems, which is one of the reasons the less flexible conventional systems are able to outperform current automatic digital systems. Unfortunately, the reason these techniques are so accurate is because they have been refined specifically for conventional photographic materials. A great deal of work will be required to develop similar techniques for the devices and material used in digital systems.
Work has just begun in this area, but significant progress is already being made. A new standard to be issued by the International Organization for Standardization (ISO), “ISO 14524, Photography—Electronic still picture cameras—Methods fo
Ahmed Samir
Eugene Stephens & Associates
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