The standardization work of formal metrics for digital imaging artefacts is not so mature than the traditional image quality metrics. Camera Phone Image Quality (CPIQ) group published Phase 1 in 2007 which included proposals for a color uniformity and flare testing (CPIQ Phase 1 2007) and Phase 2 in 2009 with geometric distortion and lateral chromatic aberration test proposals (CPIQ Phase 2 2009). Several ISO standard proposals are currently under development or have been published during recent years and the CPIQ group plans to publish the first official version of their standard in 2016 including many metrics for the imaging artefacts.
4.3.1 Lens distortions
There are several metrics to validate the geometric distortion of a lens system.
European Broadcast Union (EBU) defined a picture height distortion metric in 1995, the work was based on ISO 9039 standard and uses same metric (EBU Tech
3249–E 1995; ISO 9039 1994 and 2008). Standard Mobile Imaging Architecture (SMIA) organization has defined a slightly different metric. The ISO version is based on the ratio between the vertical distortion error of the corners towards a non-distorted image, whereas the SMIA version defined a ratio between the biggest vertical distortion and the smallest vertical distortion. A slightly moderated version of the SMIA metrics is used in CIPA DCG-002 standard.
CPIQ group has defined another approach to the geometric distortion metric, which is strongly based on the test chart used: a white test chart filled with black dots whose locations are known precisely as shown in Figure 21. There are several benefits from the approach; the geometric distortion can be calculated from several locations of the image and therefore it can be modelled by a polynomial. Using the polynomial the distortion can be defined as a function of the distance from the image center point. The function can be used to correct the artefact. Moreover, the same approach and chart can be used for measuring lateral chromatic aberration.
CPIQ group describes s a metric called Local Geometric Distortion (LGD) which can be calculated from every dot of the test chart. (CPIQ 2016)
Figure 21 Lateral chromatic aberration and geometric distortion in a dot test chart.
Moreover, the ISO organization has been published very recently a new standard for digital cameras called ISO 17850, Photography - Digital cameras - Geometric distortion (GD) measurements. The standard follows the metrics of ISO 9039 but instead of a single lens system, it takes into account the whole camera system. It is obvious that CPIQ standard and ISO 17850 have been written simultaneously,
because several chapters of both documents are almost equal. In addition to LGD metric, ISO 17850 defines another geometric distortion metric called line geometric distortion, which is not part of the CPIQ standard. The line geometric distortion is the same as in SMIA standard, it describes the ratio between minimum and maximum vertical or horizontal geometric distortion error (ISO 15780 2015).
In case of lateral chromatic aberration, the positions of red and blue color channels are measured from the green channel, and the difference is modelled again as a function of the distance from the image center point. The worst distortion between the channels proportion to image height is defined as the lateral chromatic displacement metric (LCD). (CPIQ Phase 2 2009)
On the other hand, the ISO organization has been published very recently a standard for the lateral chromatic aberration: ISO 19084, Photography - Digital cameras - Chromatic displacement measurements. ISO 19084 defines chromatic displacement and radial chromatic displacement metrics, where the chromatic displacement equals to CPIQ LCD metric. The radial chromatic displacement metric is equal for chromatic displacement but it is described as a function of distance of the green channel to the image center. Moreover ISO 17850 introduces another test chart, a V pattern chart. (ISO 17850 2015) Again, a significant equality between CPIQ standard and ISO 17850 can be noted.
4.3.2 Vignetting and color shading
Vignetting and color shading metric examples were defined already in CPIQ Phase 1 documentation (CPIQ Phase 1 2007). Since then, they have acquired more details in Phase 2 and finally in ISO 17957 standard. Vignetting and color shading are calculated from a neutral gray chart using several light sources. Whereas the latest CPIQ version proposes two light sources, an outdoor light and incandescent light, ISO 17957 adds a fluorescent light to the list. Both standards divide the captured image into 18-20x15-32 blocks, depending on the aspect ratio of the image and the color shading is calculated as defined in (7).
(7) D(i)= (a(i)−a)2 +(b(i)−b)2
Here D(i) is a deviation of a block, aand bare averages of the whole image and a(i) and b(i) are average values of blocks using the L*a*b* color space. The maximum color shading of all blocks is calculated and the maximum values is reported as the color shading metric of the image. The same method can be used for vignetting measurement by using the L* component of the L*a*b* color space (ISO 17957 2015).
4.3.3 Flare and blooming
Two different flare metrics are defined in ISO 9358 standard. A veiling glare index (VGI) and glare spread function (GFS). The veiling glare index is captured using a white test chart which has a small, complete black element in the middle of the chart. The veiling glare is calculated from the ratio of the black element’s lumination value towards the lumination value of the white area. Both values should be normalized by removing the gamma correction. (ISO 9358 1994)
The VGI test is quite simple and straightforward to build and execute, for example Imatest and Image Engineering and CPIQ Phase 1 use variations of the VGI test.
The glare spread function is a more complicated metric, requiring moving test equipment and a sensor, with a dynamic range of 13-19 f-stops. The method is based on a small, moving light source directed towards the camera system. When the light hits the camera system at different angles, even outside the camera’s field of view, the flare effect can be calculated from the luminance values, which are a function of the light source’s angle. (ISO 9358 1994)
CIPA DCG-002 standard defines also a flare metric, of which the test environment is identical to the VGI metric but measured using several exposure times (CIPA DCG-002 2012).
There seems to be a lack of blooming metrics. Theuwissen has proposed a metric based on a calculation of light spread across the image, where the scene contains a bright object (Theuwissen Blooming). However, no such standardized metrics can be found for still imaging. International Electrotechnical Commission (IEC) has defined blooming measurements for video cameras, however, the standard dates from 1997 and has not been updated since (IEC 61146-2 1997).
4.3.4 Other artefacts
There are no specific metrics for noise related artefacts like bad pixels, green imbalance and maze patterns. They are part of the noise results defined in section 4.2.2. Moreover, artefacts causing defocus and blur are validated in the resolution measurements as well as the over sharpening artefact.