IEEE Camera Phone Image Quality (CPIQ) Support

Imatest’s CPIQ Implementation

Imatest 4.4+  supports IEEE CPIQ P1858^TM (Camera Phone Image Quality) 2016 measurements. As members of CPIQ, we are strongly committed to providing convenient CPIQ-compliant and equivalent image quality measurements.

NOTE: You should use at least Imatest 4.4.9 for the latest corrections to visual noise calculations.

This page has similar order to chapters the CPIQ Standard, “P1858^TM/D1 Standard for Camera Phone Image Quality“ document. The first four chapters are 1. Overview, 2. Normative references, 3. Definitions, acronyms, and abbreviations, and 4. Test conditions and apparatus. Measurement chapters are:

5. Spatial Frequency Response (SFR)
6. Lateral Chromatic Displacement
7. Chroma Level
8. Color Uniformity
9. Local geometric distortion
10. Visual noise
11. Texture blur
12. Auto Exposure

For each of these sections we will discuss charts, measurement techniques, and (perceptual) Quality Level results. The last section on this page lists the JSON output locations for all CPIQ metrics.

Test Lab Environment

Current draft requirements (to be approved) for a IEEE certified test lab are as follows:

Charts

• Dot Pattern chart (Measures distortion and chromatic displacement)
• ESFR-ISO chart – SFR and visual noise
• Colorchecker SG – chroma level
• Spilled Coins/Dead Leaves chart – Measures texture acutance
• Blank 18% gray chart (Measures color uniformity, this could be a wall painted with an ~18% gray paint)

Lighting

All lighting must be able to achieve at least 80% uniformity across the largest chart (e.g. no more than 20% deviation from center to corner)

The following illuminants:

• 3050k Incandescent tungsten illuminant at 25 lux
• D55 illuminant at 1000 lux
• TL84 illuminant at 100 lux

Environment

• Temperature must be 23C +/- 2C
• Room should be set up to reduce reflections and prevent stray light from entering the scene. Ideally, the walls should be painted with a low-reflection, low-chroma paint such as ~18% gray or black
• Room should be light-tight so no outside light sources could affect measurements

Spatial Frequency Response (SFR) – Chapter 5

Chapter 5 of the CPIQ document describes slanted-edge SFR (MTF) measurements. It includes Acutance, which is a perceptual measurement of sharpness, derived from MTF and the human visual system’s Contrast Sensitivity Function (CSF).

For measuring SFR in compliance with CPIQ, we recommend the eSFR ISO chart. eSFR ISO has sufficient slanted-edges to map sharpness over the image surface, has a grayscale OECF chart that can be used for the Visual noise measurement, and is fully compliant with the chart requirements of the ISO 12233:2014 standard.

Enhanced eSFR ISO chart

For CPIQ, sharpness is measured as Spatial frequency response (SFR) (also called Modulation Transfer Function (MTF)). The eSFR ISO module can be used for this purpose.

Acutance is a perceptual measurement designed to answer the question, “how sharp does the image look?” It is closely related to Subjective Quality Factor (SQF), differing only in the details of the equation. It is calculated from MTF (SFR), a model of the human visual system Contrast Sensitivity Function (CSF), and viewing conditions (image height and viewing distance). Imatest displays it in exactly the same way as SQF, which provides many of options. To obtain Acutance measurements, open eSFR ISO (or other Rescharts module), then click on the SQF/Acutance Options button in the Settings window (More settings in SFRplus). Click on the CPIQ Acutance button to select the acutance calculation. Plot type can be SQF or Acutance (standard plot) or JND (Just Noticeable Difference: CPIQ Phase 2 equation. You’ll have to make some assumption on the viewing conditions. See SQF/Acutance for details.

SQF/Acutance settings window

Here are results from the eSFR ISO module display #3. We have assumed that viewing distance is proportional to the square root of the print height. 

Acutance results for viewing distance = 20*sqrt(PH/10) or 25 cm, whichever is larger for Picture height from 10 to 40 cm

Lateral Chromatic Displacement – Chapter 6

Lateral Chromatic displacement is also known as Lateral Chromatic Aberration.  Often this is characterized using a tangential slanted edge, but the CPIQ standard obtains a much more detailed measurement through use of the dot pattern test chart:

An image of the dot pattern test chart  is analyzed using the dot pattern module of Imatest.

Chroma Level – Chapter 7

The Chroma level (Saturation) test uses the X-Rite Digital Colorchecker SG chart, shown properly framed on the right. (The chart should not fill the image. If it does, light falloff may reduce accuracy of the results.) The measurement uses only the chromatic patches: it omits the black, white, and grayscale patches near the edges and center of the chart. The Colorchecker SG is analyzed using the Color/Tone Interactive (interactive) or Color/Tone (fixed) modules. The regions selection must be manually set so that the analysis regions are properly aligned with the test chart patches. Color/Tone Interactive fine adjustment window for Colorchecker SG

The mean CPIQ chroma and quality level results are shown in the 2D a*b* plot. This figure shows a*b* values (from CIELAB color space) for patches for the 4 row x 6 column Colorchecker replica, just above the chart center (rows 2-5 from the top; columns 5-10).

a*b* reference and camera colors for the 24-patch Colorchecker Classic replica inside the Colorchecker SG, showing the mean CPIQ Chroma metric (measured on all chromatic patches).

Color Uniformity – Chapter 8

The CPIQ color variability metric, maximum ΔC = D_c, is displayed in Uniformity, Uniformity-Interactive, and Blemish Detect when Grid size is set to 20×15 for a 4:3 aspect ratio, 24×16 for a 3:2 aspect ratio, or 32×18 for a 16:9 aspect ratio and Grid calculation is set to 2. Delta-C and Delta-E. Delta-C and Delta-E are calculated for each grid region i by comparing L*a*b* values in the region with the mean for the image as a whole). It is calculated and saved in the JSON and CSV output whenever the CPIQ calculation box is checked. Here is an example of the settings.

Grid plot settings for CPIQ color variability metric D_c

Here is the resulting grid plot.

Uniformity (or Uniformity-Interactive) 15×20 grid plot for ΔC and ΔE

The CPIQ color uniformity metric is displayed on the lower-right: Maximum ΔC = 2.606 is the CPIQ metric of color variability. The Quality loss associated with this metric is

QL = subjective quality loss in JNDs for color uniformity = 0;  Dc ≤ 0.8636 \( = \frac{(D_c – a)}{b} – \frac{c \ln \bigl( 1 + \frac{b(D_c – a)}{c}\bigr)}{b^2}\);    D_c > 0.8636

where  a = 0.8636,  b = 2.3952,  c = 0.9771