1. Page 1 Visual calibration for (mobile) devices Lode De Paepe 2013-09-12 GOAL: Calibrate the luminance response of a display (transfer function of the display system) without the use of a measurement device (not considering our eyes a measurement device). In order to do this, we need to ‘ Measure ’ the native curve of the display system using a visual feedback algorithm.

2. Page 2 Basic principle L black L white L dither = 50% L black + 50% L white = (L black + L white ) / 2 or L50% = (L0% + L100%)/2 + 50%=50% L dither L0%L100% L50% The only points we ‘ know ’ (relative) on the native curve are black and white: [ 0 ; L0% ] and [ 255 ; L100%] L0% and L100% are not know in absolute value, we only know that L0% corresponds to a black and L100% to a white field. By mixing both, we create a luminance level that is the average of both.

3. Page 3 Basic principle Black White Black + White dot-on-off Gradient Black to White Cross-over point (DDL(L50%)) indicates DDL (Digital Driving Level) that corresponds to 50% luminance The value of DDL(L50%) depends on the native curve. In fact, it defines the first point (not considering the black and white level) of our native curve characterization: [ L50% ; DDL(L50%) ] 0 127255

4. Page 4 Basic principle L50% L white L dither = 50% L50% + 50% L white = (L50% + L100%) / 2 = L75% + 50%=50% L dither In a similar as was done for the L50% patch, a L75% patch can be constructed using the DLL(L50%) value (being the value we ‘measured’ in the previous step) and white. DDL(L50%)

5. Page 5 Basic principle DDL(L50%) (e.g. = 180) White DDL(L50%) + White dot-on-off Gradient Black to White Cross-over point indicates DDL (Digital Driving Level) that corresponds to 75% luminance. Corresponding point on native curve: [ L75% ; DDL(L75%) ] 0 127255

6. Page 6 Basic principle L75% L25% L100% L87.5% L67.5% L50% L37.5% L12.5% L0% Starting from black and white (DDL:0 and DDL:255), we can determine several DDL / Lxx% combinations. These are points on our native curve.

7. Page 7 Measurement example The graph shows the DDL measurements as function of the Luminance. Note that: -The Luminance is relative: [0..100%] mapped on [Lmin..Lmax] -Lmin (L0%) includes the ambient light influence.

8. Page 8 Using the data  The data can be used as is to calculate gamma curves and corrections that don ’ t need absolute Luminance values.  For DICOM calibration, we need at least a good estimate of the min and max luminance values of the display in order to have acceptable results.

9. Page 9 That was simple. But…  White and black level are not known (problem for DICOM calibration, not for gamma)  Ambient light changes frequently. Black level compensation is needed (problem for all types of calibration).  The interpolation between the points is not trivial (especially in the darker regions)  Panels are not ‘perfect’: things we can handle  Panels are not ‘perfect’: things we cannot handle (in an easy way)

10. Page 10 Unknown white level  To be able to calculate the calibration LUT for according to DICOM, we need the actual value of the black and white level.  Our native curve is only a relative one (no actual luminance levels)  On most handheld devices, there is no access to data that allows to know the actual luminance of the display. Solution: Use the device in a ‘known’ state (e.g.: full brightness and with ambient light control off), and use the typical values.

11. Page 11 White level clipping All iPad models and also other handheld devices tend to show some clipping in the white (native ddl 252 to 255 generate the same luminance). This is handled by adding a step at the beginning of the algorithm, during which the user needs to adjust until a difference is visible between ddl 255 and the adjusted level. The value found is the maximum ddl we can use.

12. Page 12 Black Level: Ambient light (and clipping)  As with white level clipping, we add a step in the beginning of the algorithm to find the first ddl that actually shows up as different.  Extracting the ‘real’ ambient light value is based on the White level and the darkroom contrast of the device.

13. Page 13 Curve interpolation We need to limit the number of points in the native curve determination:  from practical point of view: it should not take long to do a visual calibration  Because of accuracy reasons: every additional point relies on previously determined points, hence errors propagate. This is especially problematic in the dark region, where the native curve is very flat. A good interpolation is needed.

14. Page 14 Curve interpolation Difference between a good and bad interpolation (see difference in % in the dark levels)

15. Page 15 Panels are not perfect: things we cannot handle  Color shift when dot-on-off mixing different levels. –Because panels show color shift along their greyscale. Hence, mixing white and black will create a result color that is slightly different from the nearest (brightness) solid grey.  Viewing angle effects –Changing the viewing angle changes the contrast. As a result, different results will be obtained.  Native curve defects. –As the algorithm uses only 8 points to measure the native curve, local defects (highly improbable) will not be detected However, QA assurance checks can cover this to the extend that the defect levels are being tested.

16. Page 16 Barco Calibration on iPad: QAWeb Mobile  IPad app providing: –Visual calibration –Visual QA check –Only demo viewer. For strategic reasons, Barco choose not to provide viewer functionality.  iOS does not allow changing the global (hardware) LUT => we cannot calibrate the device… => viewer applications have to apply the correction within the app, so we need to work together with viewer developers to have them use our calibration / QA data

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