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- Note which version of standard you are using. Each color standard values vary based on the version and the manufacturer of the standard. The XRite Colorchecker 2019 (MacBeth standard, Figure 20) is the preferred card to use for calibration. The program is set up to use the White and Black squares on the XRite Colorchecker (MacBeth standard). On the MacBeth standard the RGB values of white is 243 White and Black are listed here. White RGBs are 242, 242 and 236 and black is 52RGBs are 49, 49, 50. For the QP 101 v4 card, the RGB values are 235, 111, and 80 (Figure 21). All SHIL calibration standards are found in drawer PP-2B.
- Obtain the 3D standard (Figure 20), the gray silicone mat standard and the lens cap from PP-2B.
- Set camera f/stop to either F/16 or F/22 (Figure 22). The manufacturer suggested F/22 as the preferred for scanning with the current light set up however we have found F/16 works well for our section halves and is the most used F/stop for calibration and scanning sections. For hard rock cruises, where 360° whole round scanning is required, a larger F/stop number is required.
- If you haven’t set the camera’s height, now is the time to do so! See the section Camera Height Adjustment at the end of the calibration section.
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1. Place the 3D calibration standard on track as shown (Figure 25). Be sure to use the XRite Color checker 2019. The color squares must be oriented as pictured below, butted against the red reflection bar.
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4. In the Tiff Correction tab adjust the LUT polynomial order values for the Red, Green, and Blue channels (Figure 29-5). Adjust these values to create the lowest residual error with the smoothest curve in the Uncorrected Image tab. Poly values should be around 4. Make sure that the curve does not wave about. If it does, the values need to be lowered. Also check that the corrected ROI and MacBeth values should be very close. Make sure that the white does not exceed the MacBeth value (243values (RGB = 242. 242. 236). If you are unable to produce a reasonable correction curve, it is necessary to redo your white balance correction in the Calibration section below.
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3. Click JPEG Corrections tab (Figure 30-3). Adjust the Brightness, Contrast, and Gamma levels (Figure 54-4) to achieve a straight line in the Applied Corrections tab and the ROI Corrected box should have values near 243 242 for the white square and near 50 for the black. We want a linear relationship between the measured and given values. Each BCG setting adjusts the line in different ways and there are many different ways to adjust the values to achieve a linear relationship. You want to achieve a good image with good brightness, where the image has good saturation and not too washed out. (see JPEG corrections cheat sheet in SHIL lab notebook, to be added). The Applied Corrections Graph should be a straight line and the ROI Corrected Box should have values near 243the RGB values of 242. 242. 236. These may change depending on the instance of extreme colors, extremely white or extremely dark cores, in which the settings may have be tweaked more to get a user friendly consumer image.
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Resetting Gains and Corrections
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9. Adjust the Red Exposure Interval value until the RED value is 243242.
10. Adjust the Green Exposure Interval value until the RED/GREEN value is as close to 1 as you can achieve.
11. Adjust the Blue Exposure Interval value until the BLUE/GREEN value is is as close to 1 as you can achievenear 1 (actual ratio 0.98, Blue is 236, Green is 242). You just completed the White Balance of the upper limit of the camera.
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5. Adjust the Blue Black gain until the BLUE/GREEN value is near 1 (actual ratio 1.02, Blue is 50, Green is 49).
6. Keep an eye on the histogram graph on the bottom left corner (Figure 39). We want all the colors to overlay each other pretty closely. Adjusting the RedGain and BlueGain will move the colors (histographs) in the graph in the lower left, move until they are over lapping.
7. Adjusting the gain likely changed the RGB values in the White square of the MacBeth Color checker. Draw an ROI box in the White square. If the values aren't 243 near 242 go back to the Rates and Exposure tab and adjust the the values until you hit 243242 (RGB 242, 242, 236). Check back in the Black square and see its still about 15. Adjust the gains and/or exposure intervals until the Black reads near 15 and White reads 243near 242 (RGB 242, 242, 236). This is a balancing act and can be tedious. Remember do not let the temperature to go about 35 °C.
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Figure 55: Image Correction Window. Steps are indicated in the figure. New values for Color checker Values, RGB = 242, 242, 236)
Tiff Correction
1. Click TIFF Correction Mode (Figure 56-2)
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4. In the Tiff Correction tab adjust the LUT polynomial order values for the Red, Green, and Blue channels (Figure 56-5). Adjust these values to create the lowest residual error with the smoothest curve in the Uncorrected Image tab. Values should be around 4. Make sure that the curve does not wave about. If it does the order values need to be lowered. Also check that the corrected ROI and MacBeth values should be very close. Make sure that the white does not exceed the MacBeth value (243RGB 242, 242, 236). If you are unable to produce a reasonable correction curve, it is necessary to redo your White Balance by Shutter Correction in the Calibration section above.
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3. Click JPEG Corrections tab (Figure 57-3). Adjust the Brightness, Contrast, and Gamma levels (Figure 57-4) to achieve a straight line in the Applied Corrections tab and the ROI Corrected box should have values near 243 242 for the white square. We want a linear relationship between the measured and given values. Each BCG setting adjusts the line in different ways and there are many different ways to adjust the values to achieve a linear relationship. You want to achieve a good image with good brightness, where the image has good saturation and not too washed out. The Applied Corrections Graph should be a straight line and the ROI Corrected Box should have values near 243242. The BCG may change depending on the instance of extreme colors, extremely white or extremely dark cores, in which the settings may have be tweaked more to get a user friendly consumer image.
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