Match Reference

The Match Reference transformation lets you color correct an image based on a photograph of a standard color target taken under the same conditions as the image to be corrected. For example, if you take a picture of a Macbeth ColorChecker (see below) and then take additional photographs under the same conditions, you can color correct the images using the image of the ColorChecker.

Currently two standard targets are supported: the Macbeth ColorChecker and the Kodak Q-13 Gray Scale:

The Macbeth ColorChecker is a rigid board containing 24 patches: 18 assorted colors and 6 graduated neutral grays. It comes in two sizes – a convenient miniature version about 2x3 inches and the standard size about 9x13. Versions manufactured prior to Nov-2014 have slightly different color characteristics from those manufactured after Nov-2014, so they are treated as different targets.

The Q-13 is a printed strip of heavy coated paper about 2.5x8 inches containing a graduated set of 20 gray patches with densities running from 0.00 to 1.90 in steps of 0.10. The Q-13 comes with a companion set of color control patches but these are currently not supported. You can print out your own gray scale by printing out a simulated Q-13 image (see Transformations/Special Effects/Test Patterns) using black and white inks only.

For best results, the test target image should be created in exactly the manner as the images you want to correct, including lighting, camera settings and post-processing.

Using the gray patches from either target, Picture Window can create a set of color curves that cause each of the patches to be translated to a neutral gray of the correct brightness. If you use a ColorChecker, the additional 18 colored patches can also be used to perform selective color corrections to fine-tune the resulting color match. If the image of the reference target is so overexposed or underexposed that some of the gray patches are the same or nearly the same color (indicating loss of highlight or shadow detail), Picture Window may have trouble creating a usable set of curves, so it is important to expose the photograph of the target to keep the darkest and lightest patches from going pure black or pure white.

You can save and reload the target settings for later re-use with different input images. If you wish, you can also save the color curves and the selective color corrections as files you can use with the Color Curves and the Selective Correction transformations. If you save the selective correction settings, note that you must apply the color curves before applying the color correction to get the correct results.

This transformation can be used to convert color negatives (scanned as positives) to color positives. Simply photograph a ColorChecker using the color negative film of your choice and scan the negative using your flatbed or film scanner as a film positive. This will create a reference image you can then use to correct other negatives scanned in the same manner.

Regardless of the input image color profile, the color profile associated with the output image is always the working color space. This is different from most other transformations where the output image profile is inherited from the input image. The white point and primary colors of working color space provide the standard to which the reference image is matched.

Reference Image Type

This control lets you select the type of target you want to use (ColorChecker pre Nov-2104, ColorChecker post Nov-2014 or Q-13). If you select Q-13, some options are grayed out as this target cannot be used to make selective color corrections.

Reference Image

This control lets you select the image of the color target. Once you select a target image, Picture Window displays an overlay over the target image window showing where it expects to find each color patch. You can reposition the sample points as necessary in the target image by dragging the corners, sides, or interior of the rectangle displayed around all the patches. Initially this rectangle just covers the entire target image. You can drag each corner independently of the others to correct for any skew in the target image.

Black and White Density

These sliders let you compensate for the fact that the darkest achievable black ink on paper is never totally black and that paper is never totally white since even the blackest inks reflect a little light and the lightest papers absorb a little light. The darkest achievable black on a print usually has a density around 2.2 or 2.3 and may be lower for prints of lower quality. The lightest paper typically has a density of 0.02 or 0.03. If these deviations from ideal conditions are not taken into account when scanning images on a flatbed scanner, compression of the dynamic range will result. This in turn can cause weak blacks and impure whites in the corrected image giving an overall impression of poor contrast. Transparency film, on the other hand, has a much larger dynamic range which black densities of around 4.0 and negligible film density.

When using the Match Reference transformation to correct images scanned from a flatbed scanner, you should set the black density to somewhere between 1.9 and 2.3. Adjust the value as necessary to obtain good shadows—too low a value will cause a loss of shadow detail and too high a value will turn blacks to grays. Similarly, the white density should be adjusted to somewhere between 0.02 and 0.05 for the best compromise between loss of highlight detail and reduced highlight brightness. When working with transparencies, set the black density to 4.0 or the DMAX of your scanner, whichever is lower, and set the white density to 0.00.

The Match Reference transformation lets you calibrate your scanner for use as a densitometer. To do this, make sure the Black Density is set as high as it will go and the White Density is set to zero. Then use the transformation as usual to correct the input image. To read out density values, use the Readout Tool and select Den as the color space.

Status

A status line is displayed at the bottom of the dialog box to indicate whether or not a reference image has been selected. If an image has been selected, warning messages about problem patches may also be displayed here. A problem patch is one where the color over the sample area is not reasonably uniform. This normally indicates the sample point locations are misaligned with the target image – adjusting the sample point locations may eliminate the problems. If your reference image is very grainy, problem patches may also be reported, but you can ignore the warning and continue anyway.

Settings Menu

Save Curves

Selecting this option lets you save a set of color curves that will correct the neutral balance of the target image or any other image taken under the same conditions. This file can subsequently be used with the Color Curves transformation to correct images without further reference to the reference image.

Save Color Correction

Selecting this option lets you save a set of color corrections that will refine the color balance of the target image or any other image taken under the same conditions. This file can subsequently be used with the Color Correction transformation to correct images without further reference to the reference image. If you elect to do this, you must first apply the color curves (see above) before applying the color correction.

Apply Curves Only

Selecting this option suppresses the color correction process and limits the correction to applying curves to correct the neutral balance only. This causes Picture Window to compute corrections based on the gray patches of the ColorChecker only and ignore the 18 colored patches.

Apply Curves & Color Correction

Selecting this option enables the selective color correction process that is performed after correcting the neutral balance. Using this option, only the hue and saturation of the individual colors are adjusted – the brightness is left unchanged. This option is not available when using the Q-13 target as it has no color patches. Occasionally, this produces better results than using the Apply Curves & Color/Brightness Correction option (see below).

Apply Curves & Color/Brightness Correction

Selecting this option enables the selective color correction process that is performed after correcting the neutral balance. Using this option, the hue, saturation, and brightness of the individual colors are all adjusted. This option is not available when using the Q-13 target as it has no color patches.