(Precision RGB Color Adjustment)
In a workflow that includes color management, we can precisely control the color from the capture of the image to the digital proofing process; however, if the original itself is somewhat color cast, it needs to be corrected in Adobe Photoshop. This article presents two examples demonstrating accurate color correction techniques. The advantage of editing images in the RGB color space is that neutral reds are obtained by mixing equal amounts of red, green and blue primary colors. We can see how the RGB color space works in ColorMatch and Adobe RGB.
Adjustment of curves with the Color sampler tool In Photoshop 5.0 and later, color sampling tools can be used in combination with the Info panel to obtain accurate, measurable results in RGB images. Color correction. First, click the Window menu and select “Show Option†from the drop-down menu. The “Option†panel strip will pop up. Select the sampling area from the Point Sample drop-down list, such as 3×3 or 5×5 pixels. . Point Sample is usually not useful because it only calculates the value of a pixel. The area of ​​a pixel is too small. The resulting color value is not accurate. The color value we want is not the value of the scanned pixel. , but really representative sampling color values.
Open a .jpg image and select the "Color Sampler Eyedropper" tool from the toolbox. Click on the image and select four color samples from the highlight, midtone, and shadow sections. The selected color point is surrounded by a small bullseye, and each point appears on the Info panel. The color value. We can calculate the adjustment curve based on these values ​​and use this curve to correct the color cast of the image.
Click on Image and select the Curves command under Adjust from the drop-down menu. This will bring up the dialog for the dialog box. To place a point on the curve, just click anywhere on the curve. You can click the mouse; if you want to add a point that you want to base on a specific tone on the image, you can click the cmd (Mac) or ctrl (PC) key while clicking on a point in the image. The mark changes to a crosshair and the point is added to the corresponding position of the curve. In this example, the four color sample points previously taken should be added to each color channel curve.
This process is not as difficult as described. The cursor and the sample cross are coincident. Press the cmd (Mac) or ctrl (PC) key and click the mouse to see the RGB curve. The corresponding point.
Now let's analyze the sample readings on the Info panel to see how the image's color cast is corrected. Assume that the image we choose is a neutral gray image. The color values ​​of the RGB primary colors in the highlight, midtone, and shadow regions of the image should be substantially equal.
Let's start with the midpoint sample 2 and hope that the red and blue color values ​​match the green color values. Remember, we have added the corresponding sample points in each channel, open the “Channel†pull-down menu at the top of the curve dialog box and select each channel individually. Click on the spot on the image to see which channel on the selected point the point is surrounded by a circle. Select a point with the mouse to modify it, there are two ways to modify, or click and drag with the mouse, or use the arrow keys on the keyboard to move the point on the curve up and down. You can also enter new values ​​in the "Output" field in the lower part of the dialog box so that the three color values ​​are essentially equal. Each channel is operated in this way, and each sample point is also operated in the same way, so that the color cast correction can be completed.
Load this curve's settings into the Load Curve dialog box. This curve can then be used in the same application.
Hue/Saturation Next we describe a method for locating a color in an image and changing its hue. This method does not require complex selection. Open a picture with a yellow pattern, use the Hue/Saturation tool to change the yellow of the pattern, select Adjust from the Image drop-down menu, and then select the Hue/Saturation command to open Hue ( Hue) / Saturation dialog box, in this dialog box we can determine a particular range of colors, from the Edit drop-down menu in the upper part of the dialog box, select the yellow, then at the bottom of the dialog two Two sliders appear between the color bands, the upper color bar does not change, representing the hue of the input color; the lower color bar represents the hue of the output color and shows the relationship with the input color. We can drag the slider to a new position and change the color range. The color range is affected by other parameters in the dialog box. In addition to the Master mode, other color modes can be directly sampled in the image, and the slider moves accordingly to the selected color.
Use the Hue/Saturation filter to locate a narrower color range, that is, move the two sliders to very close positions. Changing these values ​​changes the value of the corresponding color in the image accordingly.