Crack generation and elimination
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Cracks are stained interference patterns caused by overlapping data grids. Cracks are usually produced in three stages of image processing.
1. Scan highly detailed textures: For example, when scanning an image on a textured fabric, a moiré is created, which is caused by the scanner itself. The scanner is not sensitive enough to the details read, and the resulting moiré is present in the scanned image and can hardly be removed.
2. Output incorrect screen angle: When a multi-color image is output at the wrong screen angle, a moiré appears on the image printed on the press. In addition, if the screen angle is not used, the crack will inevitably occur.
3. Scan pre-screened images: When scanning an already printed image, the continuous tone information on the photo or transparencies is not scanned, but the mesh is adjusted. The scanner partially recognizes the space between the dots, but cannot copy it as it is or convert it to a continuous tone. As a result, the image contains moiré. In this case several techniques can be used to reduce or eliminate moiré.
In the following, according to the degree of damage to the overall quality of the image, several methods for eliminating cracks are introduced in turn.
1. Reduce the crack when scanning the image of the mesh
When scanning black and white mesh images, the following steps usually eliminate the space between the dots.
1. Scan the image at a resolution that is twice the normal resolution and then subtract 10 pixels. For example, if the mesh image is scanned at a resolution of 300 dpi, it is now scanned at 590 dpi.
2. In Photoshop, select Filter\Blur\Gaussian Blur (Filter\Blur\Gaussian Blur). This filter is damaging to the image, so only lower values should be used, and pixels with a radius of 0.7 to 1.2 should be sufficient.
3. Select Image\Image Size.
4. In the Image Size dialog, select the “Constrain Proportions” and “Resample Image” options to change the resolution without affecting the width and height, then set the interpolation to "Bicubic" (secondary cube).
5. Enter 300 in the Resolution field.
6. Click "OK" to halve the resolution to remove 3/4 of the image pixels. Usually this will also remove most of the pattern.
7. Evaluate the image. If the dot pattern is sufficiently reduced, continue with other editing work. If not, go to the next step.
8. Re-evaluate the image. If there is still a small amount of texture, apply Filter\Noise\Despeckle (Filter\Mixed\Despeckle).
Second, eliminate the crack by rotating
When scanning a pre-screened color image, the moiré appears not only in one channel (black) but in one or more color channels. Too much adjustment of the entire image can have an irreversible effect on the information in the channel that does not contain the pattern at the beginning.
The image is not placed at right angles to the scanning platform during scanning, but at a different angle. This can force the scanner to misinterpret the more obvious component colors, such as cyan and black, and when rotating the image in Photoshop, blur the edge of the dot shape during the interpolation process, resulting in a more continuous effect. It is usually scanned at 15°, 30° or 45°.
After scanning the image, there are two ways to rotate the scanned image. First, select Image\Rotate Canvas\Arbitrary, enter the angle at which the image is to be rotated, and then crop the extra canvas that is produced after the entire image is rotated. Second, rotate the image while trimming the image.
Third, use a fuzzy method to eliminate cracks
The basic steps:
1. Scan the image at twice the resolution of normal resolution minus 10 pixels. Thus, when the first round of the blur filter is applied, the image contains more information than the standard scan, with less damage to important details. Also, when the resolution of the image that has been blurred is reduced later, more conflicting dot structures may be eliminated.
2. Blur the image. When you open a scan in Photoshop, use a Gaussian blur filter to slightly distort the dot shape. Generally close to 0.7 pixels can do very well.
3. Verify the color channel. In many cases, the crack affects only one or two channels, and click on the color channel in the Channels panel to check it. After finding the problematic color channel, apply a Gaussian blur or speckle filter again using the above method to find the abnormal dot values and average them.
4. Reduce the resolution. After blurring the image, reduce the image resolution to the desired value. Select Image\Image Size to select the Constrain Proportions and Resample Image options and enter a new value in the Resolution field. When Photoshop interpolates at the new resolution, each pixel value must be recalculated, which further removes the number of moiré. If there is still a slight pattern, check the color channel and do another Gaussian blur with a small value (0.3 to 0.5 pixels).
5. Sharpen the image. When applying a virtual mask to a de-networking filter, it is necessary to strike a balance between emphasizing the details of the image and exaggerating cracks that are not easily removed. When previewing the effect of the filter, set the number value to before the texture begins to protrude. a value.