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Plug-ins; description and function

In this and following chapter we'll take a walk down the Filters menu, and see what kind of goodies are hidden there. This time it will be Image plug-ins

Image plug-ins

Alien Map

This filter applies trigonometrical functions on each of your RGB channels. Let's take the "Red channel" and see what it we can do with it. First let Green and Blue have "None" (linear) check. If you now check "Cosine", the image will get a lot redder. Why? As you can see in Fig X, cos will boost the "low" red values, that's what makes the difference, because even the tiniest little redness in a pixel will get a lot redder, the parts that already are quite red will also be boosted, but the parts with middle value for red are subdued. The latter will not really be as noticed as the boost in low values, because you already saw that they were red. If you check "Sine" you will notice that it will get a lot less redder, that is because Sine subdues the "low" red values and boosts the already "high" red values. As with Cos it's a lot easier to detect when a low value is altered. If you haven't checked "None", you can also set the intensity of each channel.

Auto-Strech HSV

This filter makes an automatic contrast stretch of your image. It does this by finding the lowest and highest values of each HSV channel, and stretch this to the full contrast range. This is similar to Contrast Auto-stretch but Contrast Auto stretch works in RGB space. Auto-stretch HSV is a great filter to enhance old pictures with. If you test it on some old images, and it's not working, you can always try Equalize to see if that does the trick. The filter can also be used to do HSV stretching for other purpose than image enhancing, if that what you want. If you really want to know how it works, just bring up a histogram before and after using it.

Color exchange

This filter is a lot like Color Rotate. Well, it is much simpler, but it is also faster and slimmer. You define the color to exchange in the upper field by setting the slide for each RGB channel, and a threshold fuzziness like in Select by Color. A tip is to have a color dialog up so you can see what color you are choosing. In the lower field you select the color you want to exchange to.

Contrast Auto-Stretch

Works just like Auto-stretch HSV, but in the RGB space. This plug-in does a great job removing undesired color tints. As you may have noticed, this plug-in is also a wonderful image enhancer of bad or old images. It can also be good for old scanned images.

Convolution Matrix

Curtain:

With this filter you'll get the illusion that you are looking at your image through a thin curtain (as it happens, this curtain already has your image printed on its thin fabric!). What happens if you use this filter vertically, is that the image is copied. The copy is rotated 180 deg, thinly slashed and then combined with the original image. The same will happen if you choose "Horizontal" but from a horizontal perspective. You can also combine it so it will do both.

Deinterlace

This filter will fix images captured by video cards. Sometimes the even or odd field doesn't get captured correctly, then you can use this filter to fix it. Since we don't have a capture card we can't write so much about it. You can of course use it on non-captured images to get certain effects (see figure).

Despeckle

This is the filter to use if your image is noisy, or if you scanned a image and there was some dust in the image scan, or if you got a moire effect when you scanned a printed image from a magazine, or if your image has physical damage, like scratches.

If you have a noisy image or moire effects, use Despeckle over the whole image. If dust or scratches is your problem, select the damage with the free selection tool, and use Despeckle on that selection.

You have the following choices; "Only Radius", "Radius and Recursive", "Adaptive" or "Adaptive and Recursive". Radius is the window size from 1 (3x3 pixels) to 20 (41x41). What we mean is that the image will be chopped up in several windows, with a size of for example 3x3 pixels. In each of these windows the filter will try to smooth the color range, and thereby remove unwanted defects like scratches or noisy pixels.

If you use Only Radius, a general algoritm is used to smooth the color range in the window. If you set the radius high, the blur will also be high. If you use Radius and Recursive you can use a smaller radius to get the same effect as with a large radius and no recursive. Be careful though, Recursive can easily get you some unwanted blurring. If you use Adaptive filter the algoritm will try to calculate the best window size by itself, and use a general algoritm to smooth the color range in the window. If you use Adaptive and Recursive, a recursive algoritm will be used in the windows, calculated by the adaptive algoritm.

If you want to remove a scratch or other defect by selecting it and then despeckle it, then you can successfully use Recursive together with Radius to get rid of the scratch. If you want to remove some noise, then Adaptive or maybe a combination of Adaptive and Recursive is the way to go. If you used Recursive and/or Radius on the whole imag,e it will very easy get too blurred, this is of course of no importance when you have a tiny selection.

Destripe

It's mainly for if you have badly scanned images with stripes on them. You just have to experiment to get rid of the stripes in your original.

Engrave

Lets you make a etching out of your image. The slide More Height determines how deep your etching will be. I find it more realistic if you have low values of height (or more exactly 3). Limit line width is a limit in how many "shades of gray" you want, you will get more contrast if you don't check this button, the etching will also get more realistic, on the other hand if you check it, the outcome can be quite artistic.

Gradient Map

This is a really cool filter. It lets you map the image against the active gradient in the gradient editor. The lightest pixel in the image will get the color which is to the right in the gradient editor, and the darkest pixel in the image will get the color to the left (not transparant areas though) in the gradient editor.

You can see it as mapping the luminosity of your image against the gradient in the gradient editor. Say, that in your image the luminosity stretches from 50 (dark) to 235 (light). Then pixels with value 50, will get the left end color, the pixel with a luminosity of 51 will get the next color to the left in the gradient editor, and so on until you reach the value 235, which will get the right end color. Here are some examples to make you understand it better: 1. A grayscale image in RGB mode and Cold_Steel as gradient. 2. The same with the gradient Blurring_Transparity. 3. And now the same image in grayscale mode mapped against Cold_Steel and Blurring_Transparity.

Hot

This filter will identify pixels which can be hard or troublesome to show on a NTSC or PAL monitor. You can do this on the original image, or as a new layer on top of your image if you just check the "Create New Layer" check box. You can reduce the pixels Luminance or Saturation or simply make them black. It's generally good to use this filter when you publish your images on the web, but there is one drawback; you can never know the monitor of the surfing person.

Max RGB

This plug-in detects the RGB channel with the highest or lowest value and changes every pixel to either Red, Green or Blue - with maximum value!. If you have checked "Hold the maximal channels" you choose to select by maximal value. If there was Cyan, Magenta or Yellow in your image, those colors will remain even after the filter has been applied, because those colors already have maximal values for two RGB colors (the filter doesn't know which of those to choose). If you check "Hold the minimal channels" you choose to select by minimum value. With this button checked, Cyan will not show, because the filter will choose the lowest value, which is 0 for Red. Black and White will remain in both cases, because they have no value which is lower or higher than another.

NL Filter

NL stands for non liner, and this filter uses a 7 hexagon pixel block which you can adjust with the radius slider instead of e.g. a fixed 3x3 pixel block to do it's filter algoritms on. There are three filters in this plug-in: "Alpha trimmed mean", Optimal estimation smoothing" and "Edge enhancement".

"Alpha trimmed mean" is the filter to use when removing "pop" and single noise spots in the image ( if you set alpha to 0.5). Radius is the strength of the filter, and Alpha determines whether the filter will just smooth or reduce noise. Recommended values are from 0.0 to 0.5, if you use values over 0.5 funny thing will happen (can be quite artistic).

"Optimal estimation smoothing" This filter is a bit different from the first one and can with success be used to reduce dithering noise. With low Alpha values the smoothing will be subtle and with high (1.0) it will smooth all parts of the image. Radius should be from 0.8 to 1.0 for the filter to work correctly.

"Edge enhancement" is the opposite of the latter filter. It enhance edges. Low Alpha values will do little enhancement while high will enhance a lot. Radius stands for the effectiveness of the filter, and useful values range between 0.5 and 0.9.

Normalize

This is a filter similar to Contrast auto-stretch, but it won't allow the RGB channels to stretch independently. Instead they are treated as a union. Technically, all channels will not stretch over the whole range from 0 to 255. This is a filter you will use a lot if you are dealing with scanned images and other image enhancements. To get the whole picture, also check out Contrast autostretch and Auto-stretch HSV.

Pixelize

It will simply make your image look as if it was made of big pixels (like on TV when they don't want to show a certain person's face etc.). You have to set the new pixel size, e.g. "3" will make one pixel out of 3x3 pixels in the original image.

Sharpen

This filter will sharpen your image. You can set the amount of sharpness, and you can get a preview. This is a truly amazing tool, and even if it's simple, it is one of the more useful tools in Gimp when it comes to enhancing bad photos.

Scatter HSV

Threshold Alpha

This filter lets you remove pixels due to thier alpha value (transparity) the slide sets tha valus that you don't want to keep after the filter is applied. If you set a high value only pixels with a alpha value over that ( more solid) will be there after you run the filter.

Universal

Value Invert

This filter inverts the image in HSV space. It will not alter Hue and Saturation. It inverts the brightness value without changing the basic color in your image. This can be quite interesting if you are using Curves to change a certain value dramatically, and you get stuck with the wrong (inverted) color. Try it yourself and see what you can use it for.