| Effects Of Filters on Detail
Lets take a look at the effects of some common filters on the histogram. Before I do, I want to mention something about detail. Where do you get detail from? The detail in any image is contained in the subtle difference between adjacent pixels. If you have a row of white pixels with a row of black pixels beside them, you will see this as an edge. Most edges are not this clear cut, though. Most are more like a gradual change across a few pixels, with each being a little different from its neighbor. You have to be very careful with some of the filters that shift color, since they can remove the differences between these pixels, thereby destroying detail.
| Let's go back to the image I used in the color shift. Here is the original image with a histogram of luminosity.
I chose this image because it fills almost the whole range of the histogram (OK, and because I'd rather look at a woman than a tree). It is fairly even, as far as the color distribution is concerned. The first filter I am going to use is the contrast filter. The contrast filter changes the range of luminosity values of the pixels in your imag e.
Contrast Filters
Decreased Contrast |
In this image, I have reduced the contrast by 50. The image is washed out, faded. This is because reducing contrast tends to narrow the total range of luminosity in an image. Notice how all the information is now squeezed into the center of the graph.
This is typical of a low contrast image. If you would do this to your image and then save it, you would be losing some detail. Try this. Load any image into your paint program and reduce the contrast to 0. The whole image will go gray. That is because when you are reducing contrast, you are reducing the difference in luminosity value between pixels. Since detail is contained in those differences, you are thereby discarding detail.
Increased Contrast |
In this image I have increased contrast by 50. This will increase the range of luminosity values in your image. Notice how the mid-range has been suppressed, and the spikes at the high and low end. If you save this image, you are also discarding detail. Any subtle differences in the highlights and shadows have now been squeezed together. The effect of this is to lose detail in these areas.
There are two other filters that work about the same as this one, the equalize and stretch filters. These filters are essentially automatic contrast filters, and I have never found much use for them, nor do I recommend using them. If you are going to adjust your image, don't let the computer make your choices for you. It can not see the image, and can't control the effect.
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