Conversion of a Color Image to a Binary Image

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Conversion of a Color Image to a Binary Image - Article by asif
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Date: 4/18/2005 7:51:11 PM

Conversion of a Color Image to a Binary Image

What is a Binary Image?

   Binary images are images whose pixels have only two possible intensity values. They are normally displayed as black and white. Numerically, the two values are often 0 for black, and either 1 or 255 for white.

   Binary images are often produced by thresholding a grayscale or color image, in order to separate an object in the image from the background. The color of the object (usually white) is referred to as the foreground color. The rest (usually black) is referred to as the background color. However, depending on the image which is to be thresholded, this polarity might be inverted, in which case the object is displayed with 0 and the background is with a non-zero value.

   Some morphological operators assume a certain polarity of the binary input image so that if we process an image with inverse polarity the operator will have the opposite effect. For example, if we apply a closing operator to a black text on white background, the text will be opened.

What are Pixels?

   In order for any digital computer processing to be carried out on an image, it must first be stored within the computer in a suitable form that can be manipulated by a computer program. The most practical way of doing this is to divide the image up into a collection of discrete (and usually small) cells, which are known as pixels. Most commonly, the image is divided up into a rectangular grid of pixels, so that each pixel is itself a small rectangle. Once this has been done, each pixel is given a pixel value that represents the color of that pixel. It is assumed that the whole pixel is the same color, and so any color variation that did exist within the area of the pixel before the image was discretized is lost. However, if the area of each pixel is very small, then the discrete nature of the image is often not visible to the human eye.

   Other pixel shapes and formations can be used, most notably the hexagonal grid, in which each pixel is a small hexagon. This has some advantages in image processing, including the fact that pixel connectivity is less ambiguously defined than with a square grid, but hexagonal grids are not widely used. Part of the reason is that many image capture systems (e.g. most CCD cameras and scanners) intrinsically discretize the captured image into a rectangular grid in the first instance.

What are Intensity values?

   Each of the pixels that represents an image stored inside a computer has a pixel value which describes how bright that pixel is, and/or what color it should be. In the simplest case of binary images, the pixel value is a 1-bit number indicating either foreground or background. For a grayscale images, the pixel value is a single number that represents the brightness of the pixel. The most common pixel format is the byte image, where this number is stored as an 8-bit integer giving a range of possible values from 0 to 255. Typically zero is taken to be black, and 255 is taken to be white. Values in between make up the different shades of gray.

   To represent color images, separate red, green and blue components must be specified for each pixel (assuming an RGB colorspace), and so the pixel `value' is actually a vector of three numbers. Often the three different components are stored as three separate `grayscale' images known as color planes (one for each of red, green and blue), which have to be recombined when displaying or processing.

   Multi-spectral images can contain even more than three components for each pixel, and by extension these are stored in the same kind of way, as a vector pixel value, or as separate color planes.

   The actual grayscale or color component intensities for each pixel may not actually be stored explicitly. Often, all that is stored for each pixel is an index into a colormap in which the actual intensity or colors can be looked up.

   Although simple 8-bit integers or vectors of 8-bit integers are the most common sorts of pixel values used, some image formats support different types of value, for instance 32-bit signed integers or floating point values. Such values are extremely useful in image processing as they allow processing to be carried out on the image where the resulting pixel values are not necessarily 8-bit integers. If this approach is used then it is usually necessary to set up a colormap which relates particular ranges of pixel values to particular displayed colors.

What is meant by the Term RGB(RGB)?

   It is a useful fact that the huge variety of colors that can be perceived by humans can all be produced simply by adding together appropriate amounts of red, blue and green colors. These colors are known as the primary colors. Thus in most image processing applications, colors are represented by specifying separate intensity values for red, green and blue components. This representation is commonly referred to as RGB.

   The primary color phenomenon results from the fact that humans have three different sorts of color receptors in their retinas which are each most sensitive to different visible light wavelengths.

   The primary colors used in painting (red, yellow and blue) are different. When paints are mixed, the `addition' of a new color paint actually subtracts wavelengths from the reflected visible light.

C# Sample Program:

Guidelines for Use

To illustrate Conversion of Color Image to Grayscale image, we start with a simple image containing some distinct artificial objects(specifically text)

Now we apply Grayscale conversion to the image to convert it to Grayscale image.

Now we apply Binary conversion to the image to convert it to Binary image.

Attachments:

  Project Files: binary.zip

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