CS3162 Introduction to Computer Graphics
Helena Wong, 2001
7. Image Processing
Image processing concerns about modifying or transforming images.
Applications may include enhancing an image or adding special effects to an
image. Here we will learn some of the image processing functions. The
description will be based on grey-scale images. However, extending the
techniques to color images is usually straight-forward.
Histogram Modifications
- A histogram is a plot of the frequency of occurrence, p(g), against the gray
level value, g.
- The distribution of gray scale values affects the quality of the image. The
following diagram shows some examples:
1
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
2
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
3
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
(a) If most of the gray level values in an image are small, the image will be
dark.
(b) If most of the gray level values in an image are large, the image will be
bright.
(c) If most of the gray level values in an image are within a small region,
the image will have a low contrast. Therefore, both (a) and (b) are also
considered as low contrast images.)
(d) A high contrast image has a wide range of gray level values.
- Modifying the histogram by changing the frequency of occurrence of each
gray scale value may improve the image quality and enhance the contrast.
4
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
- Brightness Correction is used to increase the brightness of a dark image as
follows:
Pixel(x,y) = Pixel(x,y) + brightness
Brightness correction is therefore equivalent to shifting the histogram
horizontally.
- Contrast Correction is used to modify the contrast of an image as follows:
Pixel(x,y) = contrast * (Pixel(x,y) – gmean) + gmean
Where gmean is the mean gray scale value of the original image.
Hence contrast correction is equivalent to scaling the histogram. We can
reduce the contrast of an image by setting the factor contrast in the above
equation to a value smaller than 1 and we can increase the contrast by
setting it to a value higher than 1.
5
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
Editing of Intensity / Color of an Image
- Very often, it may be desirable to be able to edit the gray scale values in a
linear as well as non-linear way.
- To do this, a curve is usually provided which indicates the relationship of
the original and new values.
- This curve can be edited interactively by moving the control points.
- For a color image composed of red,
green and blue channels, three curves
can be provided so that each of the
channels can be modified independently.
6
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
Texture Mapping
Since it is still very difficult for the computer to generate realistic textures, a
method called texture mapping is developed in which a photograph of real
texture is input into the computer and mapped onto the object surface to create
the texture for the object.
7
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
- Texture pattern is defined in an MxN array of texels or a texture map
indiced by (u,v) coordinates.
- For each pixel in the display:
- Map the 4 corners of pixel back to the object surface (for curved surfaces,
these 4 points define a surface patch)
- Map the surface patch onto the texture map (this mapping computes the
source area in the texture map)
- The pixel values is modified by weighted sum of the texels' color.
8
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
Warping
- Warping is the use of image mapping functions to produce geometric
distortions of images.
- The basic technique is that the user specifies the contour of the object in the
image to be transformed and the final contour that the object is to fit into.
- A texture mapping operation is performed to map the source object into the
destination shape.
9
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
Morphing
- Morphing is used to provide a smooth transition from one image to another
and thus create an illusion of image transformation.
- A simple morphing may use a fade or dissolve process that gradually
decreases the contribution of the source image to the output image while
increases the contribution of the destination image as follows:
10
CS3162 Introduction to Computer Graphics
Helena Wong, 2001
- Depending on applications, the decrement (or increment) in contribution of
the source (or destination) image into the final image may not have to be
linear as in the diagram.
- More advanced morphing techniques may use a warping process in addition
to the fade / dissolve to slowly bring into alignment the shapes of the start
and end pictures.
11