1. June 14, ‘99
COLORS IN
MATLAB

2. UNDERSTANDING COLOR
To understand color we must first understand color spaces
We come in contact with color through:
Video(computers,TV’s)
Printed matter(magazines, brochures)
Real World

3. ADDITIVE COLOR MODEL
Formation of colors on video monitors is through an additive process of 3 color components
Red
Green
Blue
Concern is white balance, meaning how white is the white displayed on the monitor
Color=w1*red+w2*green+w3*blue

4. RGB COLOR CUBE Cyan Green White 1 Yellow Color is represented
by a point in
the RGB cube
Blue 1 1
Red
Magenta

5. RGB CUBE

6. SUBTRACTIVE COLOR MODEL
Color formation in printing industry is through the subtractive CMY model
Cyan
Magenta
Yellow
Challenge is black balance;to get real black
Often black is added as the 4th color, CMYK

7. HUMAN COLOR MODEL
The most accurate model here is hue, saturation and value(brightness) HSV
white
brightness
saturation
green
red
blue
hue
black

8. NET REFERENCE ON COLOR
There is a comprehensive FAQ on color. You can reach it at
colour.html

9. UNDERSTANDING COLORMAPS
At the heart of color control in MATLAB is the concept of colormap
Colormap is a 3-column matrix with entries between 0 and 1. Each row defines a particular combination of (R,G,B) to define a color.
black 1 1
magenta

10. FEW MATLAB’s COLORMAPS
Image intensity maps to color

11. Try it! MATLAB comes with few images. To load one of them type
load clown
Now check your workspace. You will see image data stored in X and colormap stored in map
To display the image type
image(X)
What’s wrong?

12. Digital image Before going on, let’s define a digital image.
A digital image is simply a 2D matrix. Each element of the matrix, pixel, is an integer. For a black and white image pixels are between 0 (black) and 255(white)
MATLAB’s matrix oriented structure makes minced meat out of images and image processing

13. Role of colormap
Type out a few values of X. You will see integers. But what is the relationship between these numbers and their actual color?
Enter colormap. Colormap is lookup table. It takes an intensity value, say 25, and connects it with some color.
To display true colors, you must set monitor’s colormap to the map used at the time the image was created

14. WORKING WITH COLORMAPS
Command colormap(map) sets the colormap to one of preset or user defined values
For clown image ,type colormap(map) and watch the result.
Now try these
colormap(hot)
colormap(hsv)
colormap(gray)

15. RECALLING AND SETTING COLORMAPS
Current colormap in effect can be found by
map=colormap
Any (3x1) matrix can be defined by the user and defined as the current colormap by
colormap(usermap)

16. COLOR ASSIGNMENT How is intensity connected to color? last row highest
Colormap
Data Values
maps to
lowest
first row

17. COLOR MAPPING RULE
Let data range between Zmin and Zmax. Let the colormap rows k run from 1 to m.
Then
Zmin k=1, row 1
Zmax k=m, row m
The middle Z’s follow a linear mapping rule.

18. Defining your first colormap
Plot the peaks function using
surf(peaks)
Define a colormap, mymap, so that the first row is blue, the second row is green, third row is white and fourth row red.
Then set MATLAB’s colormap to mymap. See code next

19. Try it!
Interpret colors you see

20. ARBITRARY COLOR ASSIGNMENT
Color mapping rule is simple:
take the minimum of the function and assign it the color of the first row of colormap
take the maximum of the function and assign it the color of last row of colormap
in-between values take shades of color obtained by a linear mapping of the above two colors
Next slide shows you more

21. ASSIGNING COLORS TO RANGES OF DATA
This is what we want to do:
assign blue to z<a
assign red to z>b
assign a continuous color tone(blue to red) for a<z<b b Z a

22. How to do it? clim property
clim is a two element vector [cmin,cmax]. It can be set as follows
set(gca, ‘clim’,[cmin cmax])
Colormapping is done as follows
z<cmin ---->first row of colormap
z>cmax ---->last row of colormap

23. Try it! Plot the peaks function and display it with view(90,0)
Set cmin and cmax to [-2 2],[-3 3] and [-4 4] and see if you can explain the results?

24. MATLAB’S DEFAULT COLORMAP
MATLAB’s default colormap is HSV and can be set via
colormap(‘default’) or colormap(‘hsv’)
Unless specified otherwise, colormaps are 64x3, i.e. 64 colors
Other sizes can be specified, e.g.
colormap(hot(128))

25. Try it!
Plot the peaks function in the range(-4:0.2:4) and v=[90,0]. Go through hsv, hot, flag, copper etc. colormaps. Check map’s sizes ?

26. DEFINING YOUR OWN COLORMAP
Want to display the peaks profile in only two colors. Negative values in red and positive values blue. Define a colormap for this purpose.

27. Pseudorandom color :pcolor
Command pcolor(z) draws a rectangular grid of cells with colors determined by z; function height above each cell. Mapping from z to color is determined by colormap

28. OVERLAYING CONTOURS Do the following:
get rid of the grid lines(try shading command)
Superimpose black colored gridlines(lookup help on contour)

29. pcolor and colormap
The following will display 32 bins of current colormap
z=[1:32;1:32]’
pcolor(z)

30. WORKING WITH IMAGES
A two dimensional image can be modeled by z=f(x,y) where (x,y) is the pixel position and z the corresponding height
The height is then mapped to either a grayscale or color range

31. LOADING IMAGES Images are loaded like sound file. For example
load clown
Now check your workspace to see which variable is “clown” loaded in.
Check the size of the image array and its min and max

32. DISPLAYING IMAGES
There are a number of ways to display an image. For the start use
image(z)
z is the array your image is loaded in. Check
Each element of z specifies the color of a rectangular patch in the image
Elements of z are used as indices into the current colormap to determine the color

33. INDEXED IMAGES
MATLAB supports a number of image formats one of which is “indexed”
“intensities” in indexed images are not really intensities; they are simply entries into the colormap
If image value at (x,y)=32, this simply means that pixel color at (x,y) comes from the row 32 of the colormap

34. Example of indexed image: clown
When you loaded clown you got two arrays
X: image data
map:colormap
Now check image value at position (100,100). I get 4. What color is this pixel?. MATLAB goes to the 4th row of map
This row has 3 columns, red=0.29, green=0 and blue=0

35. How to display the color of a single pixel?
In the previous slide we found out that X(100,100) has no green or blue component and is 29%red. But what color is it?
Here is a way to do get it. Type
image(X(100,100))
Is it what you expect?. Is it reddish?

36. PLAYING WITH COLORMAPS
Unless a digital image is accompanied by a colormap, it is impossible to determine pixel colors
Check the colormap that accompanies the loaded image; its size and its contents.
Change the colormaps to hot, hsv, cool, gray and see the results

37. HOMEWORK
Plot z=sin(sqrt(x^2+y^2)) in the range(-pi to pi)in increments of 0.2
Make the function appear red for z<-0.5, white for -0.5<z<0.5 and blue for z>0.5
Do the same except for making the colors between red and white follow 32 gradations of colormap hot