Question 1: Colour mixing
Computer graphics colours are created by mixing the three primary colours red, green and blue. For this assignment, we will be using the normalised RGB model for computer colours. In this context, every colour is given by specifying three scalars 0 ≤ α ≤ 1, 0 ≤ β ≤ 1 and 0 ≤ γ ≤ 1, where α encodes the brightness of the red light, β the brighness of the green light, and γ the brightness of the blue light used to mix your colour. We will be working with the colours turquoise, dark yellow, brown, dark blue, light blue and purple, obtained by mixing the primary following proportions of the primary colours:
Turquoise is obtained by mixing 0.1 red, 0.5 green, and 0.5 blue.
Dark Yellow is obtained by mixing 1 red and 0.8 green.
Brown is obtained by mixing 0.5 red and 0.1 green.
Dark blue is obtained by mixing 0.2 red and 0.7 blue
Light blue is obtained by mixing 0.2 red, 0.5 green, and 0.85 blue
Purple is obtained by mixing 0.415 red, 0.175 green, and 0.615 blue. When mixing colours, you are forming linear combination using positive scalars – you cannot subtract a colour. The outcome needs to again be a valid colour. If you wish to visualise your colour mixing in MATLAB, the code
x = [0 1 1 0]
y = [0 0 1 1]
a = [.2 .4 .2]
b = [0 .5 .3]
fill(x,y,a)
fill(x,y,b)
fill(x,y,(1/2)*a+b)
creates a square with the x- and y-coordinates of the four corners specified in x and y and fills it with the colour a, respectively b or the specified linear combination of a and b. If you are interested to read more deeply about why our colour vision can be modelled by vectors, the Feynman Lecture on Colours is a great classic, see also the summary on our Canvas page.
For each of the following, decide whether it is possible or not. If it is possible, find the mixing proportions. If it is not possible, explain the reason why. Present your computation and then write a sentence explaining what your computation shows.
(a) Is it possible to mix brown, dark blue and turquoise to obtain purple?