Image Manipulation

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Homework Assignment

One of the most useful applications for linear algebra in data science is image manipulation. We often need to compress, expand, warp, skew, etc. images. To do so, we left multiply a transformation matrix by each of the point vectors.
For this assignment, build the first letters for both your first and last name using point plots in R.

Then, write R code that will left multiply (%>%) a square matrix (x) against each of the vectors of points (y). Initially, that square matrix will be the Identity matrix.

Use a loop that changes the transformation matrix incrementally to demonstrate 1) shear, 2) scaling, 3) rotation , and 4) projection in animated fashion.

library(dplyr)
library(plotly)
library(gifski)

Build letter C

x=c(seq(-2,-1,length.out=500),rep(-2,1000), seq(-2,-1,length.out=500))
y=c(rep(-1,500),seq(-1,1,length.out=1000), rep(1,500))
plot(y ~ x, xlim = c(-3, 3), ylim = c(-3, 3))

Build letter A

x = c(rep(0, 500), seq(0, 1, length.out = 500), seq(0, 1, length.out = 500), rep(1, 500))
y = c(seq(-1, 1, length.out = 500), rep(0, 500),rep(1, 500), seq(-1, 1, length.out = 500))
plot(y~x, xlim=c(-3,3), ylim=c(-3,3))

Build letter G

x=c(seq(2,3,length.out=500),rep(2,1000), seq(2,3,length.out=500), seq(3,3, length.out = 500), seq(2.5, 3, length.out = 500))
y=c(rep(-1,500),seq(-1,1,length.out=1000), rep(1,500), seq(0,-1, length = 500), seq(0, 0, length.out = 500))
plot(y ~ x, xlim = c(-3, 3), ylim = c(-3, 3))

Plot CAG

x = c(seq(-2,-1,length.out=500),rep(-2,1000), seq(-2,-1,length.out=500), 
      rep(0, 500), seq(0, 1, length.out = 500), seq(0, 1, length.out = 500), rep(1, 500),
      seq(2,3,length.out=500),rep(2,1000), seq(2,3,length.out=500), seq(3,3, length.out = 500), seq(2.5, 3, length.out = 500))
y = c(rep(-1,500),seq(-1,1,length.out=1000), rep(1,500),
      seq(-1, 1, length.out = 500), rep(0, 500),rep(1, 500), seq(-1, 1, length.out = 500),
     rep(-1,500),seq(-1,1,length.out=1000), rep(1,500), seq(0,-1, length = 500), seq(0, 0, length.out = 500))
z = rbind(x, y)
plot(y~x, xlim = c(-3,3), ylim = c(-3,3))

Function to left multiply (%>%) a square matrix (x) against each of the vectors of points (y)

leftMultiply  <- function(x,y){
   x %*% y
}
leftMultiply(matrix(rep(seq(-2,3, length.out=6),3), nrow = 3, ncol = 6),diag(6))

##      [,1] [,2] [,3] [,4] [,5] [,6]
## [1,]   -2    1   -2    1   -2    1
## [2,]   -1    2   -1    2   -1    2
## [3,]    0    3    0    3    0    3

Shear

for (i in seq(0,1,length.out=20)) {
  z1<-apply(z,2,function(x) leftMultiply(x,matrix(c(1,i,0,1),nrow=2,ncol=2)))
   plot(z1[2,]~z1[1,], xlim=c(-3,3), ylim=c(-3,3))
}

Scaling

for (i in seq(1,3,length.out=20)) {
  z1<-apply(z,2,function(x) leftMultiply(x,matrix(c(i,0,0,i),nrow=2,ncol=2)))
   plot(z1[2,]~z1[1,], xlim=c(-3,3), ylim=c(-3,3))
}

Rotation

for (i in seq(0,pi*2,length.out=20)) {
  z1<-apply(z,2,function(x) leftMultiply(x,matrix(c(cos(i),-sin(i),sin(i),cos(i)),nrow=2,ncol=2)))
   plot(z1[2,]~z1[1,], xlim=c(-3,3), ylim=c(-3,3))
}

Projection

for (i in seq(0,2*pi,length.out=20)) {
  tempZ<-rbind(z,rep(0,ncol(z)))
  z1<-apply(tempZ,2,function(x) leftMultiply(x,matrix(c(1,0,0,0,cos(i),-sin(i),0,sin(i),cos(i)),nrow=3,ncol=3)))
   plot(z1[2,]~z1[1,], xlim=c(-3,3), ylim=c(-3,3))
}

Source: RPubs