1. Kyoungju Park kjpark@cau.ac.kr http://graphics.cau.ac.kr
Computer Graphics
Kyoungju Park

3. Image Processing 2D generalization of signal processing
Image as a two-dimensional signal
Point processing: modify pixels independently
Filtering: modify based on neighborhood
Compositing: combine several images
Image compression: space-efficient formats
Related topics (not in this lecture or this course)
Image enhancement and restoration
Computer vision

4. Outline Point Processing Filters Dithering Image Compositing
Image Compression

5. Point Processing f(v) = v identity; no change
f(v) = 1-v negate an image
(black to white, white to black)
f(v) = vp, p<1 brighten
f(v) = vp, p>1 darken
f(v) v
Processing
Examples -> Topics -> Image Processing->Brightness

6. PImage img;
void setup() {
size(200, 200);
frameRate(30);
img = loadImage("wires.jpg"); }
void draw() {
loadPixels();
for (int x = 0; x < img.width; x++) {
for (int y = 0; y < img.height; y++ ) {
int loc = x + y*img.width;
float r = red (img.pixels[loc]);
float maxdist = 50;
float d = dist(x,y,mouseX,mouseY);
float adjustbrightness
= 255*(maxdist-d)/maxdist;
r += adjustbrightness;
color c = color(r);
pixels[loc] = c; }
updatePixels();

7. Convolution filters
gaussian
box
tent

8. PImage img;
int w = 80;
float[][] matrix = { { -1, -1, -1 },
{ -1, 9, -1 },
{ -1, -1, -1 } };
void setup() {
size(200, 200);
frameRate(30);
img = loadImage("end.jpg"); }
void draw() {
image(img,0,0);
int matrixsize = 3;
loadPixels();
for (int x = width/4; x < 3*width/4; x++) {
for (int y = height/4; y < 3*height/4; y++ ) {
color c = convolution(x,y,matrix,matrixsize,img);
int loc = x + y*img.width;
pixels[loc] = c;
updatePixels();

9. color convolution(int x, int y, float[][] matrix,int matrixsize, PImage img){
float rtotal = 0.0;
float gtotal = 0.0;
float btotal = 0.0;
int offset = matrixsize / 2;
for (int i = 0; i < matrixsize; i++){
for (int j= 0; j < matrixsize; j++){
int loc = (x + i - offset) + img.width*(y + j - offset);
rtotal += (red(img.pixels[loc]) * matrix[i][j]);
gtotal += (green(img.pixels[loc]) * matrix[i][j]);
btotal += (blue(img.pixels[loc]) * matrix[i][j]); }
return color(rtotal,gtotal,btotal);

10. Blurring Filters
A simple blurring effect can be achieved with a 3x3 filter centered around a pixel,
More blurring is achieved with a wider nn filter:
Original Image
Blur 3x3 mask
Blur 7x7 mask

11. Image Filtering: Blurring
original, 64x64 pixels
3x3 blur
5x5 blur

12. Blurring Filters Average values of surrounding pixels
Can be used for anti-aliasing
What do we do at the edges and corners?
For noise reduction, use median, not average
Eliminates intensity spikes
Non-linear filter
Processing
Examples -> Topics -> Image Processing->Blur

13. Example: Noise Reduction
Image with noise
Median filter (5x5)

14. Example: Noise Reduction
Original image
Image with noise
Median filter (5x5)

15. Example of Edge Filter
Original image
Edge filter, then brightened

16. Image Filtering: Edge Detection

17. Dithering
Dithering takes advantage of the human eye's tendency to "mix"
two colors in close proximity to one another.

18. Dithering
Dithering takes advantage of the human eye's tendency to "mix"
two colors in close proximity to one another.
original
no dithering
with dithering
Colors = 224
Colors = 28
Colors = 28

19. Floyd-Steinberg Error Diffusion

20. Floyd-Steinberg Error Diffusion
Enhances edges
Retains high frequency
Some checkerboarding
From