CS 40 - Lecture 16

Cover page image

Cay S. Horstmann

Lecture 15 Recap

Loop through pixels: for (Pixel pix : getPixels())
Try out program: Load image, call new method, explore result
Negative: Invert each color, value = 255 - value
Grayscale: average color values, 0.299 * red + 0.587 * green+ 0.114 * blue

Edge Detection

Edge Detection

This is what Stanley does to find where the road is
Simplified version: Only compare pixel with its neighbor below
If colors are very different, it's an edge => color black
If colos similar, not an edge => color white
How do we find the neighbor below?
How do we find whether two colors are very different?

Getting the Neighbor Below

Get x and y coordinate of pixel:
```
int x = pix.getX();
int y = pix.getY();
```
Get pixel below:
```
Pixel below = getPixel(x, y + 1);
```
Doesn't work if pixel is in the last row.
```
if (y < getHeight() - 1)
{

}
```

Testing Colors for Similarity

Get average of red/green/blue values
```
double pixAverage = pix.getAverage();
```
Do the same for the pixel below

Check if the two averages are < 10 apart

if (Math.abs(pixAverage - belowAverage) < 10)
   pix.setColor(...);
else
   pix.setColor(...);

To set a pixel to black, use pix.setColor(Color.BLACK);

Lab 1

Start Greenfoot
Make a subclass MyPicture6 of Picture
Add method edgeDetect
Implement method
Compile. Fix errors. Compile. Fix Errors....
Try it out with a picture of your choice.
Submit your solution at the end of class

Bored?

mona lisa picture

Why 10 as the threshold? What happens when you use 5 or 20?
Why the pixel below? Try comparing with the pixel to the right. Is the result different? Better?

Mirroring

Mirroring

Reflect image along horizonal or vertical line
Useful for fixing symmetric images
Potentially useful for fun effects
Vertical mirroring: Find out if pixel is to the right side of the mirror line
If so, replace its color with the matching pixel to the left
```
pix.setColor(mirrored.getColor());
```

Mirror Math

```
int mirrorLine = getWidth() / 2;
```

Pixel is to the right:

int x = pix.getX();
if (x > mirrorLine) 
{
   . . .
}

Get distance to the mirror line
```
int d = x - mirrorLine;
```

Go the same distance to the left

Pixel mirrored = getPixel(mirrorLine - d, y);

Lab 2

Start Greenfoot
Make a subclass MyPicture7 of Picture
Add method mirror
Implement method
Compile. Fix errors. Compile. Fix Errors....
Try it out with a couple of images: girl.jpg, temple.jpg
Submit your solution at the end of class

Ok, but...

mona lisa picture

What's wrong with the reflected temple?
Mirroring at width/2 doesn't seem to work here
At which coordinate should you mirror? Make a guess and try it. Or load the image into a paint program to find out for sure.
Ok, change your program and run it again. What happens?
How do you fix it?

Reminders

Upload your labs
Homework 6 is due tonight
Exam Thursday