Changing overlays

worksheet2.md

@@ -24,9 +24,9 @@ In this resource you will make a tweeting touch screen photo booth using a Raspb
 
 1. We will need the `gpiozero` library. At the start of your Python file add an import statement:
 
-```python
-from gpiozero import Button
-```
+  ```python
+  from gpiozero import Button
+  ```
 
 1. Next we will set up the buttons. On the [previous worksheet](worksheet.md) we wired our buttons to pins 23 and 25. Let's go ahead and set both buttons up.
 
@@ -64,7 +64,7 @@ from gpiozero import Button
   from picamera import PiCamera
   ```
 
-1. Locate the line `take_pic_btn.when_pressed = take_picture` and below it add the following code to set up the camera object:
+1. Locate the existing line `take_pic_btn.when_pressed = take_picture` and below it add the following code to set up the camera object:
 
   ```python
   camera = PiCamera()
@@ -73,12 +73,12 @@ from gpiozero import Button
   camera.start_preview(alpha=128)
   ```
 
-  This code creates a camera object with the resolution set to 800x480 which is the resolution of the Raspberry Pi touchscreen. We also tell the camera to flip the preview horizontally (`hflip`) because otherwise it will be mirrored and this makes it hard for people to align themselves with the overlays! We then start the preview with alpha set to `128` so that it is semi-transparent in case we get an error and need to see what is happening underneath. When you are confident your code works you can remove the `alpha=128` to make the preview fully opaque.
+  This code creates a PiCamera object with the resolution set to 800x480 which is the resolution of the Raspberry Pi touchscreen. We also tell the camera to flip the preview horizontally (`hflip`) because otherwise the preview image will be mirrored and this makes it hard for people to align themselves with the overlays! We then start the preview with alpha set to `128` so that it is semi-transparent in case we get an error and need to see what is happening underneath. When you are confident your code works you can remove the `alpha=128` to make the preview fully opaque.
 
 
 ## Take a picture when the button is pressed
 
-1. Since we will probably take lots of pictures with the All Seeing Pi, we need filenames containing the date and time they were taken to avoid overwriting pictures each time a new one is taken. To do this, we will need the `gmtime` and `strftime` functions from the time library, so import them with this line of code:
+1. Since we will probably take lots of pictures with the All Seeing Pi, we will put the date and time the picture was taken within the filename to avoid a picture being overwritten each time a new one is taken. To do this, we will need the `gmtime` and `strftime` functions from the time library, so add this line with the other `import` statements:
 
   ```python
   from time import gmtime, strftime
@@ -90,13 +90,14 @@ from gpiozero import Button
   output = strftime("/home/pi/allseeingpi/image-%d-%m %H:%M.png", gmtime())
   ```
 
-  This will create a variable called `output` which contains the location of where we will save our picture file, including the filename. The `%d`, `%m` (etc) characters are how we specify the time format - `%d` means the day and `%m` means the month, for example. If you would like the date format in your filename to be different, there is a full [strftime reference](https://docs.python.org/2/library/time.html#time.strftime) available. The current date and time itself comes from the function `gmtime()`.
+  This will create a variable called `output` which contains the location and filename of where the captured photo will be saved. The `%d`, `%m` (etc) characters are how we specify the time format - `%d` means the day and `%m` means the month, for example. If you would like the date format in your filename to be different, there is a full [strftime reference](https://docs.python.org/2/library/time.html#time.strftime) available. The current date and time is provided by calling the function `gmtime()`.
 
 1. Now we will add some proper code to the `take_picture()` function, so that it actually takes a picture instead of just printing a message. Locate the line `def take_picture()`. Delete the line `print("Take a picture")` and in its place, add the following lines, making sure they are indented:
 
   ```python
-  camera.capture(output)
-  camera.stop_preview()
+  def take_picture():
+    camera.capture(output)
+    camera.stop_preview()
   ```
 
   This code captures a picture, saving it to the location we just defined in the variable `output`. It then stops the camera preview.
@@ -113,7 +114,7 @@ from gpiozero import Button
 
 1. Create a subfolder within your `allseeingpi` folder called `overlays` and place your overlay images inside it.
 
-1. We will need some functions to be able to work with our overlays. If you would like to use our [pre-written overlay functions](code/overlay_functions.py), download a copy of the file and save it as `overlay_functions.py` in your `allseeingpi` directory. If you would like to see a full explanation of what these functions do, head to the [overlay functions explanation page](worksheet3.md) to find out how they work.
+1. We will need some functions to be able to work with our overlays. If you would like to use our [pre-written overlay functions](code/overlay_functions.py), download a copy of the file and save it as `overlay_functions.py` in your `allseeingpi` directory. If you would like to see a full explanation of what these functions do, or you would prefer to write them yourself, head to the [overlay functions explanation page](worksheet3.md) to find out how to do this.
 
 1. Next to the other `import` statements in your program, add another one to import this file:
 
@@ -125,6 +126,18 @@ from gpiozero import Button
 
 ## Change overlays with a button
 
+1. The other button you wired up to your All Seeing Pi (called `next_overlay_btn`) will be the one we use to switch between the various overlays. Locate the `def next_overlay():` line and delete the line `print ("Next overlay")`. In its place, add the following code, making sure the lines are indented:
+
+  ```python
+  overlay = next(all_overlays)
+  preview_overlay(camera, overlay)
+  ```
+
+  The first line gets the *next overlay* from the list of `all_overlays` which is defined within the `overlay_functions.py` file. Then, the function `preview_overlay()` is called to add the overlay by giving it both the camera object and the overlay we want.
+
+1. Save your program and run it by pressing `F5`. Check that when you press the button to change between overlays, the overlays change. (Ensure you have at least one overlay image in your overlays folder!)
+
+
 ## Save overlay on your picture
 
 ## Tweet picture