Capturing still images in Raspberry Pi - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic command to capture a still image using the Raspberry Pi camera module?

The basic command is raspistill -o image.jpg, which captures an image and saves it as image.jpg.

beginner

What does the -o option specify in the raspistill command?

The -o option specifies the output filename where the captured image will be saved.

intermediate

How can you add a delay before capturing an image with raspistill?

Use the -t option followed by the delay time in milliseconds. For example, raspistill -t 5000 -o image.jpg waits 5 seconds before taking the picture.

beginner

What Python library can you use to capture still images programmatically on a Raspberry Pi?

You can use the picamera Python library to control the Raspberry Pi camera and capture images in your Python scripts.

beginner

How do you capture a still image using the picamera library in Python?

Example code:<br><pre>import picamera
with picamera.PiCamera() as camera:
    camera.capture('image.jpg')</pre>This captures and saves an image named <code>image.jpg</code>.

Which command captures a still image immediately and saves it as 'photo.jpg'?

A raspistill -t 5000 -o photo.jpg

B raspivid -o photo.jpg

C raspistill -o photo.jpg

D raspivid -t 1000 -o photo.jpg

raspistill is used for still images; the -o option sets the output file.

What does the -t option do in the raspistill command?

A Sets a delay before capturing the image

B Sets the image resolution

C Specifies the output filename

D Starts video recording

The -t option sets the delay time in milliseconds before the image is taken.

Which Python library is commonly used to control the Raspberry Pi camera?

A opencv

B picamera

C pygame

D matplotlib

picamera is designed specifically for Raspberry Pi camera control.

In Python, which method captures an image using picamera?

A camera.record()

B camera.snap()

C camera.take_photo()

D camera.capture()

The capture() method takes a still image and saves it.

What file format does raspistill save images in by default?

A JPEG

B BMP

C PNG

D GIF

raspistill saves images in JPEG format by default.

Explain how to capture a still image on a Raspberry Pi using both the command line and Python.

Think about the command line tool and the Python library.

Describe the steps to add a delay before taking a picture with the Raspberry Pi camera.

The delay is set in milliseconds using a command option.

Practice

1. What is the main purpose of the capture() method in the PiCamera library when capturing still images?

easy

A. To take a photo and save it to a file

B. To start the camera preview on the screen

C. To stop the camera and release resources

D. To adjust the camera's brightness settings

Solution

1. Step 1: Understand the role of capture()

   The capture() method is used to take a still photo with the Raspberry Pi camera.

2. Step 2: Identify what capture() does with the photo

   It saves the captured image to a file path you specify, storing the photo on disk.

3. Final Answer:

   To take a photo and save it to a file -> Option A

4. Quick Check:

   capture() saves image = C [OK]

Hint: Remember: capture() means take and save a photo [OK]

Common Mistakes:

• Confusing capture() with start_preview()
• Thinking capture() only shows image without saving
• Mixing capture() with camera stop methods

2. Which of the following is the correct syntax to start the camera preview using PiCamera in Python?

easy

A. camera.startPreview()

B. camera.preview.start()

C. start.camera_preview()

D. camera.start_preview()

Solution

1. Step 1: Recall the PiCamera method to start preview

   The correct method to start the camera preview is start_preview() called on the camera object.

2. Step 2: Check method name and syntax

   The method is lowercase with underscore: camera.start_preview(). Other options have wrong casing or order.

3. Final Answer:

   camera.start_preview() -> Option D

4. Quick Check:

   Correct method name = A [OK]

Hint: Use underscores and lowercase for PiCamera methods [OK]

Common Mistakes:

• Using camelCase instead of snake_case
• Reversing method and object order
• Adding extra dots or missing underscores

3. What will be the output of this code snippet when run on a Raspberry Pi with a camera connected?

from picamera import PiCamera
from time import sleep
camera = PiCamera()
camera.start_preview()
sleep(2)
camera.capture('/home/pi/image.jpg')
camera.stop_preview()

medium

A. The program throws an error because sleep is too short

B. The camera preview shows for 2 seconds, then saves image.jpg

C. No image is saved because stop_preview() is called too early

D. The preview never starts because start_preview() is missing

Solution

1. Step 1: Analyze the preview and sleep timing

   The preview starts, then the program waits 2 seconds allowing the camera to adjust.

2. Step 2: Capture and save the image after preview

   After waiting, the image is captured and saved to '/home/pi/image.jpg', then preview stops.

3. Final Answer:

   The camera preview shows for 2 seconds, then saves image.jpg -> Option B

4. Quick Check:

   Preview + sleep + capture = A [OK]

Hint: Preview then sleep before capture ensures good image [OK]

Common Mistakes:

• Assuming sleep is too short causes error
• Thinking stop_preview() prevents saving
• Believing start_preview() is missing

4. Identify the error in this code snippet that tries to capture an image:

from picamera import PiCamera
camera = PiCamera()
camera.capture('photo.jpg')
camera.start_preview()

medium

A. The preview should start before capturing the image

B. The capture method requires a full file path, not just filename

C. PiCamera must be imported after starting preview

D. The camera object is not created correctly

Solution

1. Step 1: Check the order of preview and capture

   The preview should start before capturing to allow camera to adjust and focus.

2. Step 2: Identify the mistake in code order

   Here, capture is called before start_preview(), which can cause poor image quality or errors.

3. Final Answer:

   The preview should start before capturing the image -> Option A

4. Quick Check:

   Preview before capture = B [OK]

Hint: Always start preview before capture for proper image [OK]

Common Mistakes:

• Calling capture before preview starts
• Assuming filename alone is invalid path
• Thinking import order affects capture

5. You want to capture multiple images with unique filenames using PiCamera in a loop. Which code snippet correctly saves images as 'image1.jpg', 'image2.jpg', and 'image3.jpg'?

hard

A. for i in range(1,4):
camera.capture('image{}.jpg'.format(i+1))

B. for i in range(3):
camera.capture('image{i+1}.jpg')

C. for i in range(1,4):
camera.capture(f'image{i}.jpg')

D. for i in range(1,4):
camera.capture('image' + i + '.jpg')

Solution

1. Step 1: Understand how to create unique filenames in a loop

   Using an f-string with f'image{i}.jpg' correctly inserts the loop variable into the filename.

2. Step 2: Check each option for correct syntax and output

   for i in range(1,4): camera.capture(f'image{i}.jpg') uses f-string with range(1,4) producing 'image1.jpg', 'image2.jpg', 'image3.jpg'. for i in range(3): camera.capture('image{i+1}.jpg') uses single quotes without f-string, so filename is literal. for i in range(1,4): camera.capture('image' + i + '.jpg') tries to add string and int without conversion, causing error. for i in range(1,4): camera.capture('image{}.jpg'.format(i+1)) adds 1 to i incorrectly, producing 'image2.jpg', 'image3.jpg', 'image4.jpg'.

3. Final Answer:

   for i in range(1,4): camera.capture(f'image{i}.jpg') -> Option C

4. Quick Check:

   f-string with correct range = D [OK]

Hint: Use f-strings and range starting at 1 for numbered files [OK]

Common Mistakes:

• Using string concatenation without converting int to str
• Forgetting f-string syntax for variable insertion
• Incorrect range causing wrong filenames