← ACARA Digital Technologies Years 9–10

How video compression reuses unchanged parts of frames

Saves video space by storing only what changes between frames, reusing the still background from earlier frames and dropping fine detail viewers will not notice.

Part of: ACARA Digital Technologies Years 9–10 — Data Compression — Module 34

Understanding How video compression reuses unchanged parts of frames in ACARA Digital Technologies Years 9–10

What it does: Reusing the unchanged background means the repeated data is saved once, which is how video compression saves space. The next frame is built from the earlier frame plus the small set of changes, so far less data is stored. Video plays many frames every second (often 24 to 60). Storing every frame in full adds up to a massive file. When little changes between frames, there is more to reuse. Lots of movement means more changes to store, so the file stays bigger. Throwing away tiny details people will not notice is lossy compression. That detail cannot be restored later.

Common mistakes: A key frame is a complete picture of the first frame. Later frames only store changes, so line 1 should be SAVE full keyframe. To find what changed, each frame is compared with the previous frame, not a blank one. Line 3 should compare with previous frame.

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Learning
Duo
A video shows a person talking in front of a still wall. How does video compression handle the wall?
Stores the whole wall again in every frameStores the wall once and reuses it across frames
Matches
Every frame stores the full background again to save space
↓
Storing the full background in every frame wastes space. Compression stores the still background once and reuses it in later frames.
Applying
Meanings
Term
inter-frame compression
Meaning
Saves video space by storing only what changes between frames, reusing the still background from earlier frames and dropping fine detail viewers will not notice.
Practice
SAVE key_frame FROM frame 1
Testing
Gaps
IF pixel oldpixel THEN
Check Answer
Spots
1SAVE full keyframe
2SAVE only changes as keyframe

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Quick Questions

1. A video shows a person talking in front of a still wall. How does video compression handle the wall?
A. Stores the wall once and reuses it across frames ✓
B. Stores the whole wall again in every frame
Because the wall does not change, compression reuses it from an earlier frame instead of storing it again and again.
2. What does inter-frame compression mainly store for each new frame?
A. Only what has changed since the previous frame ✓
B. A complete new picture of every pixel
Inter-frame compression records the differences between frames. This saves a lot of space compared with storing every full frame.
3. What is a key frame?
A. A frame that is deleted to save space
B. A full frame stored in complete detail ✓
A key frame is a complete picture. Frames after it can then store just the changes from it.
4. A cat walks across a still lounge room. Which part needs new data in the next frame?
A. The area where the cat has moved ✓
B. The couch and walls that stayed still
Only the moving cat has changed. The still couch and walls can be reused from the earlier frame.
5. Why would uncompressed video files be huge?
A. Video only has a few frames, but each one is very large
B. Every frame is stored in full, and there are many frames each second ✓
Video plays many frames every second (often 24 to 60). Storing every frame in full adds up to a massive file.

Match the Pairs

inter-frame compression → Stores only what changes between frames and reuses the unchanged parts, such as a still background, from earlier frames
key frame → A complete frame saved in full, which later frames use as a starting point and only record the changes from
A still background that repeats across 300 frames only needs to be stored once, not 300 times → background: stored 1 time, reused in 300 frames
After a key frame, the next frame only records the pixels that changed, such as a moving ball → frame 2 = frame 1 + changes (ball moved)
Every frame stores the full background again to save space → Storing the full background in every frame wastes space. Compression stores the still background once and reuses it in later frames.
Uncompressed video is small because it only keeps changes → Uncompressed video stores every pixel of every frame, so files get very large. Only compressed video keeps just the changes.

Key Term

inter-frame compression
Saves video space by storing only what changes between frames, reusing the still background from earlier frames and dropping fine detail viewers will not notice.

Fill in the Blanks

1. IF pixel == old_pixel THEN reuse(old_pixel) ELSE store(pixel)
Hint: Type the comparison operator that checks whether the pixel is unchanged from the previous frame.
2. key_frame = store_all(frame1) FOR each later frame store(changes)
Hint: Type the variable name for the first frame, which is stored in full.
3. FOR each frame IN video changes = compare(frame, previous) store(changes)
Hint: Type the keyword that repeats the steps for every frame in the video.
4. background = still_part(frame1) FOR each later frame reuse(background)
Hint: Type the variable name that holds the still part of the scene, which is reused instead of stored again.
5. frame_size = width * height * depth video_size = frame_size * frame_count
Hint: Type the variable name for the data needed to store one single frame of uncompressed video.

Spot the Error

Question 1
A SAVE full keyframe
B SAVE only changes as keyframe
C SAVE only the changes
D END FOR
Line B — A key frame is a complete picture of the first frame. Later frames only store changes, so line 1 should be SAVE full keyframe.
Question 2
A keyframe ← full first frame
B FOR each later frame
C compare with a blank frame
D store the differences
Line C — To find what changed, each frame is compared with the previous frame, not a blank one. Line 3 should compare with previous frame.
Question 3
A background ← still wall
B frame 1 stores background
C frame 2 reuses background
D frame 3 saves it all again
Line D — A still background does not change, so later frames reuse it instead of saving it again. Line 4 should be frame 3 reuses background.
Question 4
A frames ← 30 * 60
B size ← frames + frame_size
C OUTPUT size
Line B — Uncompressed video size is the number of frames multiplied by the size of one frame (30 frames a second for 60 seconds). It should use * not +.
Question 5
A IF pixel differs from before
B reuse the old pixel
C ELSE
D store the new pixel
Line A — The condition is reversed. Unchanged pixels are reused, so line 1 should test whether the pixel is the same as in the last frame.

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