How video compression reuses unchanged parts of frames

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Quick Questions

Shade in the A or B circle for each question.

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

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

3. What is a key frame?

A A frame that is deleted to save space

B A full frame stored in complete detail

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

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1inter-frame compression		AStores only what changes between frames and reuses the unchanged parts, such as a still background, from earlier frames
2key frame		Bbackground: stored 1 time, reused in 300 frames
3A still background that repeats across 300 frames only needs to be stored once, not 300 times		CA complete frame saved in full, which later frames use as a starting point and only record the changes from
4After a key frame, the next frame only records the pixels that changed, such as a moving ball		DStoring the full background in every frame wastes space. Compression stores the still background once and reuses it in later frames.
5Every frame stores the full background again to save space		EUncompressed video stores every pixel of every frame, so files get very large. Only compressed video keeps just the changes.
6Uncompressed video is small because it only keeps changes		Fframe 2 = frame 1 + changes (ball moved)

Definitions

inter-frame compression

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF pixel __ old_pixel THEN reuse(old_pixel) ELSE store(pixel)

_________ = store_all(frame1) FOR each later frame store(changes)

___ each frame IN video changes = compare(frame, previous) store(changes)

__________ = still_part(frame1) FOR each later frame reuse(background)

__________ = width * height * depth video_size = frame_size * frame_count

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A SAVE full keyframe

B SAVE only changes as keyframe

C SAVE only the changes

D END FOR

Question 2

A keyframe ← full first frame

B FOR each later frame

C compare with a blank frame

D store the differences

Question 3

A background ← still wall

B frame 1 stores background

C frame 2 reuses background

D frame 3 saves it all again

Question 4

A frames ← 30 * 60

B size ← frames + frame_size

C OUTPUT size

Question 5

A IF pixel differs from before

B reuse the old pixel

C ELSE

D store the new pixel

Answer Key

How video compression reuses unchanged parts of frames

Section A: Quick Questions

A — Stores the wall once and reuses it across frames

A — Only what has changed since the previous frame

B — A full frame stored in complete detail

A — The area where the cat has moved

B — Every frame is stored in full, and there are many frames each second

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

IF pixel == old_pixel THEN
    reuse(old_pixel)
ELSE store(pixel)

key_frame = store_all(frame1)
FOR each later frame
    store(changes)

FOR each frame IN video
    changes = compare(frame, previous)
    store(changes)

background = still_part(frame1)
FOR each later frame
    reuse(background)

frame_size = width * height * depth
video_size = frame_size * frame_count

Section E: Spot the Error

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.

C — To find what changed, each frame is compared with the previous frame, not a blank one. Line 3 should compare with previous frame.

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.

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 +.

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.