The digital image and its formats
Pixel, definition, resolution, colour depth and alpha channel; raster or vector images; lossless and lossy compression; BMP, PNG, GIF, JPEG and SVG, and what each is used for.
- 28 cards
- 5 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Questions
- Sort and link
- The numbers
- Scenarios
A few cards from the deck
Pixel: what is it?
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The smallest element of a raster image: one cell of the grid, carrying a single colour.
A raster image is just an array of pixels; zooming in far enough shows the cells and their flat colour.
How many distinct colours can a pixel take with an 8-bit indexed palette?
- 256
- 8
- 65,536
- 16,777,216
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256
8 bits give 2⁸ = 256 values, hence at most 256 colours. 65,536 matches 16 bits, 16,777,216 matches 24 bits.
Run-length encoding (RLE) replaces each sequence of identical pixels with its … and its value: the line BBBBNNBBB becomes ….
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Run-length encoding (RLE) replaces each sequence of identical pixels with its length and its value: the line BBBBNNBBB becomes 4B 2N 3B.
It saves a lot on large flat areas, but on a line that alternates BNBN every pixel costs a count: the file grows.
Raster image: what is it?
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An image described by a grid of pixels, each with its own colour.
It suits photos, but enlarged beyond its definition it turns blurry or blocky: the missing pixels are invented.