A museum is creating an online catalogue. One photograph is stored as an uncompressed bitmap before it is uploaded. The bitmap has a width of 1600 pixels, a...
Question 1 Report
A museum is creating an online catalogue. One photograph is stored as an uncompressed bitmap before it is uploaded. The bitmap has a width of 1600 pixels, a height of 1200 pixels and a colour depth of 24 bits. Ignore any file header, metadata and compression when completing the calculations. The museum wants to compare the raw file size with the size shown by a graphics package, which uses mebibytes (MiB), where 1 MiB = 1,048,576 bytes.
| Stage | Value |
|---|---|
| Number of pixels | ................................ |
| Number of bits used for pixel data | ................................ |
| Number of bytes used for pixel data | ................................ |
(a) State what is meant by colour depth. [2]
(b) Complete the table. [3]
(c) Convert the number of bytes in the completed table to MiB. Give your answer to two decimal places. [2]
(d) Give two items that could make the actual bitmap file larger than the pixel data size. [2]
(e) Identify the type of compression that guarantees the original data can be reconstructed exactly. [1]
(f) Explain why lossy compression is usually suitable for a catalogue photograph but unsuitable for a computer program file. [3]
(g) State the resolution of the bitmap in pixels. [2]
Answer Details
(a) Colour depth is the number of bits used to represent the colour of each pixel. It determines the number of possible colours [2].
(b)
| Stage | Calculation and value |
|---|---|
| Pixels | \(1600\times1200=\textbf{1,920,000}\) |
| Bits | \(1,920,000\times24=\textbf{46,080,000 bits}\) |
| Bytes | \(46,080,000\div8=\textbf{5,760,000 bytes}\) |
[3]
(c) \[5,760,000\div1,048,576=5.493\ldots\text{ MiB}\] To two decimal places, this is 5.49 MiB [2].
(d) A file header and metadata, such as camera/date/location data, can increase actual file size [2].
(e) Lossless compression guarantees exact reconstruction [1].
(f) A photograph can lose some detail without a noticeable visual effect, so lossy compression can reduce its size greatly. A program requires every bit to be restored exactly; changed or missing instructions could alter it or stop it running [3].
(g) The resolution is 1600 × 1200 pixels [2].
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