Invitation Card, Pixels and Attachment Limits
Ananya is making a digital invitation for the school annual function. She selects a photograph that is 1600 pixels wide and 1200 pixels high, saved at 24-bit colour depth. The school will email the invitation to 300 parents, and the email service allows attachments up to 2 MB. Her home broadband gives an upload speed of 512 kbps. She also wants the heading in a custom blue that she has noted as RGB (47, 79, 255). While saving, the design software offers JPEG quality 90%, PNG-24 and BMP. The computer teacher warns that the attachment may be rejected and asks her to show the calculation before choosing a format. Ananya also plans to reduce the photograph to 800 × 600 pixels if the file is still too large.
1. Calculate the uncompressed size of the 1600 × 1200 image at 24-bit colour and the ideal upload time at 512 kbps. Does it fit the 2 MB limit?
Read the worked answer
Each pixel at 24-bit colour needs 24 bits = 3 bytes, one byte each for red, green and blue.
- Pixels: 1600 × 1200 = 1,920,000.
- Uncompressed size: 1,920,000 × 3 = 5,760,000 bytes.
- In binary units: 5,760,000 ÷ 1,048,576 ≈ 5.49 MiB; in decimal units it is 5.76 MB.
- Upload time: 5,760,000 × 8 = 46,080,000 bits. Divide by 512,000 bits per second = 90 seconds at the ideal rate, or about 2 minutes with overhead.
It does not fit a 2 MB limit, because 5.76 MB is about 2.75 times the allowance. A BMP would be even larger. JPEG compression at quality 90% can normally reduce a photograph by much more than 3:1 with little visible loss, but the result must be checked, not assumed. The upload time is acceptable, so size, not speed, is the deciding constraint.
2. Convert RGB (47, 79, 255) to hexadecimal and explain what 24-bit colour means in terms of the number of colours available.
Read the worked answer
Convert each decimal value to two hexadecimal digits:
- 47 ÷ 16 = 2 remainder 15 → 2F.
- 79 ÷ 16 = 4 remainder 15 → 4F.
- 255 → FF.
So the colour is #2F4FFF. In HTML/CSS order this is red 2F, green 4F, blue FF - a strong blue with almost no red or green.
24-bit colour means 8 bits per channel, and 8 bits allow 2⁸ = 256 levels per channel. The total number of combinations is 256 × 256 × 256 = 2²⁴ = 16,777,216 colours. That is why 24-bit colour is called true colour. Adding an 8-bit alpha channel gives 32-bit colour, which adds transparency but does not increase the number of visible colours. For a printed invitation, the RGB value must also be converted to CMYK by the printer, and the printed blue may look duller than on screen; a proof print is therefore advisable.
3. Recommend a file format and settings for emailing the invitation, and explain why reducing the photograph to 800 × 600 helps.
Read the worked answer
Choose JPEG at quality 85–90% for the photograph and keep sharp text as a separate PNG or as vector text where possible. JPEG is designed for photographs and gives small files; PNG-24 preserves sharp edges but is usually too large for a 2 MB limit; BMP is uncompressed and unsuitable.
Reducing 1600 × 1200 to 800 × 600 reduces the pixel count from 1,920,000 to 480,000, exactly one quarter. An uncompressed image would therefore be about 1.44 MB, and a JPEG at the same quality would also fall roughly fourfold. The trade-off is that fine detail and small text may become blurred, especially when printed. Since the invitation is mainly viewed on phones, 800 × 600 is reasonable for the photograph, but the heading should be added as text after resizing, not resized as part of the photograph. Check the final file size, open it on a phone, and print one copy before sending 300 emails.