Image, audio and moving image file formats: R093 revision

A raster (bitmap) image is a grid of pixels, each one a single colour. Raster images can show the fine detail and subtle colour of a photograph, but they have a fixed resolution: enlarge one beyond its size and it turns blurry or blocky (pixelated). JPG, PNG, GIF, BMP and TIFF are raster formats.

A vector image is stored as shapes, lines and curves described mathematically, so it can be scaled to any size, from a business card to the side of a van, without losing quality, and simple graphics give very small files. That makes vector right for logos, icons, illustrations and maps, but wrong for photographs. SVG is the vector format of the web; EPS and AI (Adobe Illustrator) are common in print and design.

• JPG (JPEG): lossy compression, millions of colours and small files, so it is the standard for photographs on websites and social media. No transparency, and quality drops a little each time it is re-saved.
• PNG: lossless compression and full transparency, so it suits logos, graphics with sharp edges, screenshots and images placed on coloured backgrounds. Photographs saved as PNG are much larger than as JPG.
• GIF: limited to 256 colours, but supports simple animation and basic transparency, so it suits short looping animations and simple graphics, not photographs.
• TIFF: uncompressed or losslessly compressed, high quality and large, used for professional print and archiving.
• BMP: uncompressed and large; rarely the best choice today.
• SVG: vector, scalable and small for simple shapes, supported by web browsers.
• PSD: Photoshop's working format, which keeps layers so the image can be edited later.

• Uncompressed: WAV and AIFF. The full recording with nothing removed, so they are the best quality but very large. Used for recording, editing and master copies.
• Lossy compressed: MP3 and AAC. Much smaller files with a little quality removed, usually not noticeable to listeners. Used for streaming, podcasts and downloads.
• Lossless compressed: FLAC. Smaller than WAV with nothing lost, but larger than MP3 and supported by fewer devices and apps.

MP4 is the most widely supported video format: it plays on almost every phone, browser, computer and social platform and is compressed for fast streaming, so it is the usual choice for the web. MOV is Apple's QuickTime format, common on Apple devices and in editing. AVI and WMV are older Windows formats. Professional cameras can record uncompressed or lightly compressed video, which gives the best quality for editing but produces enormous files.

Animation can be exported as video (MP4), as an animated GIF for a short silent loop, or kept in the animation software's own project format so it can be edited later.

The specification sorts formats into uncompressed and compressed, and expects you to know the limits of each. Uncompressed formats, such as BMP, uncompressed TIFF, WAV and AIFF, keep every bit of the original, so they are the best quality to edit from, but the files are huge and slow to share. Compressed formats are smaller. Lossy ones (JPG, MP3, most MP4 video) throw some detail away for much smaller files; lossless ones (PNG, FLAC) shrink less but lose nothing. The compression page explains how each works.

The specification asks how a format choice relates to its use and context. Ask: where will it be used (web, print, broadcast, a phone)? What quality is needed? How big can the file be for the platform and the audience's connection? Will every device the audience uses play it? Does it need transparency, animation or scaling? Will it be edited again later?

So a band's logo for T-shirts and a website starts as an SVG; a photographer sending previews to a client sends JPGs; a podcast goes out as MP3; the radio jingle is mastered as WAV; and a promotional video for social media is exported as MP4. A good answer names the format and gives a reason tied to that use.

Image, audio and moving image files together were worth between 1 and 7 marks on each of the four papers we analysed. Questions ask you to name a suitable format for a product in the scenario and justify it, to say whether an image should be raster or vector, or to give an example of a compressed or uncompressed format.

For how the long answers and pre-production documents are marked, see how R093 is marked.