JPEG is a still-image format that uses lossy compression非可逆圧縮 to store color-rich images such as photos in small files by throwing away information the eye hardly notices.
Original photo with many colors
Thin out fine, barely visible detail
Save as a small file
Opens slightly different from the original
Discarded information cannot come back, so the opened image never fully matches the original.
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A method that lets compressed data be restored exactly to the original is called lossless compression, while one that cannot restore it but shrinks the data much further is called lossy compression. JPEG belongs to the second group and handles full color of about 16.77 million colors, so it is widely used for digital camera and web photos. On text or sharp-edged graphics, however, blurring and blocky noise tend to appear around the edges.
AnalogyIt is like squeezing your travel luggage into a vacuum bag and pulling out things you probably will not need along the way. The bag gets lighter, but what you took out is gone for good.
How to rememberThe name is the initials of the standards group Joint Photographic Experts Group, and the word Photographic points straight at what it does best.
Exam points
Mix-upJPEG is a compression format for still images, while MPEG is a compression method for video and audio. The names look alike, but one handles pictures that stand still and the other handles moving footage.
Mix-upThe harder JPEG compresses, the smaller the file, but more information is discarded and image quality drops. Unlike lossless formats, every edit-and-resave adds another layer of degradation.
GIF is a still-image format that limits a single image to at most 256 colors and stores it with lossless compression that causes no degradation.
The colors in use are first gathered into a color table (palette) of no more than 256 entries, and each pixel is stored as an index into it. Low-color icons and graphics can be saved small without loss, and a file can also mark one color as transparent or switch between several frames for a simple animation.
AnalogyIt is like filling in a coloring book with a chosen set of up to 256 colored pencils. That is plenty for a picture made of flat colors, but a photo of a gradient sky can only be approximated with the pencils you have.
How to rememberIt stands for Graphics Interchange Format, and it was first built to exchange (Interchange) images over early online networks.
Exam points
ExamGIF tops out at 256 colors per image, and its compression is lossless可逆圧縮. It suits graphics and logos with few colors.
Mix-upSaving a photo as GIF produces visible color banding when the original colors are cut down to 256. This loss comes from the color limit, not from the compression method.
PNG is a still-image format that can store even full-color images without degradation using lossless compression可逆圧縮.
It was created as a web format to replace GIF and can save either with a palette of up to 256 colors or in full color. It supports an alpha channel that gives each pixel its own degree of transparency, so it also works for laying a semi-transparent logo over a background.
AnalogyIt is like packing a high-quality scanned original into a box so carefully that not a single crease forms. It arrives exactly as sent, but the box cannot be made very small.
How to rememberIt stands for Portable Network Graphics, and the word Network shows it was born for use on networks and the web.
Exam points
Mix-upPNG transparency lets each pixel have its own level of see-through. GIF transparency only removes one chosen color, so PNG handles edges that blend smoothly into the background better.
BMP is an image format that records the colors of the dots making up an image (pixels画素) in sequence, essentially without compression.
Bits per pixel
Colors available
Pixel data size at 640×480
1 bit
2 colors
38,400 bytes
8 bits
256 colors
307,200 bytes
24 bits
About 16.77 million colors
921,600 bytes
It is the standard image format used in Windows. Because it is not compressed, quality never drops, but files tend to be large as a result. Since the picture is drawn by laying out pixels, its edges look jagged when it is enlarged a lot.
AnalogyIt is like copying a picture by coloring every single square on graph paper. The copy is accurate, but it uses a lot of paper.
How to rememberThe name shortens Bitmap to the three letters B, M and P, and the file extension is .bmp as well.
Exam points
Mix-upThe size of a BMP file is roughly the number of pixels across and down multiplied by the bits used for each pixel's color. For the same image it is usually larger than a compressed format.
TIFF is an image format that records attributes such as resolution and color type in tagsタグ and lets you choose whether and how to compress when saving.
It can save either uncompressed or with lossless compression, so it is used for scanner captures and print-ready originals where images must be kept without losing quality. One file can also hold images of several pages, which suits scanning a stack of paper documents and keeping them together.
AnalogyIt is like a storage box with a sticky note on each item explaining it. Whoever opens it can read the notes and see how everything was packed.
How to rememberIt stands for Tagged Image File Format, and the leading word Tagged announces that it is a format with tags.
Exam points
Mix-upTIFF is not the name of a particular compression method but a container format that holds images. Whether the image inside is uncompressed, and which method compresses it, varies from file to file.
EPS is a file format that describes graphics and images in PostScript, a page description languageページ記述言語 for printing, so they can be placed inside other documents.
A page description language expresses where on the page to draw what, and how, as a series of commands, and PostScript was developed by Adobe. When shapes are drawn in vector form, as coordinates and commands, their outlines stay clean however far they are enlarged. In DTP (desktop publishing), it has long been used to place illustrations and logos onto printed pages.
AnalogyIt is like a set of drawing instructions that says "draw a circle with a 3 cm radius from here." The reader does the drawing, so the lines stay crisp even on a large sheet.
How to rememberIt stands for Encapsulated PostScript, and Encapsulated (sealed in a capsule) reflects its job as a component dropped into other documents.
Exam points
Mix-upEPS can hold a graphic as drawing commands rather than a collection of dots, so scaling it up or down hardly hurts quality. It can also contain images drawn in pixels.
PDF is an electronic document電子文書 format that bundles text, images and layout so it displays and prints the same as when it was made, whatever the OS or device.
Adobe developed it from PostScript, and it is now an ISO international standard. The fonts used can be embedded in the file, so letter shapes and line breaks stay the same even if the recipient does not have those fonts. Because free reader software and web browsers can open it, it is widely used to distribute application forms and handouts.
AnalogyIt is like the final proof you hand to a print shop. Whichever shop prints it, the result looks the same, so you can send it without worrying about the other side's machines.
How to rememberIt stands for Portable Document Format, and Portable captures the idea that it looks the same wherever you carry it.
Exam points
Mix-upEPS is for artwork placed into other documents as a component, while PDF is for distributing and reading documents in their finished form, and it can hold many pages.
EPS vs. PDF
Aspect
EPS
PDF
Key differenceMain role
Artwork placed into other documents
Distributing and reading documents in finished form
Pages
Basically a single graphic
Can combine many pages
Main users
Designers producing print material with DTP
General users who receive and read documents
Key points
Lossy JPEG stores photos compactly, but the opened image never fully matches the original.
Within its 256-color limit, GIF can mark a transparent color and hold a simple animation.
BMP records pixel colors almost as they are, so it never degrades but makes large files.
TIFF is a container format that records attributes in tags, and each file can choose whether and how to compress.