A Glossary of Image Format Terms (Chroma, Alpha, Color Profiles)

Image formats are quietly complex. This glossary defines the terms you'll meet resizing, compressing, or debugging images. Try each concept with Zro7 Compress Image.

The major raster formats

• JPEG: 1992. Lossy, no alpha, universal.
• PNG: 1996. Lossless, alpha, larger than JPEG for photos.
• GIF: 1987. 256 colors, animation. Superseded by APNG/WebP for animation.
• WebP: 2010, Google. Both lossy and lossless modes, alpha, ~25-35% smaller than JPEG.
• AVIF: 2019. AV1-based, ~50% smaller than JPEG at equal quality. Modern browsers only.
• HEIC: iPhone default. Same lineage as AVIF (HEVC vs AV1). Weaker cross-platform support.
• TIFF: pro workflows. Lossless, huge, supports layers.
• RAW (CR2, NEF, ARW, DNG): sensor data — not really an 'image' until processed.

Vector formats

• SVG: XML vector, scales infinitely. Ideal for logos and icons.
• PDF: can carry vector art; often exported from Illustrator.

Quality controls

• Quality (q): 0-100 scale, format-specific curve. JPEG q80 ≠ WebP q80.
• Lossless vs lossy: lossless preserves every pixel; lossy discards imperceptible detail.
• Effort/speed: WebP/AVIF encoders trade time for size.

Color

• Bit depth: 8-bit per channel is standard (16.7M colors). 10/12/16-bit for HDR and RAW.
• Color space: sRGB (web default), Display P3 (Apple), Adobe RGB, ProPhoto.
• ICC profile: metadata declaring which color space applies. Missing profile = browser assumes sRGB.
• Gamma: nonlinear encoding matching human perception (~2.2).

Chroma subsampling

• 4:4:4: full color detail per pixel. Screenshots, text.
• 4:2:2: half horizontal color. Broadcast.
• 4:2:0: quarter color. JPEG default, invisible on photos, ruins colored text.

Alpha (transparency)

• Binary alpha: pixel is opaque or transparent (GIF).
• 8-bit alpha: 256 opacity levels (PNG, WebP, AVIF).
• Premultiplied alpha: RGB values pre-scaled by alpha — avoids fringing when compositing.

Metadata

• EXIF: camera, lens, timestamp, GPS. See EXIF Editor.
• IPTC: editorial metadata — captions, credits, copyright.
• XMP: Adobe's XML-based metadata; supersedes IPTC in modern tools.

Progressive vs baseline

• Baseline JPEG: top-to-bottom decode.
• Progressive JPEG: coarse-to-fine — user sees a blurry preview immediately.
• Interlaced PNG (Adam7): same idea for PNG.

Updated January 3, 2027 · Zro7 editorial team.