Image Optimization Glossary
Key terms and concepts for web image optimization, explained simply.
A
Alpha Channel
The transparency layer in an image format (like PNG or WebP). Allows parts of an image to be partially or fully transparent, essential for logos and overlays.
AVIF
AV1 Image File Format — a next-generation image format based on the AV1 video codec. Achieves 40–50% smaller file sizes than JPEG at equivalent quality. Supported by 92%+ of modern browsers.
B
Batch Conversion
Processing multiple images at once in a single operation. BulkPicConv supports unlimited batch conversion using Web Workers for parallel processing.
BMP
Bitmap — an uncompressed raster image format native to Windows. Files are very large. Converting BMP to WebP or AVIF typically reduces file size by 80–90%.
C
CLS (Cumulative Layout Shift)
A Core Web Vitals metric measuring visual stability. Images without explicit width/height attributes are a common cause of layout shift. Fix by always setting width and height on <img> tags.
Compression
The process of reducing file size. Lossy compression removes imperceptible data for smaller files. Lossless compression preserves every pixel exactly but produces larger files.
Core Web Vitals
Google's set of metrics measuring real-world user experience: LCP (loading), INP (interactivity), and CLS (visual stability). Image optimization directly impacts LCP and CLS.
E
EXIF
Exchangeable Image File Format — metadata embedded in images by cameras, including camera model, exposure settings, GPS coordinates, and orientation. BulkPicConv strips EXIF by default for privacy.
G
GIF
Graphics Interchange Format — supports animation but limited to 256 colors. BulkPicConv converts GIF to a static first frame in WebP/AVIF/JPEG/PNG format.
H
HEIC
High Efficiency Image Container — the default photo format on iPhones since iOS 11. Based on HEVC (H.265) video codec. BulkPicConv can convert HEIC to WebP for universal web compatibility.
J
JPEG
Joint Photographic Experts Group — the most widely used image format on the web. Uses lossy compression. Good for photos but typically 25–50% larger than WebP or AVIF at equivalent quality.
L
LCP (Largest Contentful Paint)
A Core Web Vitals metric measuring how long it takes the largest content element to render. Hero images are often the LCP element — optimizing them with WebP/AVIF directly improves LCP scores.
Lossless Compression
Compression that preserves every pixel exactly. PNG and WebP lossless are examples. Best for screenshots, diagrams, and images with text where pixel accuracy matters.
Lossy Compression
Compression that removes imperceptible visual data to achieve much smaller file sizes. WebP and AVIF at quality 80–85 are lossy but visually indistinguishable from the original.
P
PNG
Portable Network Graphics — supports lossless compression and alpha transparency. Best for logos, screenshots, and images requiring pixel-perfect accuracy. Typically 2–3x larger than WebP.
Q
Quality Setting
A parameter (0–100) controlling the trade-off between file size and visual fidelity in lossy compression. Quality 80–85 is the industry sweet spot for photos.
R
Responsive Images
Serving different image sizes based on the user's screen size using srcset and sizes attributes. Combined with modern formats, this can reduce mobile image bandwidth by 60–80%.
S
srcset
An HTML attribute that provides multiple image sources at different resolutions. The browser picks the best one based on screen size and pixel density. Essential for responsive images.
W
Web Worker
A browser API that runs JavaScript in background threads. BulkPicConv uses Web Workers for parallel batch conversion, preventing UI freezing even with hundreds of images.
WebAssembly (WASM)
A binary instruction format that runs at near-native speed in the browser. BulkPicConv uses WASM-compiled versions of libwebp and libavif to process images entirely on your device.
WebP
An image format developed by Google. Provides 25–35% smaller files than JPEG at equivalent quality. Supports lossy, lossless, transparency, and animation. Supported by 97%+ of browsers.