Image and PDF Terminology Glossary

Size and shape

Pixel: The smallest unit of an image: one square of a single colour. Everything else on this page is ultimately a statement about pixels — how many there are, how they are stored, and how faithfully they are reproduced.
Resolution: The width and height of an image in pixels, written as 1920×1080. It describes how much detail the file can hold, and nothing about physical size.In practice: Resolution is the biggest single driver of file size after format. Halving both dimensions removes three quarters of the pixels.
Megapixel: One million pixels — width times height, divided by a million. A 4000×3000 photo is 12 megapixels.In practice: Useful for judging memory cost. Every megapixel needs about 4 MB of RAM while being processed, which is why very large photos fail on phones.
Aspect ratio: The proportional relationship between width and height, such as 16:9 or 4:3. Two images can share an aspect ratio at wildly different resolutions.In practice: Leave the aspect ratio locked when resizing unless you deliberately want a stretched image. Unlocking it is how photos end up looking squashed.
DPI (dots per inch): How densely pixels are packed when an image is printed at a given physical size. It is a property of printing, not of the file — changing the DPI number alone adds no detail.In practice: The PDF Converter renders pages at 144 DPI, which is sharp on screen. Commercial print generally expects 300 DPI.

How files get smaller

Lossy compression: A method that permanently discards image detail to reach a smaller file. JPG and lossy WebP work this way. What gets discarded is chosen to be what the eye notices least.In practice: Each re-encode loses a little more. Editing a JPG repeatedly and re-saving compounds the damage; work from the original whenever you can.
Lossless compression: A method that shrinks the file with no loss at all — decompressing reproduces the original pixels exactly. PNG and lossless WebP work this way.In practice: Lossless is not automatically smaller. On a photograph, PNG can be more than ten times the size of a JPG of the same image.
Quality setting: A dial on lossy formats controlling how aggressively detail is discarded, usually expressed from 0 to 1 or 0 to 100.In practice: The scale is not linear. On our test photo, moving from quality 0.8 to 0.95 roughly tripled the file for a difference most people cannot see.
Compression artifact: Visible damage introduced by lossy compression: blocky squares in smooth gradients, halos around hard edges, or smeared fine texture.In practice: Artifacts appear first around text and sharp lines, which is why lossy formats suit photographs and suit screenshots badly.
Chroma subsampling: A trick used by JPG where colour information is stored at lower resolution than brightness, because human vision is far more sensitive to brightness detail than to colour detail.In practice: It is part of why fine red text on a JPG can look muddier than the same text in a PNG.
Target file size: An option where you name a size in KB and the converter searches for a quality setting that lands under it, re-encoding several times and keeping the closest result.In practice: It is an approach, not a calculation. A noisy photograph has a much higher achievable floor than a flat graphic does.

Colour and transparency

Alpha channel: A per-pixel transparency value stored alongside colour. It is what lets a logo sit cleanly on any background instead of arriving in a white box.In practice: PNG and WebP have one. JPG does not, and never will — converting a transparent image to JPG fills the transparent pixels with a solid colour.
Colour depth: How many bits describe each pixel. Ordinary web images use 8 bits per channel, giving 256 levels each of red, green and blue.In practice: Low colour depth causes banding — visible steps in what should be a smooth gradient, most noticeable in skies.
Colour profile: A tag describing which colour space the numbers in a file refer to, such as sRGB or Display P3. The same RGB values mean different colours in different spaces.In practice: A photo that looks vivid in one app and flat in another is usually a profile mismatch rather than a conversion fault.

Metadata and privacy

EXIF metadata: A block of data cameras and phones write inside an image: capture time, camera model, exposure settings, an orientation flag, and often GPS coordinates.In practice: Invisible in the picture but easy to read with common tools. See the EXIF and privacy guide, which tests what survives a conversion.
Orientation flag: An EXIF field telling viewers which way up the image should be displayed, used because phones often store the sensor image as captured rather than rotating the pixels.In practice: Software that ignores the flag shows the photo sideways. BravoConvert reads it and bakes the rotation into the pixels, so the output needs no flag.

PDF

Rendering (PDF): Drawing a PDF page's vector instructions onto a pixel canvas so it can be saved as an image, at a chosen scale relative to the page's own coordinate system.In practice: PDF measures pages in points at 72 to the inch, so a scale factor of 2 produces 144 DPI.
Vector vs raster: Vector content is stored as shapes and text that can be redrawn at any size; raster content is a fixed grid of pixels. PDFs usually hold vector text; images are always raster.In practice: Converting a PDF page to an image turns vector into raster, which is why the result softens when enlarged and why the text stops being selectable.

Working with files

Batch conversion: Converting several files to the same output format in one operation instead of one at a time.In practice: Results arrive as a ZIP when there is more than one file. On iOS you have to tap the ZIP to expand it.
Non-destructive editing: An approach where edits are held as instructions against a working copy, leaving the source untouched until you export.In practice: It is why Reset returns the original image intact, and why nothing you do in the editor changes the file on your disk.
Client-side processing: Work performed by your own browser rather than by a server. The file is read from disk into memory on your device and never transmitted.In practice: This is why there are size ceilings and why speed depends on your hardware — and why your files are never uploaded anywhere.

Several of these terms are explored with measured numbers in the format comparison and file size guides, or browse everything in the guides index.