What are Raster Graphics?

⏱ 5 min read · PDF Utils

Quick Definition

Raster graphics are images composed of a grid of individual pixels, each with a specific color value. In PDFs, raster images include photographs, scanned documents, and any bitmap-based graphics. Unlike vector graphics, raster images have a fixed resolution and can become pixelated when enlarged.

How Raster Graphics Work

A raster image is a rectangular grid of pixels. Each pixel stores color information (typically as RGB or CMYK values). A 1920×1080 image contains 2,073,600 individual pixels. When displayed, these pixels form the complete image. The resolution (measured in pixels per inch or DPI) determines how sharp the image appears at a given physical size.

Enlarging a raster image beyond its native resolution requires interpolation—the software must guess what colors the additional pixels should be. This results in blurriness or visible pixelation, as the image contains no information beyond its original pixel grid.

Why Raster Graphics Matter in PDFs

Photographs, scanned documents, and complex images with gradients or textures are best represented as raster graphics. While vector graphics excel at simple shapes and text, raster images can capture the full tonal range and detail of photographic content.

However, raster images are a primary cause of large PDF file sizes. A high-resolution photograph can add several megabytes to a PDF. Proper compression and resolution management are essential when including raster images in PDFs.

Raster vs Vector

• Resolution dependency: Raster images have fixed resolution. Enlarging them reduces quality. Vector graphics scale infinitely.
• File size: Raster images can be large, especially at high resolution. Vectors are typically smaller for simple content.
• Best use: Rasters are ideal for photographs and complex images. Vectors work better for logos, text, and diagrams.
• Editing: Raster editing works at the pixel level. Vector editing adjusts mathematical paths.

Common Raster Formats in PDFs

PDFs can embed raster images in various formats, including JPEG (lossy compression, ideal for photographs), PNG (lossless compression, supports transparency), and TIFF (uncompressed or losslessly compressed, used for high-quality scans). The PDF format can also apply additional compression to embedded images.

Resolution Considerations

For print, raster images should typically be 300 DPI at the intended output size. For screen display, 72-150 DPI is usually sufficient. Including images at higher resolution than necessary increases file size without improving visible quality. Conversely, low-resolution images appear blurry or pixelated when printed.

Common Use Cases

• Photographs: Product images, portraits, and photographic content
• Scanned documents: Digitized paper documents and forms
• Screenshots: Captured screen images and UI elements
• Complex artwork: Paintings, textures, and detailed illustrations
• Medical imaging: X-rays, MRI scans, and diagnostic images

Related Concepts

• Vector Graphics — Scalable path-based images
• Resolution — Image quality and DPI
• Compression — Reducing image file size
• Images Blurry — Why PDF images appear low quality