Bitmap images lose quality when resized, particularly when enlarged, because they are made up of a fixed grid of pixels. When an image is scaled up, the computer must create additional pixels to fill the new dimensions. Since bitmap images store fixed pixel values, the software can only guess what colour the new pixels should be, often using interpolation methods like bilinear or bicubic scaling. This leads to a blurry or pixelated appearance as the image is stretched beyond its intended resolution. On the other hand, vector graphics are made using mathematical equations that define shapes, curves, and lines. When resized, these equations are recalculated to render the shapes at the new size, maintaining sharpness and clarity at any scale. This is why bitmap formats are ideal for rich, detailed images like photos, but not ideal for scalable graphics such as logos, where vector formats are preferred for clean and consistent resizing.

The alpha channel in a bitmap image controls transparency. In images with an alpha channel, each pixel includes additional data to indicate how opaque or transparent it is. A fully opaque pixel has an alpha value of 255 (in 8-bit alpha), and a fully transparent pixel has a value of 0. Alpha channels are especially useful in image editing and web graphics where images need to be layered or blended with backgrounds. For example, a PNG image with transparency can be overlaid on a web page without showing a solid background. Including an alpha channel increases the file size because each pixel now requires extra bits — typically 8 more bits in a 32-bit image (24 bits for colour + 8 for alpha). This added data increases memory and storage requirements. However, it allows for sophisticated visual effects such as shadows, anti-aliasing, and smooth transitions between opaque and transparent areas.

Screen resolution refers to the number of pixels a display can show, commonly described as width × height, such as 1920 × 1080. It defines how many physical pixels exist on the monitor or screen. Image resolution, on the other hand, is measured in dots per inch (dpi) and indicates the pixel density when printed. When displaying bitmap images on a screen, dpi is mostly irrelevant — the image’s actual pixel dimensions determine how large it appears. For instance, a 1000 × 1000 image will take up more screen space on a lower-resolution monitor than on a high-resolution one. On high-DPI screens like Retina displays, the same image may appear smaller but sharper. If the image is larger than the screen resolution, it may be scaled down automatically. Conversely, if smaller, it may be upscaled, resulting in blur. Designers must consider both resolutions when preparing graphics for different output mediums, especially when switching between print and digital displays.

Compression reduces the file size of bitmap images by removing redundant or unnecessary data. In lossless compression, such as in PNG format, the original image can be perfectly reconstructed. It uses algorithms like DEFLATE to find and encode repeated patterns more efficiently without losing any visual detail. This is ideal for images with solid colours, text, or sharp lines. Lossy compression, like that used in JPEG, permanently discards some image data to reduce size. It works by approximating areas of similar colour and eliminating details less noticeable to the human eye. This can significantly reduce file size but may introduce visual artefacts like blurring or blockiness, especially after repeated saves. Lossy formats are best for photographs where small quality loss is acceptable. Lossless formats are preferred when exact reproduction is required. Ultimately, the choice between lossy and lossless depends on the use case — whether file size, quality, or fidelity is the top priority.

Dithering is a technique used in bitmap graphics to simulate the appearance of more colours than are actually available in a low colour depth image. When an image is restricted to a limited palette (e.g. 16 or 256 colours), it often cannot represent smooth gradients or subtle shading accurately. Dithering solves this by mixing pixels of different colours in a specific pattern to give the illusion of intermediate shades. For example, alternating black and white pixels can create the appearance of grey. The human eye blends these dots visually, perceiving a smoother transition than actually exists. Dithering is commonly used in GIF images, which are limited to 256 colours, and in retro gaming graphics where colour options were hardware-limited. While it improves the visual quality of restricted-palette images, dithering can increase file size and sometimes introduce a speckled or grainy look. It’s a trade-off between colour realism and simplicity, often applied dynamically by image processing software.