Graphics File Formats

There are many standard formats for saving bitmaps in disk files. GDI+ supports the graphics file formats described in the following paragraphs.

BMP

BMP is a standard format used by Windows to store device-independent and application-independent images. The number of bits per pixel (1, 4, 8, 15, 24, 32, or 64) for a given BMP file is specified in a file header. BMP files with 24 bits per pixel are common. BMP files are usually not compressed and, therefore, are not well suited for transfer across the Internet.

Graphics Interchange Format (GIF)

GIF is a common format for images that appear on Web pages. GIFs work well for line drawings, pictures with blocks of solid color, and pictures with sharp boundaries between colors. GIFs are compressed, but no information is lost in the compression process; a decompressed image is exactly the same as the original. One color in a GIF can be designated as transparent, so that the image will have the background color of any Web page that displays it. A sequence of GIF images can be stored in a single file to form an animated GIF. GIFs store at most 8 bits per pixel, so they are limited to 256 colors.

Joint Photographic Experts Group (JPEG)

JPEG is a compression scheme that works well for natural scenes such as scanned photographs. Some information is lost in the compression process, but often the loss is imperceptible to the human eye. JPEGs store 24 bits per pixel, so they are capable of displaying more than 16 million colors. JPEGs do not support transparency or animation.

The level of compression in JPEG images is configurable, but higher compression levels (smaller files) result in more loss of information. A 20:1 compression ratio often produces an image that the human eye finds difficult to distinguish from the original. The following illustration shows a BMP image and two JPEG images that were compressed from that BMP image. The first JPEG has a compression ratio of 4:1 and the second JPEG has a compression ratio of about 8:1.

Форматы графических файлов

Существует множество стандартных форматов файлов для сохранения растровых изображений. Ниже описаны форматы графических файлов, поддерживаемые интерфейсом GDI+.

BMP

BMP — это стандартный формат, используемый в Windows для хранения аппаратно- и программно-независимых изображений. Количество битов, сопоставленных в данном файле BMP одной точке (1, 4, 8, 15, 24, 32 или 64), указывается в заголовке файла. Наиболее часто используются файлы BMP, в которых каждой точке соответствует 24 бита. Обычно файлы BMP не подвергаются сжатию, поэтому этот формат плохо подходит для передачи данных через Интернет.

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