What Is a Transparent GIF?

The full name of GIF is Graphics Interchange Format, which can be translated into a graphics interchange format. It is used to display indexed color images in Hypertext Markup Language, and is widely used on the Internet and other online service systems. GIF is a common image file format standard, copyrighted by Compu Serve.

The name of the GIF format is the abbreviation of Graphics Interchange Format. It was developed by Compu Serve in 1987 to fill the gaps in cross-platform image formats. GIF can be supported on multiple platforms such as PC and Mactiontosh.

GIF is a bitmap. The general principle of a bitmap is that a picture is composed of many pixels, and each pixel is assigned a color. These pixels are combined to form a picture. GIF uses Lempel-Zev-Welch (LZW) compression algorithm, which supports up to 256 colors. Because of this feature, GIF is more suitable for pictures with less colors, such as cartoon shapes, company logos, and so on. If you need to use true color, then the expressive power of GIF is limited. GIF usually comes with a color palette, which stores various colors to be used. In web applications, the size of the image file size will obviously affect the download speed, so we can optimize the color palette according to the characteristics of the GIF with color palette and reduce the number of colors used by the image (some images are not used Color can be dropped) without affecting the quality of the picture.

The biggest difference between GIF format and other image formats is that it is completely designed as a common standard. Due to the popularity of the Compu Serve network, many platforms support the GIF format. Compu Serve promotes GIFs through free distribution of format specifications, but requires software that uses the GIF file format to include a description of its copyright information.

GIF has two versions of GIF87a and GIF89a.

The GIF87a version was introduced in 1987. A file stores an image and strictly does not support transparent pixels. GIF87a uses the LZW compression algorithm, which can compress the image size by 20 to 25 percent while maintaining image quality.

The GIF89a version is a very special version introduced in 1989. This version allows multiple files to be stored in one file, can implement animation functions, and allows certain pixels to be transparent. In this version, the graphic control block, notes, instructions, and application programming interface 4 blocks have been expanded for GIF files, and support for transparent colors and multi-frame animation is provided.

Among them, GIF89a has made significant improvements in transparency, interlaced and animated GIF. The first is to support transparency. GIF89a allows some parts of the picture to be invisible. This feature is very important, allowing us to take advantage of such a feature in some occasions so that the edges of the image no longer show a rectangular border, but become any shape we want. These transparent areas can be easily generated in Photoshop and Fireworks and exported as GIF pictures in GIF89a format. Of course, transparency does not mean that the border no longer exists. In fact, it does exist, but it is not displayed. This can make the inserted picture more consistent with the overall web page. ^[1]

The image file structure of GIF format is shown in Figure 1.

GIF format image files have the following characteristics:

(1) The extension of GIF format image files is ".gif".

(2) Best performance for grayscale images.

(3) There are two versions of GIF87a and GIF89a.

(4) Use the improved LZW compression algorithm to process the image data.

(5) The palette data is divided into general palette and local palette, and has different color values.

(6) Does not support 24-bit color mode, and can store up to 256 colors. ^[1]

GIF is a compressed file, which is used to reduce the time required for the file to be transmitted on the network;

GIF has a bit depth of 1-8bit, and is monochrome and transparent. It is realized by a palette of up to 256 colors. The image size is the largest; it is 64K × 64K pixels. GIF is mainly a transmission format designed for a data stream, not a file format, so it is the most complicated image file format;

Support Bitmap, Grayscale and index color mode. ^[2]