ACTIONSCRIPT 3 Developer’s Guide en

ACTIONSCRIPT 3.0 DEVELOPER’S GUIDE
Working with bitmaps
BMP
The BMP (bit mapped) format is a default image format used by the Microsoft Windows operating system. It does not
use any form of compression algorithm and as such usually results in large file sizes.
GIF
The Graphics Interchange Format (GIF) was originally developed by CompuServe in 1987 as a means to transmit
images with 256 colors (8-bit color). The format provides small file sizes and is ideal for web-based images. Because of
this format’s limited color palette, GIF images are generally not suitable for photographs, which typically require high
degrees of shading and color gradients. GIF images permit single-bit transparency, which allows colors to be mapped
as clear (or transparent). This results in the background color of a web page showing through the image where the
transparency has been mapped.
JPEG
Developed by the Joint Photographic Experts Group (JPEG), the JPEG (often written JPG) image format uses a lossy
compression algorithm to allow 24-bit color depth with a small file size. Lossy compression means that each time the
image is saved, the image loses quality and data but results in a smaller file size. The JPEG format is ideal for
photographs because it is capable of displaying millions of colors. The ability to control the degree of compression
applied to an image allows you to manipulate image quality and file size.
PNG
The Portable Network Graphics (PNG) format was produced as an open-source alternative to the patented GIF file
format. PNGs support up to 64-bit color depth, allowing for up to 16 million colors. Because PNG is a relatively new
format, some older browsers don’t support PNG files. Unlike JPGs, PNGs use lossless compression, which means that
none of the image data is lost when the image is saved. PNG files also support alpha transparency, which allows for up
to 256 levels of transparency.
TIFF
The Tagged Image File Format (TIFF) was the cross-platform format of choice before the PNG was introduced. The
drawback with the TIFF format is that because of the many different varieties of TIFF, there is no single reader that
can handle every version. In addition, no web browsers currently support the format. TIFF can use either lossy or
lossless compression, and is able to handle device-specific color spaces (such as CMYK).
Transparent bitmaps and opaque bitmaps
Bitmap images that use either the GIF or PNG formats can have an extra byte (alpha channel) added to each pixel. This
extra pixel byte represents the transparency value of the pixel.
GIF images allow single-bit transparency, which means that you can specify a single color, from a 256-color palette, to
be transparent. PNG images, on the other hand, can have up to 256 levels of transparency. This function is especially
beneficial when images or text are required to blend into backgrounds.
ActionScript 3.0 replicates this extra transparency pixel byte within the BitmapData class. Similar to the PNG
transparency model, ActionScript offers up to 256 levels of transparency.
Important concepts and terms
The following list contains important terms that you will encounter when learning about bitmap graphics:
Alpha The level of transparency (or more accurately, opacity) in a color or an image. The amount of alpha is often
described as the alpha channel value.
ARGB color A color scheme where each pixel’s color is a mixture of red, green, and blue color values, and its
transparency is specified as an alpha value.
Last updated 6/6/2012
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