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RGBA stands for red green blue alpha. While it is sometimes described as a color space, it is actually simply a use of the RGB color model, with extra information. The color is RGB, and may belong to any RGB color space, but an integral alpha value as invented by Catmull and Smith between 1971 and 1972 enables alpha compositing. The inventors named alpha after the Greek letter in the classic linear interpolation formula αA + (1-α)B. The alpha channel is normally used as an opacity channel. If a pixel has a value of 0% in its alpha channel, it is fully transparent (and, thus, invisible), whereas a value of 100% in the alpha channel gives a fully opaque pixel (traditional digital images). Values between 0% and 100% make it possible for pixels to show through a background like a glass, an effect not possible with simple binary (transparent or opaque) transparency. It allows easy image compositing. == ARGB == In computer graphics, pixels encoding the RGBA color space information must be stored in computer memory (or in files on disk), in well defined formats. In the most common format the intensity of each channel sample is defined by 8 bits, and are arranged in memory in such manner that a single 32-bit unsigned integer has the alpha sample in the highest 8 bits, followed by the red sample, green sample and the blue sample in the lowest 8 bits. This is often called "ARGB": ARGB values are typically expressed using 8 hexadecimal digits, with each pair of the hexadecimal digits representing the values of the Alpha, Red, Green and Blue channel, respectively. For example 80FFFF00 represents 50.2% opaque (non-premultiplied) yellow. The 80 hex value, which is 128 in decimal, represents a 50.2% alpha value because 128 is approximately 50.2% of the maximum value of 255 (FF hex); to continue to decipher the 80FFFF00 value, the first FF represents the maximum value red can have; the second FF is like the previous but for green; the final 00 represents the minimum value blue can have (effectively – no blue). Consequently red + green yields yellow. In cases where the alpha is not used this can be shortened to 6 digits RRGGBB, this is why it was chosen to put the alpha in the top bits. Depending on the context a 0x or a number sign (#)〔(Microsoft MSDN XAML Color Structure reference ) (XAML/WPF/Silverlight), including #aarrggbb and sc# scA,scR,scG,scB〕 is put before the hex digits.A confusing aspect is that on a little-endian CPU (such as Intel or AMD processors) the byte for B is stored at the lowest address, with the bytes representing the colors are in the order B,G,R,A. On a big-endian machine the bytes are in the order A,R,G,B. This only applies if read bytewise; if read 32 bits at a time the result will be the same in both endians. To indicate how it is to be interpreted this form of ARGB is often called ARGB32, because on both little-endian and big-endian each pixel can be read as 32 bits at a time and then interpreted as ARGB. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「RGBA color space」の詳細全文を読む スポンサード リンク
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