History behind 7-bit alpha component?

Toster Cx

Found this cryptic note in the source:

/* gd has only 7 bits of alpha channel resolution, and
 * 127 is transparent, 0 opaque. A moment of convenience,
 *  a lifetime of compatibility.
 */

May or may not have something to do with this:

While most people are used to an 8-bit (0-255) alpha channel, it is actually quite handy that GD's is 7-bit (0- 127). Each pixel is represented by a 32-bit signed integer, with the four 8-bit bytes arranged like this:

High Byte Low Byte

{Alpha Channel} {Red} {Green} {Blue}

For a signed integer, the leftmost bit, or the highest bit, is used to indicate whether the value is negative, thus leaving only 31 bits of actual information. PHP's default integer value is a signed long into which we can store a single GD palette entry. Whether that integer is positive or negative tells us whether antialiasing is enabled for that palette entry.

2014-10-01T15:38:59+00:00

Comments (2)

Toster Cx
Found this cryptic note in the source:
```
/* gd has only 7 bits of alpha channel resolution, and
 * 127 is transparent, 0 opaque. A moment of convenience,
 *  a lifetime of compatibility.
 */
```
May or may not have something to do with this:

While most people are used to an 8-bit (0-255) alpha channel, it is actually quite handy that GD's is 7-bit (0- 127). Each pixel is represented by a 32-bit signed integer, with the four 8-bit bytes arranged like this:

High Byte Low Byte

{Alpha Channel} {Red} {Green} {Blue}

For a signed integer, the leftmost bit, or the highest bit, is used to indicate whether the value is negative, thus leaving only 31 bits of actual information. PHP's default integer value is a signed long into which we can store a single GD palette entry. Whether that integer is positive or negative tells us whether antialiasing is enabled for that palette entry.
- 2014-10-01T15:38:59+00:00
Pierre Joye
1 bit was used for the antialias part, bad design years ago. full 32bit support (and other coming), stay tuned :)
- 2014-10-01T15:40:07+00:00
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