Transparency (graphic)

Most vector formats implicitly support transparency because they simply avoid putting any objects at a given point.

Outside the world of web browsers, support is fairly hit-or-miss for transparent GIF files.

Some image formats, such as PNG and TIFF, also allow partial transparency through an alpha channel, which solves the edge limitation problem.

A major use of partial transparency is to produce "soft edges" in graphics so that they blend into their background.

Partial transparency can also be used to make an image less prominent, such as a watermark or other logo; or to render something see-through, such as a ghostly apparition in a video game.

Animating the alpha channel in an image-editing program can allow smooth transitions between different images.

The process of combining a partially transparent color with its background ("compositing") is often ill-defined and the results may not be exactly the same in all cases.

While some transparency specifications are vague, others may give mathematical details of how two colors are to be composited.

Some consequences of this are: This is important as it means that when combining three or more objects with this rule for transparency, the final color depends very much on the order of doing the calculations.

Human perception of brightness is not linear - we do not necessarily consider that a gray value of 0.5 is halfway between black and white.

An alternative model is that at every point in each element to be combined for transparency there is an associated color and opacity between 0 and 1.

To work with the above formula, the opacity needs to be scaled to the range 0 to 1, whatever its external representation (often 0 to 255 if using 8 bit samples such as "RGBA").

PDF transparency is a very complex model, its original specification by Adobe being over 100 pages long.

A key source of complication is that blending objects with different color spaces can be tricky and error-prone as well as cause compatibility issues.

However, by the use of patterns, arbitrary graphics can be painted through masks defined by any vector or text operations.

In addition, this too often reached implementation limits, and few if any application programs ever offered this technique.

A semi-proprietary technique pioneered in Photoshop and adopted by a number of pre-press applications is to store a clipping path in a standard location of the EPS, and use that for display.