ImageGear for C and C++ on Linux v18.10 - Updated
Transparency
User Guide > How to Work with... > Color > Transparency

Transparency allows you to specify a palette index or a color to be transparent. When the image is displayed, all pixels having this index (color) show background rather than the pixel's color.

Transparency requires very little storage space in the file - only a few bytes. However, the pixels can't be semi-transparent; also, transparent color occupies a palette entry or a color, which reduces the possible range of colors for the image. Alpha channel requires significantly more storage space in the file, but it does not have the limitations listed above.

ImageGear provides transparency support for the following file format filters:

Format Support
GIF Supports transparency for Read and Write.
PNG Supports transparency for Read and Write.
XPM Supports transparency for Read and Write.