Member variable XbpBitmap():hasAlphaChannel Foundation
Specifies whether the image has an alpha channel.
The :hasAlphaChannel instance variable contains a logical value which specifies whether an alpha channel is present in the image. If :hasAlphaChannel is .T. (true), the raster image uses an alpha channel which means that transparency information was saved for each pixel in the image along with the pixel's color value.
Per-pixel transparency is supported by the following image formats:
|Windows Bitmap||Default Windows bitmap format. Supported in images with a color depth of 32 bits per pixel. See note below.|
|PNG||Portable Network Graphics format. Supported in images with a color depth of 32 bits per pixel.|
The value in :hasAlphaChannel is set by methods such as :load() and :loadFile(), depending on the transparency information found in the image. The value can be changed by the application before drawing the bitmap using the :draw() method, for example, for enabling per-pixel transparency for the raster image.
Note on Windows Bitmaps: Many graphics packages do not support per-pixel transparency in Windows Bitmap files. Consequently, the information stored in the alpha channel of such image files is unreliable and cannot be used to determine whether the image actually uses the alpha channel. For this reason, :hasAlphaChannel is always .F. (false) after loading Windows Bitmap files using :loadFile() or :load(). However, applications can assign .T. (true) to :hasAlphaChannel prior to output via :draw() for Windows Bitmaps known to have an alpha channel.
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