When using transparent fills, you have to know that there's a close relationship between Transparency values and output image formats.
Keep in mind the following rules regarding this topic:
Totally Transparent fill (Alpha channel is 0) Both -and only- PNG and GIF image formats support it.
Semi-Transparent fill (Alpha channel is between 1 and 254) Only PNG image format support it. Keep in mind that transparency is supported by PNG format only if the ColorDepth property is set to Format32bppAlpha; otherwise a black background will be rendered. Please, read some considerations regarding PNG Transparency in Microsoft Internet Explorer browser and ImageDraw controls.
For JPG/JPEG and BMP formats, transparency is not allowed and if a transparency value is set, a black background will be rendered.
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