PPT2000: Semi-Transparent PNG Images Appear Dithered in PowerPoint

ID: Q212420

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If you insert a Portable Network Graphic (PNG)image in a PowerPoint slide, any semi-transparent portions of the image appear grainy (dithered). If you examine the image closely, the semi-transparent areas of the picture appear to be drawn by alternating pixels of the foreground color with pixels of the slide background color.


A PNG file is a type of bitmap. In other words, the picture data is stored as a rectangular array of dots of different colors. Each dot is described as a mixture of 256 shades of red, 256 shades of green, 256 shades of blue, and (optionally) 256 shades of transparency. The transparency component is called an alpha channel.

If you import semi-transparent images into a photo composting program (such as Microsoft Image Composer), you can layer them to achieve various special effects. For example, if you place the image of a semi-transparent wine glass filled with red wine in front of the image of a gift box, you can still see the gift box through the wine. It just has a red tint to it.

When you import a picture that contains transparency information, PowerPoint generates a mask, which substitutes the slide background for the masked pixels. This mask is not semi-transparent; it either shows or hides (masks) a pixel. To simulate semi-transparency, PowerPoint generates a mask that hides alternate pixels in a semi-transparent area.

NOTE: These images will display with a higher level of fidelity during the slide show.

Additional query words: ppt9 ppt2k clear pattern moire interference Portable Network Graphic Alpha Mask broken fuzzy blurry

Keywords          : kbdraw 
Version           : WINDOWS:2000
Platform          : WINDOWS 
Issue type        : kbinfo 

Last Reviewed: June 28, 1999