How to test if a bitmap file has a transparent background?

Question

I created a bitmap with transparent background but in the Windows Picture Viewer I see the white background and even if I reopen it in Photoshop CS3. how do I make sure the white colour in bitmap is really transparent or see if contain any transparent colour?

Answer 1

Bitmaps (i.e. files with .BMP extension) do not natively support transparency: you need to save as a different format like PNG. Another format that supports transparency is GIF but it is only suitable for simple images with few colours. The best format depends on the image and where it will be used.

If the background is transparent then in Photoshop and most other image editors you would normally see a checkered background made up of small white and grey squares. But the Windows Picture Viewer itself is no good for testing transparency - it shows transparent areas as being white.

Answer 2

BMP's do support transparency. Bitmap must be set to indexed 256 color, and pixel at (0,0) is for the color that you want transparent.

Answer 3

If you have Office, start a new presentation using something other than the default blank template, then insert your picture (on the Insert tab in 2007/2010 or Insert | Picture | From File in earlier versions). If you can see through the (supposedly) transparent bits of your picture to the background behind it, then the image does have transparency. Otherwise, no.

This assumes that the image is in a format that PPT can insert (BMP, GIF, PNG, JPG etc).