How do you make a bitmap background transparent?

Answer 1

There are Bitmap-mode images, which can only contain solid white and black areas, and there are bitmap "raster" images, which can contain many colors, as in a color photo, and are commonly in CMYK or RGB color modes.

You cannot put transparency in a Bitmap-mode (Image > Mode > Bitmap) image in Photoshop, however, once the image gets placed in another software, for example Quark page-layout software, you may be able to enable a setting that makes the white areas in the image appear transparent. In Quark (a few versions back), you could set the background color of a TIFF bitmap-mode image to transparent, which would make the white in the bitmap-mode image appear transparent on the Quark page. There must be a way to do it in Indesign page-layout software, as well.

If you mean bitmap raster images, such as a color photo that may be in RGB or CMYK color mode (rather than Bitmap mode), then you can make transparency in a Photoshop layer by deleting pixels in a regular layer (not a default background layer). A default background layer will be at the bottom and have a lock symbol next to its name in the Layers panel and cannot contain transparency, so if you want it to, you must change it to a regular layer by double-clicking on its layer name, click ok, then delete pixels on it where you want transparency. I think Indesign will import even the original PSD formatted file and maintain the transparency. In preparation for Quark a few versions back, you could first select the areas in Photoshop that are supposed to be transparent , then Select > Save Selection to save an alpha channel. Save the TIF. Import into Quark, update the image inside Quark and apply the alpha channel. You could have also created a closed path in the paths panel in Photoshop that represented the outline of the non-transparent pixels, change that path to a clipping path in the Paths panel menu, then save as an EPS and include clipping paths, then import into Quark. There may be a better way or more ways with the newer versions of Quark.

In bitmap raster images (not bitmap mode) for the Web, GIF and PNG formats will support transparency. Make transparency in Photoshop layers and select transparent in the Save for Web dialog box.