How do I change the background of a bitmap from white to grey in Inkscape (or other open source photo editor)

Question

Here is a nice picture of a 4-d hypercube "sliced" into 3d and then projected onto the 2d plane.

Unfortunately its background is white and I'd like to use it on a 10% grey background:

How can change all the white to 10% grey, while (hopefully) retaining any anti-aliasing?

Answer 1

The quick way would be to use gimp to make the background of the image transparent, then it won't matter what colour your background has.

1. Add an alpha channel to the image:

2. select the colour to alpha option:

3. choose the background colour, white in your case

4. Image with transparent background:

Answer 2

You can't edit bitmaps in Inkscape. Inkscape is a vector image editor.

You could redraw the graphic in Inkscape however. It's not too complicated and shouldn't take too long.

All it consists of is two copies of the hypercube, one with a red stroke, one with a blue stroke on a layer above, with the blending mode set to "Darken". The blue layer is then distorted by selecting and editing the nodes.

In this example I created the hypercube by manually tracing over the bitmap image.