Ludovic Oddos

The bitmap files can differ on two or more machines. The bitmap outmap is generated with glReadPixels OpenGL command. 
glReadPixels returns pixel data directly from the frame buffer. With this, the pixel data can differ because the graphics card and the driver is not the same.

So, there are two ways to handle the issue:
1) Generate the reference images on the same machine where the test is executed. (In case developer machines have different hardware/driver configurations)
2) Use computers that matches in hardware and driver configuration for the developers.