This transform sets alpha values of pixels with a given color (and tolerant ones) to transparent. This transform is very useful when you need to blend two images and "upper" image does not have alpha channel, but it has a background color. For example, you draw a logo placed in the center of the image on white background. In this case you set BackgroundColor to white color. You can also adjust a Threshold parameter to make colors close to white color semi-transparent (to get smooth colors). Besides of that Threshold is useful when background color is not completely uniform (for example, you loaded JPEG file with low quality).