Subject:Masking Images
Date:2003-10-17
Rev:0.1
Sometimes subject areas that look fine on the original images can block up when the images are stacked. You can resolve this problem by masking the images before adding them. The degree of masking (or, how dense the mask should be) is a hit and miss affair hence the previous pages warning and will probably require a number of trials before just the right amount of masking is achieved. Fortunately making masks is relatively easy in Photoshop, especially when compared to doing them the old fashion way, in the darkroom!
To begin, open one of the images to be added and make a copy. With the copy active go into the change the image to Grayscale. Next open the Image menu and go to Map->Invert, this will produce a black and white negative of the original image. At this point experimentation starts. The idea is to lower the contrast on the mask until its effect is only on the desired subject area of the original color image. The mask is altered by first opening the Image menu and proceeding to Adjust...Brightness/Contrast and using the Contrast slider to lower image contrast. How much? Each image is different, you will have to experiment! Next the mask needs to be blurred to such an extent that all stars are rendered invisible or nearly so. To do this, use the Blur tool under the Filter menu. Note that the brighter stars may have to be removed using the pencil, paintbrush or eraser tools. Once the mask is completed then it is applied to each of the original color images using the Multiply funtion in Apply Image (there is no need to make a mask from each of the originals), finally the color originals are added to one another.
This brings to the end my brief look into the world of manual stacking and registration. I have found the methods in these documents to be more reliable and give better results than any of the image stacking applications we can now download. The human eye is far better at judging the result than a quadratic equation.