Start by opening the image you wish to alter. I chose to use the
background image from one of Adobe's tutorial images.
Create a new image with the same width as your image and a height
of one pixel.
Fill this image with black
Now change the canvas size by clicking IMAGE>>CANVAS SIZE.
the height to 2 pixels while keeping the width the same size. Be
sure to force the anchor upward.
You will now have an image with one pixel black and one pixel
a new selection by pressing Ctrl+A.
Now you need to make this selection the active pattern by clicking
Return to your backgound image and create a new layer. Fill this
layer with your
new pattern by clicking
EDIT>>FILL and choosing Pattern for the contents and the blending Mode
There you have it. Now play around with the Mode settings and have