Corner Perspective
I'll try to present this study as it was presented to me eons ago by an elementary school art teacher.
The best way to get a handle on perspective is to look at what you're trying to draw. That may sound a little strange but we often look and don't see. Take a chair and put it in the center of your room. Turn it so you're facing the corner. From where you sit you will see that the line where the ceiling meets the wall seems to go up from the corner as it comes toward you. The line where the floor meets the wall seems to go down.
That will be our starting point.
First right click on the two images below and save them to your disc. You will need them both for parts of this study.
1. Open a new transparent image 350 x 350 pixels. Flood fill it with any light (not white) color. Color doesn't matter right now. We can change it later.
Hide this layer (layer 1). Add a new layer.
2. Set your foreground color to black and background color to null.
With your draw tool 
set as below
3. Add a layer (layer 3) and draw a line with the same settings from 175,100 to 350,70. 
If it's a bit jaggy don't worry about it right now. We'll fix it later.
Duplicate this layer and go to Image>Flip.
Hide layer 2.
On the layer palette right click on the duplicate layer and merge visible.
4. Duplicate the merged layer and go to Image>Mirror.
Un-hide all layers and merge visible.
You should now have 1 layer with the basic perspective shape for the corner of your room like the image below.
NOTE: The angles we have used here can be changed to whatever you want.
To see the effects of changing the angles go HERE
5. Click on the ceiling area with your magic wand. 
Add a layer and name it ceiling. Flood fill the selected area with white.
Deselect it.
Don't try to fancy it up just yet and don't worry about the black line showing.
6. Open the image "myfloor.jpg".
Set your foreground to null and your background to the myfloor.jpg image with the following settings:
Go back to the merged
It's pretty close but the floor board perspective needs a little work. To correct it do the following.
6. Deselect the floor area.
Click on the maximize button to give you more working area.
Click on the deformation tool 
Hold your shift key down and pull down on the lower right node until you get the angle of the boards lined up with the lower slanted black line on the right wall.
You'll probably need to go to the center node on the left side and pull it to the left (without using the shift ket) to adjust the width to cover the floor area. You may also need to pull down on the center bottom node to get the floor area completely covered. Don't worry if some of it is outside the black lines.
Double click to accept the deformation then go to Image>Sharpen>Sharpen.
6. Now go to your base layer and select the floor area with your magic wand. Go back to your floor layer. Invert the selection and hit the delete key. Deselect.
Your floorboards should now have correct perspective.
We've now got a ceiling and a floor. Let's go to work on the walls.
© Griffin Images 2001