Photos To Textures - Part 2
Fixing Perspective Errors

What is Perspective Correction?

When photos are taken at an angle to the subject matter, edges that should be square, are not. An example is taking a photo of a skyscraper, with the camera pointed upwards. Photos like this can't be used as seamless tiles until they are "squared off", and altered as if they were taken from a point perpendicular to the building. This is called perspective correction.

Original Photo

Perspective corrected, cropped, and ready to be made seamless

Here is a photo I took for "City and Urban Exteriors: Volume 1" texture CD. It's a very interesting and modern roof, that could be used not only as a roof, but as a spaceship hull, industrial complex walls, floor, ceiling, or probably many others that only you will think of. However, before we can use this as a texture in our futuristic vision of post-apocalyptic earth, we need to correct the perspective, so that we can then make the texture a seamless tile.

If you've got Photoshop 7, then read this section. If you've got Photoshop 4,5 or 6, skip the next section. If you don't have Photoshop, then skip to your nearest computer store and buy it, as you'll be needing it if you want to gain total control over your texture maps.

Photoshop 7: Select the the crop tool, and check the "perspective" option, making sure all other fields are cleared. The perspective option allows you do not only crop, but correct perspective in one operation. Very neat! The trick is to look for features in the photo that should be square in the final texture, then drag each vertex so that the cropping area encloses this rectangular section. In this image, Roughly drag the four corners of the crop tool around these lines, thinking of how the texture will tile when laid end to end. For instance, if you align the cropping line to the centerline of the roof seam on the left side, then align the right crop mark to the center of the roof line, too. This way, it will match up better when you go to make the tile seamless (another lesson).

Next, I zoom in to an extreme level, while still in crop mode. To do this, hold the ALT key while using your mouse wheel, as a shortcut to zoom while inside of the crop tool (Windows - can anyone provide me with the Mac shortcut to zoom in while in the crop tool? Send to carl@carlsart.com).

When zoomed in to an extreme level, in this case, greater than 700%, I can adjust the cropping marquee to perfectly align with the lines of the roof. When satisfied with my selection, I simply hit enter, and voila! A square image, which would have been impossible to photograph, unless I was in a hot air balloon, or helicopter.

Photoshop 5 or 6: you wont have the perspective correction option inside the crop tool, so the procedure is slightly different: Display the ruler CTRL (Windows) or CMD (Mac) R, and drag two vertical guides inward to where you will make your selection.

Then, using the marquee tool, select a square area around the part of the image you wish to crop to. Then hit CTRL/CMD-T (Free Transform). While holding the CTRL/CMD, drag each vertex such that vertical lines in your selection, the roof lines which should be vertical, become parallel with the guides, then hit enter when done.

When satisfied that your vertical lines are vertical, and horizontal lines horizontal (drag out some horizontal guides if you need to), hit ENTER to commit your transform. Then use the crop tool to crop away the excess, leaving a texture which is ready to be made seamless. I'll teach this in a separate lesson.

Save $100 off "City and Urban Exteriors!"
I screwed up! So, I'm giving you, my loyal newsletter subscriber, the deal of a lifetime. I made a slight error when pressing the first run of CDs, and it turns out the HTML graphical guide doesn't work for MAC users on these CDs because the filenames all have greater than 8 characters. This is actually a benefit for PC users, as it makes identifying textures from their names quick and easy. All textures are 100% usable, it's just that MAC users can't use my custom graphical browser (many don't use this anyway, they use their OS to browse the files). For PC users, the CD is 100% functional, it's just like the $147 version. Due to my screw up, I'm letting this lot go for only $47 each, and once they run out, the price goes back up to $147.

This offer is NO LONGER AVAILABLE

• How to hand craft realistic bump maps
• Using alpha channels vs. transparency maps
• Creating Textures: Color Correction
• Making your textures seamless
• And whatever you want to learn - just ask me, and if it has wide enough appeal, I'll develop a tutorial for you!
  

Your feedback is critical, as it drives the content on this site. Did you find this tutorial useful? Was it too advanced, too simple, or just right? What would you like to see here next? Send your ideas to me at carl@carlsart.com.