Fake billboards with GIMP

Ever wanted to see your name in lights? In this tutorial you will learn how to replace a billboard image with your own. Below is an example where I have replaced a billboard with the logo and name of my site "Bemmu". I wish it was real!

Finding the victim

First of all we need a source image that has a billboard in it that we can replace with out own. A nice source of images is Flickr's Creative Commons. Especially images under the attribution license are nice, because they can be used in any way you want as long as you give credit to the person who took that image. Below is an image taken by justinbaeder that I found.

This is a very simple case, since there is little lighting to worry about. In the image I showed in the beginning I had to worry about light reflecting from nearby billboards and uneven lighting, but this one seems like we can just replace the image with a new one transformed using GIMP's perspective tool. The billboard here even has nice black borders, making our life a bit easier.

Measuring the billboard image with unnecessary accuracy

Just open up GIMP and create some image to replace this with. But how big should that image be? It's probably safest to choose a relatively high resolution, but what about the aspect ratio, what should the relation of width and height be? Well, you could guess, or we could try measure it, since GIMP comes with a measuring tool.

I get the result 153 x 122 pixels. Actually I know that this must be a bit wrong, because of perspective. The ratio from this would be 1.254:1. I was curious how wrong I was, so I decided to do a small transformation with the perspective tool to get the billboard facing us directly, and then measure again. Not because this is necessary, but because playing with the perspective tool is such fun :)

I selected the perspective tool, then dragged the four endpoints of the plane around with my mouse to try and align the poster so that it would face me. I also had to make sure it looks correct, as I could easily create an error in the aspect ratio while doing this.

This time I get 120 x 73 pixels, so 1.644:1. The difference is quite big, but I am more confident with this later result, so I will create a fake billboard image of 1644*1000 pixels in size.
1644 x 1000 pixels of fakeness

Not wanting to create an entirely new design for the billboard, I decided to stick with yellow gradient text on green background as in the original. I did not happen to have a wild west style font in GIMP, so I went with "Impact Condensed", which at least has a similar size (most fonts tend to be quite wide).

I just wrote the text I wanted with a really large font, then created a path out of it by clicking "create path" in the text options, then created a selection out of the path by using "select > from path" in the menu. I filled the selection with black (for the border), then shrunk the selection by a few pixels and filled the selection with a yellow gradient. I added another text below with the text tool.

Replacing the original billboard with our fake one

Thanks to GIMP's perspective tool, this part is very easy. You should now have both our fake image and the original one open in GIMP. Create a new transparent layer on the original image. Now copy all (CTRL-A "edit > copy visible") from the fake one and paste it on the new layer in the original. It will be huge, but don't worry. Next select the perspective tool from the menu (or click Shift-P). You can now move the four point of the rectangle around. You may not see them because the image is very big, but you can still grab then. Just try dragging your mouse around the image and you should get the hang of it. One annoying thing you may notice is that the source image is not visible in the background. You can make it visible by going to layer (CTRL-L) and de-selecting the visibility (the eye icon) of the floating selection.

Now you should be able to see the background while you work with the perspective tool. Try to drag the endpoints so that they match the original billboard's endpoints as well as possible. It is probably helpful to zoom in when doing this (select 400% for example from the lower left). When you are finished, resist the urge to click on the "transform" button yet! First from the perspective tool's options, make sure you have selected "interpolation: cubic (best)" and "supersampling". Now, after clicking on "transform", you will see... your work completely vanishing. That is because you still have the visibility of your floating selection disabled. Go to layers (CTRL-L) and click the eye back on. Now you can see the result? Does it look good? Maybe not. It is tricky to get a good match, you may have to attempt it a few times.

Your problems are very likely in the borders of the billboard. If you get a result such as the one pictured on the left, it may be easier to just fix it by drawing in some borders with the paint brush (you can draw lines with the paint brush by holding down shift).
Final adjustments

I drew the borders with the paint brush tool, then adjusted the hue, lightness and contrast a bit so that the billboard would pop out a bit less (even though the original does). Added slight blur. Also used the dodge tool to lighten up the upper part a bit. The end result can be seen below.