I've had some questions about how "Continue Testing..." was made, so here's a shallow (but lengthy!) overview of its progression from a pencil sketch to an Inkscape illustration and finally to a Gimp painting.

1. Continue Testing started out life as a pencil sketch. This is that sketch.
2. I imported the sketch into Inkscape and traced Chell.
3. I recreated the background using regular shapes rather than tracing. I also put a nice pattern onto the wall panels and made sure that the floor had the kind of perspective I wanted.
4. I added in the door and the glowing signs. I also put a dark gradient on the wall to put emphasis on the doorway.
5. To make sure I was on the right track, I put a temporary light bloom in (I would use this as a reference later).
6. And I popped in some shadow to make Chell look silhouetted (a version of this clipped to Chell's outline became one of the shadow layers in the final image).
7. At this point, I exported most of the elements separately (so that I could use them as masks) and imported them into the Gimp as separate layers. My first step in the Gimp was to put some clouds on the walls and floor. I applied them to each panel separately so that they wouldn't look like a continuation of the same pattern. They're not very distinct in the final image, but they definitely add some much needed variation.
8. Next came some wear and tear type detail in the form of algae and cracks in the panels. The algae was created by painting a rough shape and then using the smudge tool to make it look like it was effected by gravity. The cracks were created using the polygonal lasso tool, filled with some dark red and black clouds, and then painted over with a black grid. I also emphasised the gaps between the panels by painting over them with a fuzzy black brush.
9. With the door layer and its dark gradient over the top, the detail I'd just added looks heaps better. I dimmed out some of the door button lights and painted cracks on them to make them look a bit weathered.
10. With the picture coming together now, I repainted the light bloom and used the smudge tool to push Chell's shadow into it. I also started to paint over Chell's clothes to give them texture (the lighting for the piece of her overalls that I'd painted was wrong here, so I repainted it).
11. I added the shadow layer back on and painted over the top of all of the elements of Chell's body and clothes that I'd exported from Inkscape. I used a combination of burning, dodging and painting with specific colours. I tried to focus on making sure that Chell appeared lit from the front rather than a side or from behind (which I may have failed a little bit at).
12. I darkened Chell's shadow to really bring home the silhouette effect.
13. I rotated the finished image to give it a more energetic feel (this also helps draw attention away from the perfectly straight lines), and I also painted white around the edges of Chell's body that occluded the doorway to help the bloom effect.

When I made this, I knew nothing of Portal 2 beyond the fact that it took place at least hundreds of years after the first game and that the facility was in a pretty advanced state of decay. I felt that in some ways, Chell's situation mirrored the anticipation that we as fans had as we enthusiastically waited to know what awaited us inside the decrepit facility. With that as my vague inspiration, I was aiming to finish a short while before a mysterious countdown timer and title it "8:59" (the timer would have finished at 9:00am Pacific time in the US), but it took longer than that, and the timer ended up revealing some more pre-release stuff (read up on the Portal 2 ARG and the GLaDOS@home project if you're interested in the specifics), so I renamed it "Continue Testing", a phrase that GLaDOS utters repeatedly during the co-op bot trailer.

So there you have it - my technique and inspiration for Continue Testing. The piece also ended up becoming a part of a collection of fan created 'countdown' images (and a follow up video that showcased a lot of them - you can spot a few of our other pieces in there too).

According to the modified dates on my source files, the finished piece was saved 46 hours after I scanned the pencil sketch.