Superagentes: Day to Night

Introduction:

During the production of Superagentes Nueva Generación, we received some extra fx shots that weren’t planned on the script, or that for some reason the producers decided to solve in post-production. In this case the outside take of the villain’s house was only shot during the day, but there was a scene that required a night shot. The solution: convert the daylight shot to night with compo.

Arbol de nodos completo

Fig 1: Complete Node Tree

Fig. 2: Sky mask

The first thing I needed was a mask to be able to separate the sky from the image. The general light balance plus the cloudy, low contrast sky didn’t allow me to do a quick mask based on color or luminance. That’s when I decided to use the good old bezier curves, that picasus rotoscoped and animated for me, following the contours of the house and trees.

Usually, it doesn’t matter how much time you spend doing your masks, you will end up making some extra adjustment, specially to improve the borders or to hide some error (yes, there’s a lot of lying and hiding in the compo world). A dilate/erode node to expand or contract the mask a couple of pixels, then a small blur to soften the edges (depending on the type of mask, some times you can render them without OSA and soften with blur). In this particular job there was no time for small details, so even if the resulting mask wasn’t the ideal, it worked.

Fig. 3: First color correction

With the mask ready, I was able to start with the color correction, lowering the global intensity and grading the colors towards an afternoon-like palette (Fig. 3); at first I wasn’t sure how much dark did they wanted the night-shot to be (I only had a couple of low-quality references from the same scene, but from another place), so I decided to keep an intermediate light set between day and night, just in case I had to adjust something later.

Fig.4: Sky color correction

As for the sky, after a couple of trial and error, I decided to invert the colors with an RGB curve (always with the clipping off so I don’t loose color depth) and adjusted the gain to balance the colors and take them to a darker sky (Fig. 4).

I didn’t care about the other elements in the image because I multiply this result to the image in Fig. 3 using the mask as a factor (Fig. 5).

Fig. 5: First color correction + darkened sky

With the sky darkened and re-composed to the shot, and the first color correction done, it was time to start taking the afternoon to a night scene.

In my experience, this kind of work don’t have an automatic solution or a universal node configuration that I could re-use from another project: every image is a different world that must be solved in a particular way, depending on the predominant colors, light intensities, etc. Of course you can always try a pre-existent setting if the shots are similar and then make some adjustments for the parts that doesn’t work. I find it much more usefull to start from scratch than trying to understand a composition I made six months ago ;-)

Fig. 6: CC 2 - removing reds/magentas

The first thing I did was to remove the reds/magentas that the sky multiplication produced (Fig. 6). I could have done this using different tools but I got a nice result using the Palette node in Division mode, RGB colorspace.

The Palette node was my workhorse during the production of Superagentes, because it allows you to work in different color spaces and with different operations, what makes it a really interesting color corrector.

Fig. 7: Tint

The next step was to tint the whole image with a strong blue (following the cinema convention of the blue night) that later I use to colorize the image.

Again I used the Palette node, but this time multiplying, and a Gain node (Fig. 7).

I combined this tinted result with the second color correction using a mix node in color mode so I could raise or lower the amount of tint if it was necessary later.

Fig. 8: Contrast and gain adjustments

Then I added a couple of nodes to fine-tune the contrast and gain (Fig. 8). I always try to leave mix nodes that allow me to quickly fix important changes that the director could ask for.

Since in this scene the villains are inside the house, we had to turn on some lights.

Fig. 9: Creating the fake on windows

I started with the basic: a small model of some of the contours and windows, separated in individual render layers so I could adjust borders, glow, etc. for each one.

I used a displacement node to create a colored image with the walls details and then combined it with the window masks to simulate the light comming from inside the house (Fig. 9). To give color to this kind of effects I use color ramps and I try to not leave pure white because the screen and add nodes give better results if there is a small tint (unless the desired effect is to burn the whites).

Fig 10: Combining the lights and final adjustments

Finally I combined the fake-on-windows with the last color correction using screen nodes, and did the last gain adjustments to achieve the final night look (Fig. 10).

There are a lot of things that could have been improved, like the masks borders, or parts that could have been separated and worked in more details, but the deadline didn’t allow us to do it. All in all, the final result is at least correct.

Split view comparing the original footage and the final composite

For this project I used a couple of nodes that are missing from the official Blender trunk or are improved versions. We’ll post the patches here as soon as possible. The nodes are: