Solving Common Problems with Sky Stitching

Today I am going to do another post-processing tutorial for sticking a dark, stormy sky onto a sunlit foreground. I have covered the procedure of how to do sky-stitching in a previous post so I won’t go into this detail again. In this post I want to look at some of the problems you can encounter doing this and how to try and overcome them.

The starting picture is of the Tay Rail Bridge in Dundee. It was taken in bright sunlight during the middle of the day.

IMGP0921.jpg

I like this image because the bridge supports are well illuminated whereas the underside of the arches are in strong shadow. Most of the other sections of the foreground are also quite dark so I want to make it look like a shaft of light is hitting these supports.

Brightly Coloured

The first thing I usually like to tackle when doing this is the problem of colour. The bright, daylight sun shines white light onto everything meaning the colours are rendered as they appear and are not tinted by the light source. This type of light also tends to increase colour saturation.

Although I have not stitched on a new sky yet I have a good idea of the type of sky I am going to use and because of this I have some idea of what I want to do with the colours in the image. It may seem odd doing this at the start of the workflow before the sky is attached but I find trying to to do large scale changes to the colours later on to be more problematic as I find it harder to reduce clashing complimentary colours.

As I am going to make this dark and stormy looking, the sky is going to be very grey. Although grey is a neutral colour which sits well with every other colour, when the weather is dark and stormy with a grey sky, the resultant colours tend to be very muted. This will not sit well with the brightly saturated orange bridge supports. Thus, the first thing to do is pull down the colour saturation across the image.

In addition, I find that tinting another colour in helps to unify the overall colour scheme and prevent from clashing colours. Thus, I tint a small amount of sepia into the highlights. Here is the result. The colours are far more muted and should sit better next to the new sky when it is attached. This is not the end of all the work I will do on the colour but it is a good starting point for sticking on the new sky.

IMGP0921-2.jpg

Colour is a hugely complicated subject which I will talk about at more length in future posts. However, if you are doing dark, stormy images, desaturated and unified colours tend to work well with this.

Exposure and Contrast

Before I stick on the new sky I also like to make some adjustments to the exposure and contrast. I mainly do this early on because I initially work in Lightroom which works on the RAW files and these are more forgiving to exposure changes than the jpgs which Photoshop uses. However, Lightroom won’t allow you to stick on a new sky so I have to import it into Photoshop for that.

Anyway, the image looks a little flat so I boost the contrast a bit. Also, the bridge supports which are to be brightly lit up in the final image look a little dull, so I boost the exposure slightly.

IMGP0921-3.jpg

Cutting out Complex Objects

Before I can stick on the new sky I have to make a selection of the existing sky. Although this may look straightforward around the arches of the bridge there are some complex objects on the top. The bridge has some maintenance work going on so there is a load of scaffolding on it which will cause a problem cutting out the sky.

IMGP0921-4crop

You could, in theory, wrap the selection around all these projecting poles of the scaffold but this would take a ridiculous amount of time cutting it out and refining all the edges. Thus, it is best just to slice most of it off.

When making selections around complex objects such as this it is really a trade off between time and realism. If it is a massively complex object, it may be worth just cutting it off. Although this will not make it look as realistic, it may be passable and most wont notice. As such, it is finding a balance that works best.

Here is the bridge with the sky cut out, and below it is the detail around the scaffold.

IMGP0921-Edit3IMGP0921-Editcut

Is is not a perfect cut out by any means but it will be convincing enough when the new sky is on.

Refining Edges

Now to stick the sky on and see how it looks. Here is the section of sky I have chosen.

IMGP0921-Edit4

The reason I have chosen this section of sky is because 1. Its dark and stormy. 2. It has a light patch in the top right corner which will simulate the light hitting the right side of the bridge, and 3. Because it is taken at the same ultra wide focal length (10mm) as the bridge so it looks accurate.

Here is the image with this sky stuck on.

IMGP0921-Edi5t.jpg

Even with what I believed was an accurate selection, there are still edge problems. The left hand side of the closest bridge support, the top of the bridge in the distance, and the land way back in the foreground all have white fringing which looks poor. Thus, these edges will have to be refined further.

When working in Photoshop, the initial selection becomes a layer mask on the new layer. Thus when refining these edges I find it best just to alter the layer mask directly using the paintbrush tool. This can be done freehand or you can use further selections to help constrain the part of the mask affected, it is up to you.

Anyway, what I am looking for when doing this on such a complex object is something that is passable to the eye. It would take a very long time to get every edge perfect so it is best just to take care of the glaringly obvious edges and just leave the rest if they are not too noticeable.

Here it is with these edges refined.

IMGP0921-Edi6t

Now that the sky is stitched on it is now time to look at making two separate images look realistic together. Remember, the foreground image was taken in bright, daylight conditions and the sky image was taken in dark, stormy conditions so unless I am very lucky (which I am not in this case) there is work which has to be done to make them balance and look realistic together.

Exposure Balancing

The first place I start is with exposure balancing. For this, the exposure levels of the two images not only have to look realistic together, but also have to be at the right level for the artistic vision you want to create in the image. In this image, I want a very dark sky and the bridge supports quite light to simulate direct illumination.

The first problem I notice is the sky is too light so I have to darken it down. However, I want to leave the light patch in the top right corner to simulate the light source.

IMGP0921-Edi10t

Now the sky looks fine but the bridge does not look realistic next to it. Although I want the bridge supports to be brightly lit, the inside faces of the supports appear far too bright when sitting next to the dark sky. I therefore darken these down along the left hand sides to help them sit more comfortably next to the dark sky.

IMGP0921-Ed12it

That looks a lot better. When having to do exposure tweaks like this to small parts of an image, it can be really tricky to track down the problem areas and how to deal with them. However, a good tip is if something looks out of place, try lightening or darkening it until it is more harmonious with what surrounds it.

Colour Balancing

Although I did a lot of work on the colour at the start, it still doesn’t look quite right. It looks quite good but just needs a tweak. Although the red / orange colour of the bridge supports provides a nice contrast to the steely, blue /grey sky I still feel it is too much and looks unrealistic for the conditions I am trying to simulate.

Therefore, I desaturate the red / orange in the bridge supports further and tint some yellow into the shadows and midtones of the image to help neutralize the blues in the sky and unify the overall colours further.

As I said, I prefer doing the large scale colour alterations at the start of my workflow but you could leave them until now if you wish. I just find it easier to do it earlier as I don’t have the problem of huge colour clashes later on.

Anyway, here is the image with the colour changes.

add-yellow-desaturate.jpg

Finishing Touches

I am now happy with the exposure levels of the images and the colours in them, and I think they blend together quite realistically.

So now it is just a case of few finishing touches.

First, a contrast boost to enhance the drama in the image.

contrast.jpg

Next, I vignette the edges to draw attention into the center of the image. This also to helps darken it down to increase the overall dark, dramatic effect.

vignette.jpg

Lastly, I add some sharpening to the foreground to help it pop more. I also slightly lighten the area of water to the side of the bridge as this looked at bit flat.

sharpen-lighten-water.jpg

And that is the image finished. I feel, and hope you do to, that these two images are blended well together and create a dark, dramatic image that looks quite realistic.

Conclusion

Unfortunately, when blending two images such as this, every composition is unique and brings with it it’s own set of unique problems. I hope in this tutorial I have been able to provide some general advice on how to tackle some of the most common problems and get the different images looking realistic together.

Thanks for Reading,

Neil

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