– July 5, 2010

I showed you already in the past some photos of this wonderful town in the South of Germany, Freiburg. Here is now my second part. Beautiful street scenes at a nice and sunny summer day.

But, what doing if you are at a wonderful place but the weather is bad. Coming back again, when the weather is better? Sometimes not an option. In such a case I would replace the sky.

On a photo of a overcast day the colors are not so nice and the sky looks terrible. One advantage of HDR is, that it is possible to get some of the colors back (see here my approach). It would be of course better to do it on a sunny day, but if that is not possible I would highly recommend to do HDR and hope to get more colors back in the post processing.

To replace a sky there are several methods out there (One very easy way to do it is using the new edge refinement tool of Photoshop CS5. But this is not all the time perfect and you can get in trouble and maybe you don’t have CS5). I personally prefer a combined method of blending and masking.

Here is my initial photo to demonstrate my procedure

First, I blend my HDR version with my best normal shot (very often 66% HDR / 33% normal). This gives the shot a more realistic looks. You can see that I mask out the HDR shot where moving people are in the picture to eliminate ghosts. To ensure a smoother blending of both shots in this areas. I adjust the ground layer (my normal shot) to the lightness of the HDR shot by means of an adjustment layer.

My photoshop file:



Next, I clone out the wire in the sky (fourth layer from the bottom). The next layer is a general adjustment layer to increase the contrast. The photo till now has a nice street scene but a boring sky. Now start the really sky replacement.

Channel View:



To get a nice mask of the sky, I switch to the channel view and duplicate the blue channel. Because an overcast sky is mainly grey or slightly blue, this channel is almost complete white in the sky and a good staring point for a sky mask. In the next step, I need to make everything that is dark grey turn into black and everything that is very light turn into white. I use for the the dodge and burn tool.

Apply the dodge tool to the highlight and brush over the fringes of the white sky makes every this that is lightly grey completely white. Apply the burn tool to the shadows and brush over the houses and street scene to make everything that is dark grey pitch black. The result is my layer mask 3 and will be my mask for the sky.

Final photoshop file:



I use this layer mask to my sky layer (top layer in my photoshop document). Here I could stop my tutorial for replacing the sky. But very often this technique still results in some fringing around the houses, although the mask is almost perfect.

For better fringes I use a blending mode technique. An overcast sky is usually lighter than a blue sky, thus if you use the “darken” blend mode for the sky, you get a perfect fit between houses and sky. If this blend mode doesn’t work perfectly, you can improve the result, if you just increase the lightness of the overcast sky (second top layer).

Here is now the final photo.



I hope that blog entry was helpful for you. Please let me know what you think. Enclosed some further shots from Freiburg (with replaced sky).



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