Creating the “sky” layer and its gradient

Let's create a new layer. In the Colors window, create a blue color with R = 212, G = 232, and B = 255 (1). Invert the Primary and Secondary colors, and select the white in the palette (2). We have now white as primary color and sky blue as secondary color.

Define colors

Choose Gradient in the Tools window. By clicking at the starting point (bottom) and then moving up while holding the button down, the gradient appears, contained in the selected area, white down, blue upwards.

Tools Gradient

Now, to avoid a too sharp cut out, after deselecting the sky, let’s do Effect / Blur / Gaussian Blur / 2 (exact value to test, say 2 to 4 depending on the image size, but 2 most often gives excellent results), which has the effect of blurring the “sky” limit by blending the blue pixels with transparent pixels.

Effects Gaussian Blur


Applying the gradient, then the Gaussian blur

Save to PDN

For safety reasons and also to reserve the possibility to go back on modifications, it is better to save the complex image to the proprietary format PDN. This can be used, for example, if the mask is subsequently discovered incomplete and the “sky” pierces the object being photographed!

“Flattening” the image

Now we just have to delete the Mask layer, and to merge the Sky layer into the Background layer.

Layers Delete Merge

We can also simply do Image / Flatten, but then we get no control on the order in which the mergers or deletions are going to take place (it may have its importance).

Effect of the blur applied to the sky: if you zoom in on the limit between the model and the sky, you will see that this limit has a gradual, not too sharp transition, which would betray the “montage” and would be not really aesthetic.

Blurr effect

General view enlarged 2 times.
“Magnifiers” × 4; at the top, without blur: the aliasing is visible; at the bottom, with blur.

Save to JPEG

Let's finish by saving to JPEG. Here is the result.


Final picture

Note: for car photography, as here, it is a little more complicated, because you must reserve a different treatment to the windows. They are certainly transparent, but absorb some light; it is therefore necessary to darken a little the light crossing them. However, specular reflections, ie reflected by a very smooth surface like water or glass, must be kept. That's what I did here (see the windscreen).

This is the end of this little trip in photography. I don’t pretend to hold THE truth, but only some experience. I do not claim to have explained everything in a clear and exhaustive way! I left you a little work… Enjoy!