Extracting Blue Sky from Between Weeds

[Lobalobo]

To practice technique, I'm working on an image of a person standing in tall weeds against an (almost) uniform blue sky. I want to clone the person out of the image leaving just weeds and blue sky. Because the sky is almost but not quite uniform, cloning out the person always leaves a mark, no matter how much I reduce opacity at the edges. So in the first paragraph below, I describe the technique I'm trying and request comments or suggestions; in the second paragraph I ask a specific question about extracting the sky from between the weeds. Anyone bored by the first paragraph but with an answer to the title of the post, please just skip to the second paragraph.

1. After setting the original image as background, I create an extraction layer, eliminating the blue sky; I then duplicate this layer creating a weeds layer and erase the top of the person (the portion above the weed line) then clone the weeds over the bottom of the person (below the weed line). Next I duplicate the background again for a sky layer erasing everything below the weed line, cloning sky for everything above the weed line. (The cloned sky now shows where the person above the weeds has been turned to sky.) Next I duplicate the sky layer, creating a mask for the sky (though I may not be using the term properly) and set a portion of the sky as the primary color, then select the entire sky, erase it, and fill with the primary color on this sky mask layer, finally reducing the opacity of the sky mask layer to cover the clone marks but allow some of the differences in the actual sky to show through. If this all works, I can then flatten the sky mask layer, the sky layer, and weeds layer and have the final image. Is there a better way to do this?

2. Even if the above works, it is painstaking to extract the sky from between the weeds using the magic wand. And although this isn't strictly necessary for the process I describe above, I am curious about whether one can do this more easily in Paint.net. That is, is there something analogous to a touch up tool to remove background from behind grass, limbs, hair, etc. (There is for example in Play-with-Pictures, e.g., but I'd prefer to do this all within Paint.net.)

[Curmudgeon]

try the chroma key plugin, set to remove blue, not green

[Tanel]

You can achieve most precise control with Color to Alpha plugin:

Before using the plugin point the color picker at sky area to set primary color to blue.

Then open the plugin and check the "invert alpha" check box. Most of the sky should become transparent at this point. Move color nubs and Tolerance sliders to fine-tune the output.