In this example we create the image seen above. The task is to take a georeferenced globe image, re-project it into Orthographic projection and then add haze near the edges.
Step 1: Prepare the base image
We begin by using the methods described in Manually Georegistering an Image to georegister the globe image. We alter the contrast as described in Selection in Images and apply Relief to better highlight mountainous areas.
Next, we show the image in a map and re-project the map into Orthographic projection using Edit - Assign Projection. Click the Suggest button for reasonable values in the map projection. The map shows a georegistered image seen within a projected map. This is great for applying projections and other geographic effects but it involves a lot of computation overhead if we are doing only graphics arts work and don't care about projections or preserving geographic context.
Note: if we wanted to re-project the original image, we would use Edit - Change Projection. In that case, it is very important to use the suggest button to get sensible values for local scale, which should be the same.
Because we are interested in using this image for purely graphics arts work, we will take a moment to save it as an image. To do this, we use the Tools - Make Image command to save the component as image at screen resolution.
Clicked open in an image window the result is seen above. It is a nice image but lacks a sense of presence because there is no haze that appears near the edges. We will use the earth image in a map together with other image layers to create a more artistic effect.
Step 2: Create the haze layer mask
We will be working in a map window, so we create a map using the earth image. We will call the map earth_from_space. Next, we need to create the image layers that will modify the appearance of the earth layer. We begin with a haze layer.
To create the haze layer, we will first create a mask that will be used to apply variable transparency
In the project pane, right click onto the earth image and choose Copy. Right click elsewhere in the project pane and choose Paste. This makes a copy of the earth image. Name this copy hazemask. Double-click open the hazemask image.
Touch select the white areas in the outer part of the image.
Next, choose Edit - Select Inverse to invert the selection. This is a quick way of selecting just those pixels that make up the planet in the image. We use Image - Adjust - Hue / Saturation to reduce Lightness all the way to the left. This makes all the pixels in the planet region black.
We now apply Gaussian Blur to blur the edges. The image above shows the effect after we have applied it and have chosen Select None to clear the selection. Note that since only the region of pixels showing the planet was selected the Gaussian blur effect is blurry inward only. [A nuance: even though the Gaussian Blur effect works within the selected region only, it computes its effect globally based on pixels outside of the selection as well. The image should be large enough so that the region of white pixels outside the selected area is reasonably large all the way around the planet's disk. If the image is cropped so there are few white pixels except in the corners of the region, then there will be a visible difference in the Gaussian blur effect along the edges where there are fewer white pixels outside the selection.]
We then apply Invert to invert black and white values. We use Image - Convert To once more to convert the image to a Grayscale image (because masks must be grayscale). We can now use hazemask as a mask.
Step 3: Create a haze layer image and apply the mask
Once more we copy the earth image in the project pane and paste it as a new image in the project pane. Name this new image haze.
Open the haze image in an image window. So far, it is a copy of the earth image. Use Image - Adjust - Hue / Saturation to increase the Lightness to full. This makes the entire images white pixels.
Now we used Edit - Load Mask/Channel to load the alpha channel from hazemask. The result in the haze image is shown above. The central part of the haze image is completely transparent and the outer part is completely opaque white. The transition region from completely transparent to completely white is that part of the hazemask image that was blurred using Gaussian blur.
We can now drag and drop the haze image into our earth_from_space map as a layer above the earth layer. The result is seen above. The transparent central parts of the haze image allow the earth image underneath to be visible. The gradient from transparent to white creates a haze effect at the edges of the planet region.
Step 4: Create a "background" layer
If we want a white surrounding color we are done. However, if we wish to have a different color surrounding the "planet" we will need to add one more layer. This new layer will overlay the layers already created, but because it will be precisely shaped with a "hole" in the center for the planet it will appear to be a background.
We do this by going back to the project pane and once more copying the earth image and pasting it as a new image, which we shall call space. So far, space is an exact copy of the earth image.
We open the space image and touch select the white areas in the outer part of the image.
Next, we choose Edit - Select Inverse to invert the selection. We want to select a region just one or two pixels smaller than the "planet", so we use Edit - Modify Selection - Contract and contract the selection by 1 pixel. [This is a matter of taste. Under some circumstances we might use 2 pixels.] We then choose Edit - Delete to remove the pixels in the selection by converting them to invisible pixels .
Next, we use Image - Adjust - Hue / Saturation to reduce Lightness all the way so that all the pixels become black. The result is illustrated above. Finally, we apply Image - Effects - Gaussian Blur set to blur 2 or so pixels. The result is a black image with a circular region that is a pixel smaller than the planet region with a slight blur right at the edge. We drag and drop the space image thus created from the project pane into the earth_from_space image.
In the earth_from_space map window we can now right click on the space layer tab and set the layer's Layer Opacity to achieve the color we like for what appears to be the background. Layer opacity in the 80% to 90% range results in effects as seen above.
One of the main techniques shown here is copying an original image and pasting it as a series of images that are then in turn transformed into masks and other layers. Using the original image and then creating the masks from copies of that image guarantees that the masks and space layer will be exactly the right size to use as we desire in layers in combination with the original image.
Why did we go to the effort of contracting the selection and applying a Gaussian blur when creating the space image? If we were to simply select the planet region and make it invisible in the space layer we would end up with the "jaggies" when the space layer was placed above the haze and earth layers. The harsh transition between invisible pixels and solid black pixels in the space layer would result in a jagged edge. By making the "hole" a pixel or two smaller in size than the planet disk and applying a Gaussian blur we end up with a slight transition from invisible pixels to solid black pixels. The effect is something of a manually created anti-aliasing that gets rid of the jaggies. Using a blur in a zone a few pixels wide is a good way of making transitions in images.
Strictly speaking, only the haze and space images need to be RGBa images. The earth image could be an RGB image.
It's important to understand that except for Step 1 this entire topic is about graphics arts work using images in a non-geographic way. Only in the first step do we use the images in a geographic context to create a view of the Earth in Orthographic projection. We then make an image that is a screen shot of that view. After that, all work is done on the screen shot image and not with the original georegistered image / map.
We could have combined drawings with the image in the original map from which the original screen shot was made. In that case, the earth image with which we started could have included visual effects from drawing, like the last image in the Manually Georegister an Image topic.
Just for fun, we can go back to the original geographic map and create an Orthographic projection that is centered on 0 longitude and 90 latitude for a polar aspect view. We can then make a screen shot image and process that image as noted above to create a polar aspect view from space, with haze.