Detailed Render of the Earth from Space - Blender 2.5 - page 217

Specular Map
6. Let's start by dragging the image named EarthMask_2500x1250.
jpg onto the main window of Gimp to get it open. Then drag the image
EarthClouds_2500x1250.jpg over the previous one to get it added as a
separate layer in Gimp.
7. Now, we need to make sure that the images are correctly aligned. To do this, let's go
to View → Zoom → 4:1 (400%) , to be able to move the layer with pixel precision
easily. Now go to the botom right-hand side corner of the window and click-and-drag
over the four-arrows icon unil the part of the image shown in the viewport is one
of the corners. After looking at the right place, let's go to the Toolbox and acivate
the Move tool. Finally, we just need to drag the clouds layer so that its corner exactly
matches the corner of the water/land image. Then let's switch to another zoom level
by going to View → Zoom → 1:4 (25%) .
8. Now let's go to the Layers window, select the EarthClouds layer, and set its
blending mode to Multiply ( Mode drop-down, above the layers list).
9. Now we just need to go to the main window and go to Colors → Invert .
10. Finally, let's switch the image to RGB mode by going to Image → Mode → RGB and
we are done with the processing. Remember to save the image as EarthSpecMap.
jpg in the tex folder of the project and close it in Gimp.
The purpose of creaing this specular map is to correctly mix the specularity of the ocean
(full) with one of the clouds that is above the ocean (null). This way, we get a correct
specularity, both in the ocean and in the clouds. If we just used the water or land mask to
control specularity, then the clouds above the ocean would have specular relecion, which
is wrong.
Bump Map
The bump map controls the roughness of the material; this one is very important as it adds
a lot of detail to the final render without having to create actual geometry to represent it.
11. First, drag the EarthElevation_2500x1250.jpg to the main window of Gimp
to get it open. Then let's drag the EarthClouds_2500x1250.jpg image over the
previous one, so that it gets loaded as a layer above the first one.
12. Now zoom in by going to View → Zoom → 4:1 (400%) . Drag the image so that
you are able to see one of its corners and use the move tool to get the clouds layer
exactly matching the elevaion layer. Then switch back to a wider view by going to
View → Zoom → 1:4 (25%) .
13. Now it's ime to add a mask to the clouds layer. Right-click on the clouds layer and
select the Add Layer Mask... entry from the menu. Then select the Grayscale copy
of layer opion in the dialog box and click Add .