Marginal X-Plane Scenery Tutorials Hangar Transparency

Open Objects

The hangar that we've constructed and painted in the last two chapters has the front doors closed:

What if we want to have the doors open, like in the picture? As we shall see, open objects present extra issues in X-Plane.

First, let's give the open hangar a new name, so that we don't overwrite the closed hangar:

• Choose File → Save As...
• Type in the name VAhangar_open and press .
• Press the Save As button.

Alpha channel

In addition to specifying the colour of each pixel, the PNG file format that X-Plane uses for textures allows you to specify how transparent each pixel is - from fully transparent to fully opaque. This transparency information is called the "Alpha Channel".

The BMP file format doesn't support transparency. However X-Plane fakes it by rendering any magenta (#FF00FF) pixels that it finds in the texture as fully transparent.

Using transparency

With the doors open, the front of the hangar is quite a complicated shape:

We could construct this out of three or more rectangles, but that sounds like work. Instead we'll cheat and just make the gap made by the open doors transparent. We can do this by using a version of the texture file that has this area marked as fully transparent in the alpha channel:

• Press to put Blender in UV Face Select mode.
• Press twice to select all faces.
• In the UV/Image Editor window, choose Image → Replace:

• Choose VAhangar_open.png
• Press the Replace Image button.

The hangar now looks like this:

That's not right - the gap is black instead of transparent, even though this area is marked as transparent in the texture file's Alpha channel. Blender and X-Plane have to be told about the Alpha channel:

• Click on the front face to select it.
• Press to make the Buttons Window display the editing buttons.
• In the Texture face panel, select Alpha:

Blender now correctly shows the door space as transparent:

But wait - what happened to the left hand hangar wall? For efficiency, Blender and X-Plane normally only display one side of each face. This is usually a good thing, since it helps to reduce the work that the OpenGL renderer has to do and so keeps frame rates up. However it's not such a good thing when one is constructing open objects. We could tell Blender and X-Plane to display both sides of the hangar walls (using the Twoside button in the Texture face panel), but we want the inside of the hangar to look different from the outside, like in the picture.

Inward-facing faces

The answer is to add extra faces, and to make them face inwards. We could do this by duplicating every existing face, but that's more detail than we need. So, to keep the number of faces down and frame rates up, we'll add a new cube for the inside of the hangar like this (marked in red) instead:

In Blender it can be difficult to edit Meshes that have both inward and outward-facing faces, so we'll add this cube as a new Mesh. First we need to set things up:

• Press to exit UV Face Select mode and to put Blender back in Object Mode.
• Choose Wireframe from the Shading menu in the 3D View window's menu bar:

We want to add the new cube so that it's centred at the origin:

• Press to centre the cursor at the origin.
• Press to bring up a context menu.
• Choose Add → Mesh → Cube to add a new Mesh consisting of a cube.
• Move the mouse cursor back over the origin.
• Press to re-scale.
• Hold down and drag the mouse out until the cube is 25 times larger. Make sure you make it exactly 25.000 times larger - it would be a pain to fix it later. (You can press to cancel the scaling operation, or to undo it if you make a mistake).
• Click when the cube is 25 times larger:

Blender gives the new Mesh the name "Cube". Lets call it something descriptive, like "Internal":

• If the Buttons Window is not already showing the editing buttons, press to show them.
• In the Link and Materials panel click on the Object Name field, type in the name Internal and press .

Now we need to resize the new cube to fit according to our sketch. We'll go back to using Face select mode to do this:

• Click on the Face select mode icon:

• Press to bring up a Properties window.
• Click on the bottom face to select it.
• Press to grab the selected face.
• Hold down and move the face with the mouse to line up with the base of the hangar).
• Click when the face is the right place:

• Use the same technique to grab the left face of the cube and move it to line up with the hangar (-48 on the x axis).
• Use the same technique to grab the right face of the cube and move it to line up with the hangar (+48 on the x axis):

• Press on the numeric keypad to get a rear view.
• Grab the left and right faces of the cube and move them to line up with the hangar (±40 on the y axis):

The new cube has outwards-facing faces, just like the faces that make up the outside of the hangar. We can tell this by looking at the light blue "normals" that Blender draws in the middle of each face - they're all pointing outwards:

Whether Blender draws these "normals" and their size is controlled by the Draw Normals and NSize buttons in the Mesh Tools 1 panel:

We need the faces of the new cube to face inwards:

• Press twice to select all vertices.
• In the Mesh Tools panel click on Flip Normals:

The light blue "normals" in the middle of each face now point inwards, indicating that the faces are facing inwards:

Finally before we apply textures to the new cube, we should remove hidden faces to keep the number of faces down and keep frame rates up. This time we'll keep the bottom face to use as a concrete floor. But we will get rid of the front face, since this face would only be visible from inside the hangar:

• Use to rotate the hangar so that the front face is facing you.
• Select and delete the front face.

More textures

We need to assign textures to the inside of the hangar. First, we need to set things up for managing textures again:

• Choose UV Face Select from the Mode menu in the 3D View window's menu bar:

• Choose Textured from the Shading menu in the 3D View window's menu bar:

Start by assigning the texture file to all faces of the new cube, and then fix up each face one by one:

• Press twice to select all faces.
• In the UV/Image Editor window, choose VAhangar_open.png from the pullup menu:

Now to fix up each face's texture. For each face in the new cube:

• Select the face and use the UV/Image Editor window to fix up the texture assigned to the face:

The floor needs an extra step:

• Select the floor face.
• In the Texture face panel in the Buttons Window, click on Tiles:

In general you should use the Tiles button on all faces that lie horizontally on the ground to prevent artifacts in X-Plane. Don't press this button for other faces. ("Tiles" is not a very descriptive term for what's going on here but it was the only available button in the panel and so got co-opted for this purpose - actually this button tells X-Plane to draw these faces with "polygon offsetting" so that they are raised above taxiways and aprons).

When you're done with all of the faces:

• Press to exit UV Face Select mode and to put Blender back in Object Mode.
• Press to unselect all objects.

That's it! We've finished the open hangar.

• Use to rotate and view the finished hangar:

Now save your work:

• Choose File → Save.
• A confirmation dialog pops up:

• Press or to confirm.

Remember that Blender does not automatically save your work when you quit. Always choose File → Save before you quit Blender!

Marginal X-Plane Scenery Tutorials Hangar Transparency