Using plane textures for trees, people, etc.. in still rendering
Most of the time pixel images are more suitable for representing people, cars, trees, etc... than a 3D model. If these are not the main subject of your render it's often not worth modelling a precise 3D object. Moreover, it's very hard to make human or tree models look realistic. In addition, they require a lot of modelling which increases rendering times and memory consumption.
So, the trick is to use plane pixel images to fake 3D models in your render. We will use a cut out image and use it as texture on a plane facing the camera. The result will be the same as a postproduction in Gimp. The advantage to using this trick in Blender, rather than in postprodction, is that less time will be consumed because wherever you move you camera, images will always face you, cast shadow on your 3D model and be obscured by a 3D foreground. However, if you use a tree picture shot from an human height(~1.7m), you cannot use it to render a "from the sky view" of your model.
Prepare your texture
This is quite hard work here. You could skip this part by finding libraries of these cut out pictures of trees and people on the web.
Otherwise, take a digital camera and take shots of what's around you. Setting your lens to it's maximum zoom will help avoid the distortion caused by wider angles. When photographing people, make sure feet are horizontal on pictures. Cut out the subject in your favorite image software (Gimp, Photoshop). The best technique is to use layer masks and have a blurred transition around the cut out image. Once it's done save your picture in a format that supports an alpha layer, PNG is a good choice.
Tree cut out picture in PNG
Create your tree in Blender
To make it easier, this a Blender start scene where your tree could grow in serenity. (A green grass plane with a sunlight and a blue sky :)
First, create a vertical plane object in the center of your grass plot.
Create a new material for this plane object in the shading panel (F5). Create a new texture for this material, choose "bitmap" as type and load your tree picture. Tick "UseAlpha" and the "Alpha" button near the preview to see transparency.
Go back to the material panel. Tick Ztransp, Shadeless, put Alpha slider to 0. Tick Alpha in the "Map To" tab. That's it for the material.
Plane dimensions should be the same ratio as the texture size. For example if your texture is 500x1000px, your plane could be 2.5x5 blender units. In our example, the texture size is 477x664px so : select the plane object, press "N" and set the dimensions as follow.
This also means that our tree is 6.64 units tall. That's a bit small for that kind of tree. We can resize it by the pressing the "S" button and moving until we get 8 units tall. The ratio will be preserved ;-).
Move the plane until the base just reaches the ground.
Start a render.
Tip: If you don't see the tree transparent shadow on the ground, tick the TraShadow button of the ground material (not the tree material).
Deal with many 2D planes : a forest
This a good start, but a single tree is one thing, a forest is another. We must be careful using 2D objects.
- First, 2D images always have to face the camera.
- Second, their shadows must fit with your main shadow lamp.
- Third, you must be able to duplicate your 2D objects (in our case, trees) to spread them among your scene.
Prepare your object to constraints
Select your tree object, press N and fill RotZ: to 0. Press CTRL-A to apply size and rotation. This was to fix correct rotation and size before constraint. Go to material panel (F5). In texture map input, tick Cube because our plane is vertical now.
I have UVmapped the plane to see our tree in the 3D view so you can understand what happened easily. (Uvmapping is another topic not explained here)
Face the camera
Go to Object panel (F4) and add a "Copy rotation" constraint. Fill the target object name with the camera name. Switch off X and Y, keep Z. Ok, so the tree will follow the camera rotation to always face it.
Shadows face the main lamp
Another issue is that if your tree faces your camera but not the lamp, you will get a tiny shadow or a weak shadow. If it were a real 3D tree, the shadow would be right in any direction.
Therefore, we need to separate the tree we will see and it's shadow. Duplicate (Shift-D to duplicate without link) your tree on the same place.
In the constraint, change the target to the main lamp of your scene.
Thus, we have one tree facing the camera and one tree facing the lamp.
We must modify materials to get one showing the tree without casting shadows and the other one only casting shadow.
Select the tree facing the camera and go to material panel (F5)
Select the tree facing the lamp. Create a new Material to unlick the two tree materials. Tick "Traceable" and "OnlyCast"
Duplicate the trees
We will use the Duplivert function. A tree will grow out of earth on each vertex of an third party object.
Position your cursor to a tree origin (Shift S). Go to top view (7), add a plane to your scene. Subdivide it 4 times to get a grid. Size it to fit with the ground already there. Trees must not be too close. Select your two trees and the plane (Shift+Reight button mouse). The plane should be selected in last. Press Ctrl-P to create a parent link. While plane selected, go to object panel (F7) and tick "Dupliverts".