Finding and Using Reference Images
Blender 3d tutorial
To set up reference images in Blender 3D, we will learn how to identify good reference images on the web, prepare those references, the use the Background Image tool to display them in Blender.
Finding Reference Images
To start, let's find our references. Personally, I recommend going to the-blueprints.com for any vehicular reference images. You can find cars, trucks, trains, planes, helicopters, tanks, pretty much everything. There are a few other places listed on the links page.
Wherever you decide to search for references, there are a few things to look out for. The first is that the image should be clear and easy to see. Blurry, messy, and incorrect references will only make modeling more difficult. An example of bad references I noticed a while ago had mirrors on only two of the views and they were in different locations on the car. If you see something like that, look for something better.
A couple other tips include making sure all sides of the car/object are shown. Top, Front, Back, Side. Sometimes you don't need all, but if you do, it's harder to model without the reference. Also, if you can, find references where all of the views are the same size relative to each other. Typically, better reference images will already be the same size relative to each other.
Crop and Rotate
After you have found good reference, you may need to split up your image. If so, we will take one big image and turn it into four smaller ones, one for each view. For this you will need an image editor. Any editor works as long as you have the capability to crop your image.
Before opening your image, copy and paste your reference until you have one for each view. I also make one extra so that I have the original. Then, in your image editor, crop the image until you only have the one of the views in that image. When cropping, you want to crop until you hit the very edge of the car without cutting anything out. I've done the front view here. Then do the same for the remaining views.
Once all views are cropped and saved, we need to make sure the orientation of the images are correct. The front and back are easy, just make sure they are right side up. However, for the side you need the front of the car on the left of the image and on the top view you need the front at the bottom. In the image in the previous step, I just need to rotate the top view.
Now we need to make sure that all of the images are the same size in relation to each other. This just means the width of the car in the front view should be the same width as the back and top views as well as all other dimensions.
If you've cropped correctly and had good reference images, the front, back, and top images should all have the same width in pixels. The height of the images should be the same in the front, back, and side views. Lastly, the width of the top and side view should be the same. If you find that they are not the same, resize your images as necessary. Being off by a pixel or two is nothing to worry about, especially if the images are really big. Be precise, but there's no need to be perfect.
Setup in Blender
Now we can open up Blender. To use background images all you have to do is click on the little plus sign in the top right corner of one of the 3D viewing windows or hit the 'N' key. Somewhere on that sidebar you should see a checkbox labeled Background Image. Check that Box.
Once you have done that, click on Add Image. This will be for one of your views. To load your first image, click on the 'Open' button and find one of your reference images. In the pop out menu labeled Axis (set at All Views by default) you would chose the corresponding view. That way, the image will only appear when you are facing the right direction. Then do the same for your other images.
Keep in mind that you must be in orthographic view to see your background images in Blender. You can toggle between orthographic and perspective views by using Numpad 5 or by going to View-->View Persp/Ortho the menu of the 3D view window.
Scaling in Blender
When you load a background image into Blender, it gives the image a width (called 'Size') of 5 Blender units. It does not matter what the actual width in pixels is, Blender will make it 5 Blender units wide. However, the width of the side view should be larger than the other images. This means we need to scale it up to match our other views.
Determining the size of the side view in Blender units requires some math. First, we want to find out how much longer the car is compared to the width of the car. For example, if the width of the front view is 500 pixels and the width of the side view is 1000 pixels we get 1000/500 = 2. This means that the length of the car is twice as large as the width. To that to Blender units, we multiply by 5. 2*5 = 10, therefore the size of the side image needs to be changed to 10. Here's my equation:
Size = (width of side view / width of front view) * 5
Once you've scaled up your image, you are ready to start modeling!
No single tutorial will cover everything, so if you want to dig deeper, here are some resources for you to check out. If you have other resources you think I should add to this list, feel free to let me know!