The very simple script below shows you how you can rasterize a shapefile using the Python Shapefile Library (PSL) and the Python Imaging Library (PIL). PIL is a very old and well-developed library originally created to process remote sensing imagery however it has absolutely no spatial capability. What it does have is the ability to read and write multiple image formats and can handle very large images. It also has an API that lets you easily import and export data to and from other libraries using python strings and arrays. The PIL ImageDraw module provides an easy way to draw on an image canvas.
The following script reads in a shapefile, grabs the points from the first and only polygon, draws them to an image, and then saves the image as a PNG file with an accompanying .pgw world file to make it a geospatial image. Most modern GIS packages handle PNG images but you could just as easily change the file and worldfile extension to jpg and jgw respectively for even better compatibility. As usual I created minimal variables to keep the code short and as easy to understand as possible.
import shapefile import Image, ImageDraw # Read in a shapefile r = shapefile.Reader("mississippi") # Geographic x & y distance xdist = r.bbox - r.bbox ydist = r.bbox - r.bbox # Image width & height iwidth = 400 iheight = 600 xratio = iwidth/xdist yratio = iheight/ydist pixels =  for x,y in r.shapes().points: px = int(iwidth - ((r.bbox - x) * xratio)) py = int((r.bbox - y) * yratio) pixels.append((px,py)) img = Image.new("RGB", (iwidth, iheight), "white") draw = ImageDraw.Draw(img) draw.polygon(pixels, outline="rgb(203, 196, 190)", fill="rgb(198, 204, 189)") img.save("mississippi.png") # Create a world file wld = file("mississippi.pgw", "w") wld.write("%s\n" % (xdist/iwidth)) wld.write("0.0\n") wld.write("0.0\n") wld.write("-%s\n" % (ydist/iheight)) wld.write("%s\n" % r.bbox) wld.write("%s\n" % r.bbox) wld.close
You can download this script here:
You can download the shapefile used here:
Of course you will also need the Python Shapefile Library found here and the latest version of the Python Imaging Library from here.
The image created by this script is featured at the top of this post.
The idea of using a shapefile as a clipping mask for an image can be done with GDAL. The python API for GDAL includes integration with the well-known Python Numeric (NumPy) package using a module called "gdalnumeric". Both gdalnumeric and PIL contain "tostring" and "fromstring" methods which allow you to move image data back and forth between the packages. GDAL and NumPy make handling geospatial data as numerical arrays easier and PIL's API makes creating a polygon clipping mask much easier.
I'll cover using PIL, GDAL, NumPy, and PSL together in a future post. I'll also demonstrate a way where the above operation can be performed using pure Python.