Extracting a Raster from an Array of Pixels
A less common method for obtaining a raster is to define an array of pixels and convert the array to a raster. Listing 3 deomonstrates this technique. Figure 2 shows the possible results.
Figure 2. RasterFromPixelArray.java.
Share DataBuffers Between Rasters
Because Java allows rasters to share DataBuffers, not every raster has its own DataBuffer object. This can really help save your application resources. You can create rasters that share the same DataBuffer explicitly or you can define a "parent-child" relationship by using the following two methods from the Raster class:
-
public Raster createChild(int px, int py, int w, int h, int cx, int cy, int[] bands): This method creates a "child" of the current Raster with the following specifications:
- The
(px,py,w,h) parameters define the rectangular area that will be shared by the selected raster with its "child". The px,py parameters represents the coordinates of the current raster origins.
- The
cx,cy parameters represent the coordinates of origins for the "child" raster. This can be the same as the px,py parameters or can be different.
- The bands parameter represents a subset of color bands that belong to the current raster. This subset of bands belongs to the "child" raster, as well. If this parameter value is
null, then all the bands that belong to the selected raster will also belong to the "child" raster.
-
public Raster createTranslatedChild(int cx, int cy): This method creates a "child" raster identical to the selected raster. In this case, the "child" raster will have other coordinates of the origin, (cx,cy).
Figure 3. Parent-Child: Two rasters that share the same DataBuffer object.
You should know that the WritableRaster class also has its own methods for defining parent-child relationships with WritableRaster objects:
-
public WritableRaster createWritableChild(int px, int py, int w, int h, int cx, int cy, int[] bands)
-
public WritableRaster createWritableTranslatedChild(int cx, int cy)
To retrieve the "parent" of a raster, use this getParent method:
public Raster getParent()
Translation of a Raster's Origins
The coordinates of origin for a raster can be any cartesian coordinates. Unfortunately, the SampleModel coordinates of origin must be always (0,0). For compatibility between these two systems of coordinates, you must translate the origin of the SampleModel to the origin of the raster using the following methods:
-
public final int getSampleModelTranslateX()
-
public final int getSampleModelTranslateY()
Apply Filters to Rasters
Now it's time to learn how to apply filters to rasters. You'll be using the most common filters found in the Java2D API.
Note: If you are not familiar with the Java2D API—especially the Ops classes and interfaces—you can find some very helpful resources here.
Here's a list of the principal Java 2D API classes you can use for image filters:
-
java.awt.image.AffineTransformOp: This provides methods for filters like translations, rotations, etc.
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java.awt.image.ConvolveOp: This provides methods for filters based on the convolution operator.
- java.awt.image.LookupOp: This provides methods for filters used for colors transformations based on a table of colors.
-
java.awt.image.ColorConvertOp: This provides methods for changing the colors of an image.
-
java.awt.image.RescaleOp: This provides methods for rescaling an image.
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java.awt.image.BandCombineOp: This provides methods for creating filters based on a table of values. This class is specific only to rasters.
All of the above classes implement the java.awt.image.RasterOp interface, which defines the method you call when you want to apply a filter to a raster:
-
WritableRaster filter(Raster sourceR, WritableRaster destR): This applies a filter to the sourceR raster and returns the result to the destR raster.
Applying a filter to a raster is a two steps process. First, create an instance of one of the above classes. Second, call the filter method.
Listing 4 applies a filter (created with the ConvolveOp class) to a raster obtained from a BufferedImage. This filter is known as the "sharpen" filter.
Figure 4. RasterConvolveOp.java: A possible result.
Listing 5 shows an AffineTransformOp filter applied to a raster obtained for an array of pixels. The filter consists in scaling the raster.
Figure 5. RasterAffineTransformOp.java: A possible result.
A particulary useful set of filters you can apply to a raster are the java.awt.image.BandCombineOp filters. These filters are for use only with rasters and are based on the multiplication of a bands table with a table of values. The number of columns from the table of values must be equal or greater to the number of bands from the raster. If the table of values has the same number of columns as the number of raster bands plus 1, then a value of 1 will be added at the end of the bands table. Also, the number of rows from the table of values must be equal to the number of bands from the table bands.
Figure 6. The java.awt.image.BandCombineOp Filters: The table of values and the raster bands table.
public BandCombineOp(float[][] table, RenderingHints rh) constructs the BandCombineOp class by creating a BandCombineOp object with the specified table of values. The rh parameter represents a hint.
Listing 6 demonstrates how to apply a BandCombineOp filter to a raster to obtain a cubist style image.
Figure 7. RasterBandCombineOp.java: A possible result.
A Powerful Tool
As you can see, rasters are fundamental in image proceesing, especially when you want a full control over the image pixels. They're a very powerful tool when it comes to applications with image filters, video processing, movie tricks, photo adjusting, video games, creating transparency, and creating opacity. Digital image artists understand that an image is just a collection of 10, 100, 1000000 rasters that can give an image a new face.
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