ImageReader
. An image encoded as part of a file or stream may be thought of extending out in multiple dimensions: the spatial dimensions of width and height, a number of bands, and a number of progressive decoding passes. This class allows a contiguous (hyper)rectangular subarea of the image in all of these dimensions to be selected for decoding. Additionally, the spatial dimensions may be subsampled discontinuously. Finally, color and format conversions may be specified by controlling the ColorModel
and SampleModel
of the destination image, either by providing a BufferedImage
or by using an ImageTypeSpecifier
.
An ImageReadParam
object is used to specify how an image, or a set of images, will be converted on input from a stream in the context of the Java Image I/O framework. A plug-in for a specific image format will return instances of ImageReadParam
from the getDefaultReadParam
method of its ImageReader
implementation.
The state maintained by an instance of ImageReadParam
is independent of any particular image being decoded. When actual decoding takes place, the values set in the read param are combined with the actual properties of the image being decoded from the stream and the destination BufferedImage
that will receive the decoded pixel data. For example, the source region set using setSourceRegion
will first be intersected with the actual valid source area. The result will be translated by the value returned by getDestinationOffset
, and the resulting rectangle intersected with the actual valid destination area to yield the destination area that will be written.
The parameters specified by an ImageReadParam
are applied to an image as follows. First, if a rendering size has been set by setSourceRenderSize
, the entire decoded image is rendered at the size given by getSourceRenderSize
. Otherwise, the image has its natural size given by ImageReader.getWidth
and ImageReader.getHeight
.
Next, the image is clipped against the source region specified by getSourceXOffset
, getSourceYOffset
, getSourceWidth
, and getSourceHeight
.
The resulting region is then subsampled according to the factors given in {@link IIOParam#setSourceSubsampling IIOParam.setSourceSubsampling}. The first pixel, the number of pixels per row, and the number of rows all depend on the subsampling settings. Call the minimum X and Y coordinates of the resulting rectangle (minX
, minY
), its width w
and its height h
.
This rectangle is offset by (getDestinationOffset().x
, getDestinationOffset().y
) and clipped against the destination bounds. If no destination image has been set, the destination is defined to have a width of getDestinationOffset().x
+ w
, and a height of getDestinationOffset().y
+ h
so that all pixels of the source region may be written to the destination.
Pixels that land, after subsampling, within the destination image, and that are written in one of the progressive passes specified by getSourceMinProgressivePass
and getSourceNumProgressivePasses
are passed along to the next step.
Finally, the source samples of each pixel are mapped into destination bands according to the algorithm described in the comment for setDestinationBands
.
Plug-in writers may extend the functionality of ImageReadParam
by providing a subclass that implements additional, plug-in specific interfaces. It is up to the plug-in to document what interfaces are available and how they are to be used. Readers will silently ignore any extended features of an ImageReadParam
subclass of which they are not aware. Also, they may ignore any optional features that they normally disable when creating their own ImageReadParam
instances via getDefaultReadParam
.
Note that unless a query method exists for a capability, it must be supported by all ImageReader
implementations (e.g. source render size is optional, but subsampling must be supported).
@see ImageReader
@see ImageWriter
@see ImageWriteParam
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