PNGDecodeParam
Date: 2016-04-28

org.apache.xmlgraphics.image.codec.png

## Class PNGDecodeParam

- java.lang.Object
- - org.apache.xmlgraphics.image.codec.png.PNGDecodeParam

- All Implemented Interfaces:
    java.io.Serializable, java.lang.Cloneable, ImageDecodeParam

      public class PNGDecodeParam
      extends java.lang.Object
      implements ImageDecodeParam

  An instance of `ImageDecodeParam` for decoding images in the PNG format. `PNGDecodeParam` allows several aspects of the decoding process for PNG images to be controlled. By default, decoding produces output images with the following properties:

  Images with a bit depth of 8 or less use a `DataBufferByte` to hold the pixel data. 16-bit images use a `DataBufferUShort`.

  Palette color images and non-transparent grayscale images with bit depths of 1, 2, or 4 will have a `MultiPixelPackedSampleModel` and an `IndexColorModel`. For palette color images, the `ColorModel` palette contains the red, green, blue, and optionally alpha palette information. For grayscale images, the palette is used to expand the pixel data to cover the range 0-255. The pixels are stored packed 8, 4, or 2 to the byte.

  All other images are stored using a `PixelInterleavedSampleModel` with each sample of a pixel occupying its own `byte` or `short` within the `DataBuffer`. A `ComponentColorModel` is used which simply extracts the red, green, blue, gray, and/or alpha information from separate `DataBuffer` entries.

  Five aspects of this process may be altered by means of methods in this class.

  `setSuppressAlpha()` prevents an alpha channel from appearing in the output.

  `setExpandPalette()` turns palette-color images into 3-or 4-channel full-color images.

  `setOutput8BitGray()` causes 1, 2, or 4 bit grayscale images to be output in 8-bit form, using a `ComponentSampleModel` and `ComponentColorModel`.

  `setDecodingExponent()` causes the output image to be gamma-corrected using a supplied output gamma value.

  `setExpandGrayAlpha()` causes 2-channel gray/alpha (GA) images to be output as full-color (GGGA) images, which may simplify further processing and display.

  This class is not a committed part of the JAI API. It may be removed or changed in future releases of JAI.

  See Also:
    Serialized Form

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `PNGDecodeParam()` Constructs a default instance of `PNGDecodeParam`.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `float` | `getDisplayExponent()` Returns the current value of the display exponent parameter.
    `PNGEncodeParam` | `getEncodeParam()` If `getGenerateEncodeParam()` is `true`, this method may be called after decoding has completed, and will return an instance of `PNGEncodeParam` containing information about the contents of the PNG file just decoded.
    `boolean` | `getExpandGrayAlpha()` Returns the current setting of the gray/alpha expansion.
    `boolean` | `getExpandPalette()` Returns true if palette-color images will be expanded to produce full-color output.
    `boolean` | `getGenerateEncodeParam()` Returns `true` if an instance of `PNGEncodeParam` will be available after an image has been decoded via the `getEncodeParam` method.
    `boolean` | `getOutput8BitGray()` Returns the current value of the 8-bit gray output parameter.
    `boolean` | `getPerformGammaCorrection()` Returns `true` if gamma correction is to be performed on the image data.
    `boolean` | `getSuppressAlpha()` Returns `true` if alpha (transparency) will be prevented from appearing in the output.
    `float` | `getUserExponent()` Returns the current value of the user exponent parameter.
    `void` | `setDisplayExponent(float displayExponent)` Sets the display exponent to a given value.
    `void` | `setEncodeParam(PNGEncodeParam encodeParam)` Sets the current encoder param instance.
    `void` | `setExpandGrayAlpha(boolean expandGrayAlpha)` If set, images containing one channel of gray and one channel of alpha (GA) will be output in a 4-channel format (GGGA).
    `void` | `setExpandPalette(boolean expandPalette)` If set, palette color images (PNG color type 3) will be decoded into full-color (RGB) output images.
    `void` | `setGenerateEncodeParam(boolean generateEncodeParam)` If set, an instance of `PNGEncodeParam` will be available after an image has been decoded via the `getEncodeParam` method that encapsulates information about the contents of the PNG file.
    `void` | `setOutput8BitGray(boolean output8BitGray)` If set, grayscale images with a bit depth less than 8 (1, 2, or 4) will be output in 8 bit form.
    `void` | `setPerformGammaCorrection(boolean performGammaCorrection)` Turns gamma corection of the image data on or off.
    `void` | `setSuppressAlpha(boolean suppressAlpha)` If set, no alpha (transparency) channel will appear in the output image.
    `void` | `setUserExponent(float userExponent)` Sets the user exponent to a given value.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### PNGDecodeParam

          public PNGDecodeParam()

      Constructs a default instance of `PNGDecodeParam`.

  - ### Method Detail

    - #### getSuppressAlpha

          public boolean getSuppressAlpha()

      Returns `true` if alpha (transparency) will be prevented from appearing in the output.

    - #### setSuppressAlpha

          public void setSuppressAlpha(boolean suppressAlpha)

      If set, no alpha (transparency) channel will appear in the output image.

      The default is to allow transparency to appear in the output image.

    - #### getExpandPalette

          public boolean getExpandPalette()

      Returns true if palette-color images will be expanded to produce full-color output.

    - #### setExpandPalette

          public void setExpandPalette(boolean expandPalette)

      If set, palette color images (PNG color type 3) will be decoded into full-color (RGB) output images. The output image may have 3 or 4 channels, depending on the presence of transparency information.

      The default is to output palette images using a single channel. The palette information is used to construct the output image's `ColorModel`.

    - #### getOutput8BitGray

          public boolean getOutput8BitGray()

      Returns the current value of the 8-bit gray output parameter.

    - #### setOutput8BitGray

          public void setOutput8BitGray(boolean output8BitGray)

      If set, grayscale images with a bit depth less than 8 (1, 2, or 4) will be output in 8 bit form. The output values will occupy the full 8-bit range. For example, gray values 0, 1, 2, and 3 of a 2-bit image will be output as 0, 85, 170, and 255.

      The decoding of non-grayscale images and grayscale images with a bit depth of 8 or 16 are unaffected by this setting.

      The default is not to perform expansion. Grayscale images with a depth of 1, 2, or 4 bits will be represented using a `MultiPixelPackedSampleModel` and an `IndexColorModel`.

    - #### getPerformGammaCorrection

          public boolean getPerformGammaCorrection()

      Returns `true` if gamma correction is to be performed on the image data. The default is `true`.

      If gamma correction is to be performed, the `getUserExponent()` and `getDisplayExponent()` methods are used in addition to the gamma value stored within the file (or the default value of 1/2.2 used if no value is found) to produce a single exponent using the formula:

           decoding_exponent = user_exponent/(gamma_from_file * display_exponent)

    - #### setPerformGammaCorrection

          public void setPerformGammaCorrection(boolean performGammaCorrection)

      Turns gamma corection of the image data on or off.

    - #### getUserExponent

          public float getUserExponent()

      Returns the current value of the user exponent parameter. By default, the user exponent is equal to 1.0F.

    - #### setUserExponent

          public void setUserExponent(float userExponent)

      Sets the user exponent to a given value. The exponent must be positive. If not, an `IllegalArgumentException` will be thrown.

      The output image pixels will be placed through a transformation of the form:

           sample = integer_sample / (2^bitdepth - 1.0)
           decoding_exponent = user_exponent/(gamma_from_file * display_exponent)
           output = sample ^ decoding_exponent

      where `gamma_from_file` is the gamma of the file data, as determined by the `gAMA`, `sRGB`, and/or `iCCP` chunks, and `display_exponent` is the exponent of the intrinsic transfer curve of the display, generally 2.2.

      Input files which do not specify any gamma are assumed to have a gamma of `1/2.2`; such images may be displayed on a CRT with an exponent of 2.2 using the default user exponent of 1.0.

      The user exponent may be used in order to change the effective gamma of a file. If a file has a stored gamma of X, but the decoder believes that the true file gamma is Y, setting a user exponent of Y/X will produce the same result as changing the file gamma.

      This parameter affects the decoding of all image types.

      Throws:
        `java.lang.IllegalArgumentException` - if `userExponent` is negative.

    - #### getDisplayExponent

          public float getDisplayExponent()

      Returns the current value of the display exponent parameter. By default, the display exponent is equal to 2.2F.

    - #### setDisplayExponent

          public void setDisplayExponent(float displayExponent)

      Sets the display exponent to a given value. The exponent must be positive. If not, an `IllegalArgumentException` will be thrown.

      The output image pixels will be placed through a transformation of the form:

           sample = integer_sample / (2^bitdepth - 1.0)
           decoding_exponent = user_exponent/(gamma_from_file * display_exponent)
           output = sample ^ decoding_exponent

      where `gamma_from_file` is the gamma of the file data, as determined by the `gAMA`, `sRGB`, and/or `iCCP` chunks, and `user_exponent` is an additional user-supplied parameter.

      Input files which do not specify any gamma are assumed to have a gamma of `1/2.2`; such images should be decoding using the default display exponent of 2.2.

      If an image is to be processed further before being displayed, it may be preferable to set the display exponent to 1.0 in order to produce a linear output image.

      This parameter affects the decoding of all image types.

      Throws:
        `java.lang.IllegalArgumentException` - if `userExponent` is negative.

    - #### getExpandGrayAlpha

          public boolean getExpandGrayAlpha()

      Returns the current setting of the gray/alpha expansion.

    - #### setExpandGrayAlpha

          public void setExpandGrayAlpha(boolean expandGrayAlpha)

      If set, images containing one channel of gray and one channel of alpha (GA) will be output in a 4-channel format (GGGA). This produces output that may be simpler to process and display.

      This setting affects both images of color type 4 (explicit alpha) and images of color type 0 (grayscale) that contain transparency information.

      By default, no expansion is performed.

    - #### getGenerateEncodeParam

          public boolean getGenerateEncodeParam()

      Returns `true` if an instance of `PNGEncodeParam` will be available after an image has been decoded via the `getEncodeParam` method.

    - #### setGenerateEncodeParam

          public void setGenerateEncodeParam(boolean generateEncodeParam)

      If set, an instance of `PNGEncodeParam` will be available after an image has been decoded via the `getEncodeParam` method that encapsulates information about the contents of the PNG file. If not set, this information will not be recorded and `getEncodeParam()` will return `null`.

    - #### getEncodeParam

          public PNGEncodeParam getEncodeParam()

      If `getGenerateEncodeParam()` is `true`, this method may be called after decoding has completed, and will return an instance of `PNGEncodeParam` containing information about the contents of the PNG file just decoded.

    - #### setEncodeParam

          public void setEncodeParam(PNGEncodeParam encodeParam)

      Sets the current encoder param instance. This method is intended to be called by the PNG decoder and will overwrite the current instance returned by `getEncodeParam`.