org.apache.xmlgraphics.image.codec.png

## Class PNGEncodeParam

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

- All Implemented Interfaces:
    java.io.Serializable, java.lang.Cloneable, ImageDecodeParam, ImageEncodeParam
  Direct Known Subclasses:
    PNGEncodeParam.Gray, PNGEncodeParam.Palette, PNGEncodeParam.RGB

      public abstract class PNGEncodeParam
      extends java.lang.Object
      implements ImageEncodeParam

  An instance of `ImageEncodeParam` for encoding images in the PNG format.

  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

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static class` | `PNGEncodeParam.Gray`
    `static class` | `PNGEncodeParam.Palette`
    `static class` | `PNGEncodeParam.RGB`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected int` | `bitDepth`
    `protected boolean` | `bitDepthSet`
    `static int` | `INTENT_ABSOLUTE` Constant for use with the sRGB chunk.
    `static int` | `INTENT_PERCEPTUAL` Constant for use with the sRGB chunk.
    `static int` | `INTENT_RELATIVE` Constant for use with the sRGB chunk.
    `static int` | `INTENT_SATURATION` Constant for use with the sRGB chunk.
    `static int` | `PNG_FILTER_AVERAGE` Constant for use in filtering.
    `static int` | `PNG_FILTER_NONE` Constant for use in filtering.
    `static int` | `PNG_FILTER_PAETH` Constant for use in filtering.
    `static int` | `PNG_FILTER_SUB` Constant for use in filtering.
    `static int` | `PNG_FILTER_UP` Constant for use in filtering.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `PNGEncodeParam()`

  - ### Method Summary

    All Methods Static Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `addPrivateChunk(java.lang.String type, byte[] data)` Adds a private chunk, in binary form, to the list of chunks to be stored with this image.
    `int` | `filterRow(byte[] currRow, byte[] prevRow, byte[][] scratchRows, int bytesPerRow, int bytesPerPixel)` Performs filtering on a row of an image.
    `int` | `getBitDepth()` Returns the desired bit depth for a grayscale image.
    `float[]` | `getChromaticity()` Returns the white point and primary chromaticities in CIE (x, y) space.
    `java.lang.String[]` | `getCompressedText()` Returns the text strings to be stored in compressed form with this image as an array of `String`s.
    `static PNGEncodeParam` | `getDefaultEncodeParam(java.awt.image.RenderedImage im)` Returns an instance of `PNGEncodeParam.Palette`, `PNGEncodeParam.Gray`, or `PNGEncodeParam.RGB` appropriate for encoding the given image.
    `float` | `getGamma()` Returns the file gamma value for the image.
    `byte[]` | `getICCProfileData()` Returns the ICC profile data to be stored with this image.
    `boolean` | `getInterlacing()` Returns `true` if Adam7 interlacing will be used.
    `java.util.Date` | `getModificationTime()` Returns the modification time to be stored with this image.
    `int` | `getNumPrivateChunks()` Returns the number of private chunks to be written to the output file.
    `int[]` | `getPaletteHistogram()` Returns the palette histogram to be stored with this image.
    `int[]` | `getPhysicalDimension()` Returns the physical dimension information to be stored with this image.
    `byte[]` | `getPrivateChunkData(int index)` Returns the data associated of the private chunk at a given index, as an array of `byte`s.
    `java.lang.String` | `getPrivateChunkType(int index)` Returns the type of the private chunk at a given index, as a 4-character `String`.
    `int[]` | `getSignificantBits()` Returns the number of significant bits for each band of the image.
    `int` | `getSRGBIntent()` Returns the sRGB rendering intent to be stored with this image.
    `PNGSuggestedPaletteEntry[]` | `getSuggestedPalette()` Returns the suggested palette information to be stored with this image.
    `java.lang.String[]` | `getText()` Returns the text strings to be stored in uncompressed form with this image as an array of `String`s.
    `boolean` | `isBackgroundSet()` Returns true if a 'bKGD' chunk will be output.
    `boolean` | `isChromaticitySet()` Returns true if a 'cHRM' chunk will be output.
    `boolean` | `isCompressedTextSet()` Returns true if a 'zTXT' chunk will be output.
    `boolean` | `isGammaSet()` Returns true if a 'gAMA' chunk will be output.
    `boolean` | `isICCProfileDataSet()` Returns true if a 'iCCP' chunk will be output.
    `boolean` | `isModificationTimeSet()` Returns true if a 'tIME' chunk will be output.
    `boolean` | `isPaletteHistogramSet()` Returns true if a 'hIST' chunk will be output.
    `boolean` | `isPhysicalDimensionSet()` Returns true if a 'pHYS' chunk will be output.
    `boolean` | `isSignificantBitsSet()` Returns true if an 'sBIT' chunk will be output.
    `boolean` | `isSRGBIntentSet()` Returns true if an 'sRGB' chunk will be output.
    `boolean` | `isSuggestedPaletteSet()` Returns true if a 'sPLT' chunk will be output.
    `boolean` | `isTextSet()` Returns true if a 'tEXt' chunk will be output.
    `boolean` | `isTransparencySet()` Returns true if a 'tRNS' chunk will be output.
    `static int` | `paethPredictor(int a, int b, int c)` The Paeth predictor routine used in PNG encoding.
    `void` | `removeAllPrivateChunks()` Remove all private chunks associated with this parameter instance.
    `void` | `removeUnsafeToCopyPrivateChunks()` Remove all private chunks associated with this parameter instance whose 'safe-to-copy' bit is not set.
    `abstract void` | `setBitDepth(int bitDepth)` Sets the desired bit depth of an image.
    `void` | `setChromaticity(float[] chromaticity)` Sets the white point and primary chromaticities in CIE (x, y) space.
    `void` | `setChromaticity(float whitePointX, float whitePointY, float redX, float redY, float greenX, float greenY, float blueX, float blueY)` A convenience method that calls the array version.
    `void` | `setCompressedText(java.lang.String[] text)` Sets the text strings to be stored in compressed form with this image.
    `void` | `setGamma(float gamma)` Sets the file gamma value for the image.
    `void` | `setICCProfileData(byte[] iccProfileData)` Sets the ICC profile data to be stored with this image.
    `void` | `setInterlacing(boolean useInterlacing)` Turns Adam7 interlacing on or off.
    `void` | `setModificationTime(java.util.Date modificationTime)` Sets the modification time, as a `Date`, to be stored with this image.
    `void` | `setPaletteHistogram(int[] paletteHistogram)` Sets the palette histogram to be stored with this image.
    `void` | `setPhysicalDimension(int[] physicalDimension)` Sets the physical dimension information to be stored with this image.
    `void` | `setPhysicalDimension(int xPixelsPerUnit, int yPixelsPerUnit, int unitSpecifier)` A convenience method that calls the array version.
    `void` | `setSignificantBits(int[] significantBits)` Sets the number of significant bits for each band of the image.
    `void` | `setSRGBIntent(int srgbIntent)` Sets the sRGB rendering intent to be stored with this image.
    `void` | `setSuggestedPalette(PNGSuggestedPaletteEntry[] palette)` Sets the suggested palette information to be stored with this image.
    `void` | `setText(java.lang.String[] text)` Sets the textual data to be stored in uncompressed form with this image.
    `void` | `unsetBackground()` Suppresses the 'bKGD' chunk from being output.
    `void` | `unsetBitDepth()` Suppresses the setting of the bit depth of a grayscale image.
    `void` | `unsetChromaticity()` Suppresses the 'cHRM' chunk from being output.
    `void` | `unsetCompressedText()` Suppresses the 'zTXt' chunk from being output.
    `void` | `unsetGamma()` Suppresses the 'gAMA' chunk from being output.
    `void` | `unsetICCProfileData()` Suppresses the 'iCCP' chunk from being output.
    `void` | `unsetModificationTime()` Suppresses the 'tIME' chunk from being output.
    `void` | `unsetPaletteHistogram()` Suppresses the 'hIST' chunk from being output.
    `void` | `unsetPhysicalDimension()` Suppresses the 'pHYS' chunk from being output.
    `void` | `unsetSignificantBits()` Suppresses the 'sBIT' chunk from being output.
    `void` | `unsetSRGBIntent()` Suppresses the 'sRGB' chunk from being output.
    `void` | `unsetSuggestedPalette()` Suppresses the 'sPLT' chunk from being output.
    `void` | `unsetText()` Suppresses the 'tEXt' chunk from being output.
    `void` | `unsetTransparency()` Suppresses the 'tRNS' chunk from being output.

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

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

- - ### Field Detail

    - #### INTENT_PERCEPTUAL

          public static final int INTENT_PERCEPTUAL

      Constant for use with the sRGB chunk.
      See Also:
        Constant Field Values

    - #### INTENT_RELATIVE

          public static final int INTENT_RELATIVE

      Constant for use with the sRGB chunk.
      See Also:
        Constant Field Values

    - #### INTENT_SATURATION

          public static final int INTENT_SATURATION

      Constant for use with the sRGB chunk.
      See Also:
        Constant Field Values

    - #### INTENT_ABSOLUTE

          public static final int INTENT_ABSOLUTE

      Constant for use with the sRGB chunk.
      See Also:
        Constant Field Values

    - #### PNG_FILTER_NONE

          public static final int PNG_FILTER_NONE

      Constant for use in filtering.
      See Also:
        Constant Field Values

    - #### PNG_FILTER_SUB

          public static final int PNG_FILTER_SUB

      Constant for use in filtering.
      See Also:
        Constant Field Values

    - #### PNG_FILTER_UP

          public static final int PNG_FILTER_UP

      Constant for use in filtering.
      See Also:
        Constant Field Values

    - #### PNG_FILTER_AVERAGE

          public static final int PNG_FILTER_AVERAGE

      Constant for use in filtering.
      See Also:
        Constant Field Values

    - #### PNG_FILTER_PAETH

          public static final int PNG_FILTER_PAETH

      Constant for use in filtering.
      See Also:
        Constant Field Values

    - #### bitDepth

          protected int bitDepth

    - #### bitDepthSet

          protected boolean bitDepthSet

  - ### Constructor Detail

    - #### PNGEncodeParam

          public PNGEncodeParam()

  - ### Method Detail

    - #### getDefaultEncodeParam

          public static PNGEncodeParam getDefaultEncodeParam(java.awt.image.RenderedImage im)

      Returns an instance of `PNGEncodeParam.Palette`, `PNGEncodeParam.Gray`, or `PNGEncodeParam.RGB` appropriate for encoding the given image.

      If the image has an `IndexColorModel`, an instance of `PNGEncodeParam.Palette` is returned. Otherwise, if the image has 1 or 2 bands an instance of `PNGEncodeParam.Gray` is returned. In all other cases an instance of `PNGEncodeParam.RGB` is returned.

      Note that this method does not provide any guarantee that the given image will be successfully encoded by the PNG encoder, as it only performs a very superficial analysis of the image structure.

    - #### setBitDepth

          public abstract void setBitDepth(int bitDepth)

      Sets the desired bit depth of an image.

    - #### getBitDepth

          public int getBitDepth()

      Returns the desired bit depth for a grayscale image.

      If the bit depth has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the bit depth is not set.

    - #### unsetBitDepth

          public void unsetBitDepth()

      Suppresses the setting of the bit depth of a grayscale image. The depth of the encoded image will be inferred from the source image bit depth, rounded up to the next power of 2 between 1 and 16.

    - #### setInterlacing

          public void setInterlacing(boolean useInterlacing)

      Turns Adam7 interlacing on or off.

    - #### getInterlacing

          public boolean getInterlacing()

      Returns `true` if Adam7 interlacing will be used.

    - #### unsetBackground

          public void unsetBackground()

      Suppresses the 'bKGD' chunk from being output. For API compatibility with JAI 1.0, the superclass defines this method to throw a `RuntimeException`; accordingly, subclasses must provide their own implementations.

    - #### isBackgroundSet

          public boolean isBackgroundSet()

      Returns true if a 'bKGD' chunk will be output. For API compatibility with JAI 1.0, the superclass defines this method to throw a `RuntimeException`; accordingly, subclasses must provide their own implementations.

    - #### setChromaticity

          public void setChromaticity(float[] chromaticity)

      Sets the white point and primary chromaticities in CIE (x, y) space.

      The `chromaticity` parameter should be a `float` array of length 8 containing the white point X and Y, red X and Y, green X and Y, and blue X and Y values in order.

      The 'cHRM' chunk will encode this information.

    - #### setChromaticity

          public void setChromaticity(float whitePointX,
                                      float whitePointY,
                                      float redX,
                                      float redY,
                                      float greenX,
                                      float greenY,
                                      float blueX,
                                      float blueY)

      A convenience method that calls the array version.

    - #### getChromaticity

          public float[] getChromaticity()

      Returns the white point and primary chromaticities in CIE (x, y) space.

      See the documentation for the `setChromaticity` method for the format of the returned data.

      If the chromaticity has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the chromaticity is not set.

    - #### unsetChromaticity

          public void unsetChromaticity()

      Suppresses the 'cHRM' chunk from being output.

    - #### isChromaticitySet

          public boolean isChromaticitySet()

      Returns true if a 'cHRM' chunk will be output.

    - #### setGamma

          public void setGamma(float gamma)

      Sets the file gamma value for the image.

      The 'gAMA' chunk will encode this information.

    - #### getGamma

          public float getGamma()

      Returns the file gamma value for the image.

      If the file gamma has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the gamma is not set.

    - #### unsetGamma

          public void unsetGamma()

      Suppresses the 'gAMA' chunk from being output.

    - #### isGammaSet

          public boolean isGammaSet()

      Returns true if a 'gAMA' chunk will be output.

    - #### setPaletteHistogram

          public void setPaletteHistogram(int[] paletteHistogram)

      Sets the palette histogram to be stored with this image. The histogram consists of an array of integers, one per palette entry.

      The 'hIST' chunk will encode this information.

    - #### getPaletteHistogram

          public int[] getPaletteHistogram()

      Returns the palette histogram to be stored with this image.

      If the histogram has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the histogram is not set.

    - #### unsetPaletteHistogram

          public void unsetPaletteHistogram()

      Suppresses the 'hIST' chunk from being output.

    - #### isPaletteHistogramSet

          public boolean isPaletteHistogramSet()

      Returns true if a 'hIST' chunk will be output.

    - #### setICCProfileData

          public void setICCProfileData(byte[] iccProfileData)

      Sets the ICC profile data to be stored with this image. The profile is represented in raw binary form.

      The 'iCCP' chunk will encode this information.

    - #### getICCProfileData

          public byte[] getICCProfileData()

      Returns the ICC profile data to be stored with this image.

      If the ICC profile has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the ICC profile is not set.

    - #### unsetICCProfileData

          public void unsetICCProfileData()

      Suppresses the 'iCCP' chunk from being output.

    - #### isICCProfileDataSet

          public boolean isICCProfileDataSet()

      Returns true if a 'iCCP' chunk will be output.

    - #### setPhysicalDimension

          public void setPhysicalDimension(int[] physicalDimension)

      Sets the physical dimension information to be stored with this image. The physicalDimension parameter should be a 3-entry array containing the number of pixels per unit in the X direction, the number of pixels per unit in the Y direction, and the unit specifier (0 = unknown, 1 = meters).

      The 'pHYS' chunk will encode this information.

    - #### setPhysicalDimension

          public void setPhysicalDimension(int xPixelsPerUnit,
                                           int yPixelsPerUnit,
                                           int unitSpecifier)

      A convenience method that calls the array version.

    - #### getPhysicalDimension

          public int[] getPhysicalDimension()

      Returns the physical dimension information to be stored with this image.

      If the physical dimension information has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the physical dimension information is not set.

    - #### unsetPhysicalDimension

          public void unsetPhysicalDimension()

      Suppresses the 'pHYS' chunk from being output.

    - #### isPhysicalDimensionSet

          public boolean isPhysicalDimensionSet()

      Returns true if a 'pHYS' chunk will be output.

    - #### setSuggestedPalette

          public void setSuggestedPalette(PNGSuggestedPaletteEntry[] palette)

      Sets the suggested palette information to be stored with this image. The information is passed to this method as an array of `PNGSuggestedPaletteEntry` objects.

      The 'sPLT' chunk will encode this information.

    - #### getSuggestedPalette

          public PNGSuggestedPaletteEntry[] getSuggestedPalette()

      Returns the suggested palette information to be stored with this image.

      If the suggested palette information has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the suggested palette information is not set.

    - #### unsetSuggestedPalette

          public void unsetSuggestedPalette()

      Suppresses the 'sPLT' chunk from being output.

    - #### isSuggestedPaletteSet

          public boolean isSuggestedPaletteSet()

      Returns true if a 'sPLT' chunk will be output.

    - #### setSignificantBits

          public void setSignificantBits(int[] significantBits)

      Sets the number of significant bits for each band of the image.

      The number of entries in the `significantBits` array must be equal to the number of output bands in the image: 1 for a gray image, 2 for gray+alpha, 3 for index or truecolor, and 4 for truecolor+alpha.

      The 'sBIT' chunk will encode this information.

    - #### getSignificantBits

          public int[] getSignificantBits()

      Returns the number of significant bits for each band of the image.

      If the significant bits values have not previously been set, or have been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the significant bits values are not set.

    - #### unsetSignificantBits

          public void unsetSignificantBits()

      Suppresses the 'sBIT' chunk from being output.

    - #### isSignificantBitsSet

          public boolean isSignificantBitsSet()

      Returns true if an 'sBIT' chunk will be output.

    - #### setSRGBIntent

          public void setSRGBIntent(int srgbIntent)

      Sets the sRGB rendering intent to be stored with this image. The legal values are 0 = Perceptual, 1 = Relative Colorimetric, 2 = Saturation, and 3 = Absolute Colorimetric. Refer to the PNG specification for information on these values.

      The 'sRGB' chunk will encode this information.

    - #### getSRGBIntent

          public int getSRGBIntent()

      Returns the sRGB rendering intent to be stored with this image.

      If the sRGB intent has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the sRGB intent is not set.

    - #### unsetSRGBIntent

          public void unsetSRGBIntent()

      Suppresses the 'sRGB' chunk from being output.

    - #### isSRGBIntentSet

          public boolean isSRGBIntentSet()

      Returns true if an 'sRGB' chunk will be output.

    - #### setText

          public void setText(java.lang.String[] text)

      Sets the textual data to be stored in uncompressed form with this image. The data is passed to this method as an array of `String`s.

      The 'tEXt' chunk will encode this information.

    - #### getText

          public java.lang.String[] getText()

      Returns the text strings to be stored in uncompressed form with this image as an array of `String`s.

      If the text strings have not previously been set, or have been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the text strings are not set.

    - #### unsetText

          public void unsetText()

      Suppresses the 'tEXt' chunk from being output.

    - #### isTextSet

          public boolean isTextSet()

      Returns true if a 'tEXt' chunk will be output.

    - #### setModificationTime

          public void setModificationTime(java.util.Date modificationTime)

      Sets the modification time, as a `Date`, to be stored with this image. The internal storage format will use UTC regardless of how the `modificationTime` parameter was created.

      The 'tIME' chunk will encode this information.

    - #### getModificationTime

          public java.util.Date getModificationTime()

      Returns the modification time to be stored with this image.

      If the bit depth has not previously been set, or has been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the bit depth is not set.

    - #### unsetModificationTime

          public void unsetModificationTime()

      Suppresses the 'tIME' chunk from being output.

    - #### isModificationTimeSet

          public boolean isModificationTimeSet()

      Returns true if a 'tIME' chunk will be output.

    - #### unsetTransparency

          public void unsetTransparency()

      Suppresses the 'tRNS' chunk from being output.

    - #### isTransparencySet

          public boolean isTransparencySet()

      Returns true if a 'tRNS' chunk will be output.

    - #### setCompressedText

          public void setCompressedText(java.lang.String[] text)

      Sets the text strings to be stored in compressed form with this image. The data is passed to this method as an array of `String`s.

      The 'zTXt' chunk will encode this information.

    - #### getCompressedText

          public java.lang.String[] getCompressedText()

      Returns the text strings to be stored in compressed form with this image as an array of `String`s.

      If the compressed text strings have not previously been set, or have been unset, an `IllegalStateException` will be thrown.

      Throws:
        `java.lang.IllegalStateException` - if the compressed text strings are not set.

    - #### unsetCompressedText

          public void unsetCompressedText()

      Suppresses the 'zTXt' chunk from being output.

    - #### isCompressedTextSet

          public boolean isCompressedTextSet()

      Returns true if a 'zTXT' chunk will be output.

    - #### addPrivateChunk

          public void addPrivateChunk(java.lang.String type,
                                      byte[] data)

      Adds a private chunk, in binary form, to the list of chunks to be stored with this image.
      Parameters:
        `type` - a 4-character String giving the chunk type name.
        `data` - an array of `byte`s containing the chunk data.

    - #### getNumPrivateChunks

          public int getNumPrivateChunks()

      Returns the number of private chunks to be written to the output file.

    - #### getPrivateChunkType

          public java.lang.String getPrivateChunkType(int index)

      Returns the type of the private chunk at a given index, as a 4-character `String`. The index must be smaller than the return value of `getNumPrivateChunks`.

    - #### getPrivateChunkData

          public byte[] getPrivateChunkData(int index)

      Returns the data associated of the private chunk at a given index, as an array of `byte`s. The index must be smaller than the return value of `getNumPrivateChunks`.

    - #### removeUnsafeToCopyPrivateChunks

          public void removeUnsafeToCopyPrivateChunks()

      Remove all private chunks associated with this parameter instance whose 'safe-to-copy' bit is not set. This may be advisable when transcoding PNG images.

    - #### removeAllPrivateChunks

          public void removeAllPrivateChunks()

      Remove all private chunks associated with this parameter instance.

    - #### paethPredictor

          public static int paethPredictor(int a,
                                           int b,
                                           int c)

      The Paeth predictor routine used in PNG encoding. This routine is included as a convenience to subclasses that override the `filterRow` method.

    - #### filterRow

          public int filterRow(byte[] currRow,
                               byte[] prevRow,
                               byte[][] scratchRows,
                               int bytesPerRow,
                               int bytesPerPixel)

      Performs filtering on a row of an image. This method may be overridden in order to provide a custom algorithm for choosing the filter type for a given row.

      The method is supplied with the current and previous rows of the image. For the first row of the image, or of an interlacing pass, the previous row array will be filled with zeros as required by the PNG specification.

      The method is also supplied with five scratch arrays. These arrays may be used within the method for any purpose. At method exit, the array at the index given by the return value of the method should contain the filtered data. The return value will also be used as the filter type.

      The default implementation of the method performs a trial encoding with each of the filter types, and computes the sum of absolute values of the differences between the raw bytes of the current row and the predicted values. The index of the filter producing the smallest result is returned.

      As an example, to perform only 'sub' filtering, this method could be implemented (non-optimally) as follows:

           for (int i = bytesPerPixel; i < bytesPerRow + bytesPerPixel; i++) {
               int curr = currRow[i] & 0xff;
               int left = currRow[i - bytesPerPixel] & 0xff;
               scratchRow[PNG_FILTER_SUB][i] = (byte)(curr - left);
           }
           return PNG_FILTER_SUB;

      Parameters:
        `currRow` - The current row as an array of `byte`s of length at least `bytesPerRow + bytesPerPixel`. The pixel data starts at index `bytesPerPixel`; the initial `bytesPerPixel` bytes are zero.
        `prevRow` - The current row as an array of `byte`s The pixel data starts at index `bytesPerPixel`; the initial `bytesPerPixel` bytes are zero.
        `scratchRows` - An array of 5 `byte` arrays of length at least `bytesPerRow + bytesPerPixel`, useable to hold temporary results. The filtered row will be returned as one of the entries of this array. The returned filtered data should start at index `bytesPerPixel`; The initial `bytesPerPixel` bytes are not used.
        `bytesPerRow` - The number of bytes in the image row. This value will always be greater than 0.
        `bytesPerPixel` - The number of bytes representing a single pixel, rounded up to an integer. This is the 'bpp' parameter described in the PNG specification.
      Returns:
        The filter type to be used. The entry of `scratchRows[]` at this index holds the filtered data.

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