[FOP-2847] [PATCH] Support palette-based transparency via tRNS chunk of PNG images in PDF export

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.3
• Component/s: renderer/pdf
• Assignee: Simon Steiner
• Reporter: Tobias Hain
• Created: 27/Feb/19 00:14
• Updated: 27/Mar/19 16:07
• Resolved: 27/Mar/19 16:07

Description

TL;NR

When using palette-based transparency via tRNS chunk in PNG images, the generated PDFs are broken.

palette-based with transparency.

8.5.2. Palette-Based with Transparency

"The PNG spec forbids the use of a full alpha channel with palette-based images, but it does allow `cheap alpha` via the transparency chunk, tRNS [...] In effect, it transforms the palette from an RGB lookup table to an RGBA table [...] a single PNG transparency entry should come first"

http://www.libpng.org/pub/png/book/chapter08.html#png.ch08.div.5.2

general PDF encoding strategy

Option 1:

most libraries just undo the palettization and use a full color image + mask.

https://forums.adobe.com/message/6148131#6148131

Option 2:

SMask would be simpler since you could leave the palette in place

https://forums.adobe.com/message/6148557#6148557

what does FOP do

It creates a chroma-key mask:

 /Type /XObject
  /ColorSpace [/Indexed /DeviceRGB 255 <D9DB ...>]
  /Mask [182 182 151 151 158 158]

Acrobat Reader even fails to read those files while PDF viewers embedded into Chrome/Firefox read the file, but show visual glitches:

https://github.com/apache/fop/pull/49 fixes the Acrobat Reader error.

FOP doesn't implement Option 1 yet. None of the ImageLoaders is prepared to do that:
https://wiki.apache.org/xmlgraphics-fop/HowTo/ImageLoaderRawPNG
In fact the following results in missing Option 2:

public void setup(PDFDocument doc) {
  RenderedImage ri = getImage().getRenderedImage();
  // code deleted
  Raster raster = GraphicsUtil.getAlphaRaster(ri);
  if (raster != null) {
    AlphaRasterImage alphaImage = new AlphaRasterImage("Mask:" + getKey(), raster);
    this.softMask = doc.addImage(null, alphaImage).makeReference();
  }

https://github.com/apache/fop/blob/ff452f3685a02715aee791d651b58dd1797ddfd4/fop-core/src/main/java/org/apache/fop/render/pdf/ImageRenderedAdapter.java

In the above code raster will be null and therefore FOP fails to initialize a softMask. GraphicsUtil.getAlphaRaster() calls internally ColorModel.getAlphaRaster(), which returns null:

If this is an IndexColorModel which has alpha in the lookup table, this method will return null since there is no spatially discrete alpha channel.

https://docs.oracle.com/javase/8/docs/api/index.html?java/awt/image/ColorModel.html

The softMask Issue is fixed by:
https://github.com/apache/fop/pull/50
As a result an image such as is rendered with a transparency gradient instead of

This code will then add the chroma key mask to the image:

protected void populateXObjectDictionaryForIndexColorModel(PDFDictionary dict, IndexColorModel icm) {
  // code deleted
  Integer index = getIndexOfFirstTransparentColorInPalette(icm);
  if (index != null) {
    PDFArray mask = new PDFArray(dict);
    mask.add(index);
    mask.add(index);
    dict.put("Mask", mask);
  }
}

https://github.com/apache/fop/blob/ff452f3685a02715aee791d651b58dd1797ddfd4/fop-core/src/main/java/org/apache/fop/render/pdf/AbstractImageAdapter.java

why care to support?

Libraries such as pngquant optimize images for web usage and low footprint and can not be used directly with Apache FOP as source images:

"pngquant: Lossy PNG optimizer, reducing a PNG image down to an 8 bit color palette with dithering. It will build indexed-color PNG's with alpha transparency colors conveyed in the tRNS chunk."

https://imagemagick.org/Usage/formats/#png_non-im

sample data

$ identify -verbose sample_image_palette_tRNS.png

  Format: PNG (Portable Network Graphics)
  Type: PaletteAlpha
  png:IHDR.color_type      : 3 (Indexed)
  png:tRNS                 : chunk was found

Issue Links

• relates to: FOP-2512 java.lang.NullPointerException: Parameter alpha must not be null