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

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

Andreas Joseph Krogh (Jira)

     [ https://issues.apache.org/jira/browse/FOP-2847?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

simon steiner resolved FOP-2847.
--------------------------------
    Resolution: Fixed
      Assignee: simon steiner

http://svn.apache.org/viewvc?view=revision&revision=1856423

> [PATCH] Support palette-based transparency via tRNS chunk of PNG images in PDF export
> -------------------------------------------------------------------------------------
>
>                 Key: FOP-2847
>                 URL: https://issues.apache.org/jira/browse/FOP-2847
>             Project: FOP
>          Issue Type: Bug
>          Components: renderer/pdf
>    Affects Versions: 2.3
>            Reporter: Tobias Hain
>            Assignee: simon steiner
>            Priority: Major
>         Attachments: acrobat_reader_error_message.PNG, broken_generated_pdf.pdf, pal_bk.png, pdf_trans_broken.png, sample_image_palette_tRNS.png
>
>
> h4. TL;NR
> When using palette-based transparency via tRNS chunk in PNG images, the generated PDFs are broken.
> h4. palette-based with transparency.
> {quote}*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"
> {quote}
> [http://www.libpng.org/pub/png/book/chapter08.html#png.ch08.div.5.2]
> h4. general PDF encoding strategy
> Option 1:
> {quote}most libraries just undo the palettization and use a full color image + mask.
> {quote}
> [https://forums.adobe.com/message/6148131#6148131]
> Option 2:
> {quote}SMask would be simpler since you could leave the palette in place
> {quote}
> [https://forums.adobe.com/message/6148557#6148557]
> h4. what does FOP do
> It creates a chroma-key mask:
> {code:java}
>  /Type /XObject
>   /ColorSpace [/Indexed /DeviceRGB 255 <D9DB ...>]
>   /Mask [182 182 151 151 158 158]
> {code}
> Acrobat Reader even fails to read those files while PDF viewers embedded into Chrome/Firefox read the file, but show visual glitches:
> !acrobat_reader_error_message.PNG!
> 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:
> {code:java}
> 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();
>   }
> {code}
> [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:
> {quote}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.
> {quote}
> [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 !pal_bk.png! is rendered with a transparency gradient instead of !pdf_trans_broken.png!
> This code will then add the chroma key mask to the image:
> {code:java}
> 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);
>   }
> }
> {code}
> [https://github.com/apache/fop/blob/ff452f3685a02715aee791d651b58dd1797ddfd4/fop-core/src/main/java/org/apache/fop/render/pdf/AbstractImageAdapter.java]
> h4. 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:
> {quote}"*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."
> {quote}
> [https://imagemagick.org/Usage/formats/#png_non-im]
> h4. sample data
> {code}
> $ 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
> {code}



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