Jani Pehkonen commented on PDFBOX-4095:

{quote}some blendmodes don't work properly against the page background
The "unmarked space" simply means a raster buffer (like BufferedImage) that is 
initially fully transparent. This makes the blend modes work. No new graphics 
engine is needed.

See how Mozilla PDF.js has implemented it 
 If a PDF page uses any blend modes [other than 
 PDF.js renders everything on a fully transparent RGBA canvas. Finally when the 
page has been rendered, PDF.js draws the RGBA canvas on a white canvas.

When rendering graphics like this in a fully transparent buffer, you are 
actually implementing _page groups_, which are explained in the PDF 32000 
standard in section 11.4.7.

> Non Separable Blend Modes implementation(HUE, SATURATION, COLOR, LUMINOSITY)
> ----------------------------------------------------------------------------
>                 Key: PDFBOX-4095
>                 URL: https://issues.apache.org/jira/browse/PDFBOX-4095
>             Project: PDFBox
>          Issue Type: Improvement
>          Components: Rendering
>    Affects Versions: 2.0.8
>            Reporter: savan patel
>            Priority: Major
>         Attachments: BlendComposite.java.patch, BlendMode.java.patch, 
> COSName.java.patch, blendmodes.pdf, gs-bugzilla690477.pdf, 
> gs-bugzilla697433-1.pdf
> I have a patch for implementation of non separable blend modes...

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