Replace black color in PDF file

Question

I'm trying to replace the black (0,0,0) color in a PDF file (it's not a spot color with name, it's normal fill color) but I could not find a way to do that yet, could anyone please help me with this?

Attached the PDF file: https://gofile.io/d/AB1Bil

Answer 1

Making use of the PdfContentStreamEditor from this answer, you can replace the instructions selecting a black'ish RGB color in the page content streams like this:

float[] replacementColor = new float[] {.1f, .7f, .6f};

PDDocument document = ...;
for (PDPage page : document.getDocumentCatalog().getPages()) {
    PdfContentStreamEditor identity = new PdfContentStreamEditor(document, page) {
        @Override
        protected void write(ContentStreamWriter contentStreamWriter, Operator operator, List<COSBase> operands) throws IOException {
            String operatorString = operator.getName();

            if (RGB_FILL_COLOR_OPERATORS.contains(operatorString))
            {
                if (operands.size() == 3) {
                    if (isApproximately(operands.get(0), 0) &&
                            isApproximately(operands.get(1), 0) &&
                            isApproximately(operands.get(2), 0)) {
                        for (int i = 0; i < replacementColor.length; i++) {
                            operands.set(i, new COSFloat(replacementColor[i]));
                        }
                    }
                }
            }

            super.write(contentStreamWriter, operator, operands);
        }

        boolean isApproximately(COSBase number, float compare) {
            return (number instanceof COSNumber) && Math.abs(((COSNumber)number).floatValue() - compare) < 1e-4;
        }

        final List<String> RGB_FILL_COLOR_OPERATORS = Arrays.asList("rg", "sc", "scn");
    };
    identity.processPage(page);
}
document.save("gridShapesModified-RGBBlackReplaced.pdf");

(EditPageContent test testReplaceRGBBlackGridShapesModified)

Of course, for your example PDF one could have simply taken the content stream and replaced 0 0 0 rg by .1 .7 .6 rg for the same effect. In general, though, the situation can be more complicated, so I propose this approach.

Beware, though:

• The content stream editor only manipulates the immediate page content stream. But colors can also be set and used in XObjects and patterns used from there. So strictly speaking one has to descent into the XObjects and patterns in the page resources.
• I blindly treat all three-parameter sc and scn operation as if they set RGB colors. In truth they can also refer to Lab colors and (for scn) to Pattern, Separation, DeviceN and ICCBased colors. Strictly speaking on should test the current non-stroking colorspace here.
• I completely ignore that content added over other content with interesting blend modes may result in a displayed color differing from the current fill color.