Create CMYK+alpha bitmap, mapping black to specific CMYK color

Question

Given this arched text image, black pixels on transparent background, with antialiasing

I would ultimately like to have a CMYK rendering of this text, where every black pixel becomes a specific CMYK color, say magenta, or {72,36,9,28}, or whatever my customer specifies. The ultimate output will be PDF. Every transparent pixel needs to remain transparent, as there may be other PDF objects under the text.

I would be content with a CMYK+Alpha TIFF equivalent written to disk. Or, possibly, a System.Windows.Media.Imaging.Bitmapimage. Or something else.

My PDF library is ABCpdf.NET 10. I realize that System.Drawing does not support CMYK color format. I'm ok with other third party libraries; I'm already using AForge in another context in the library.

I am not looking to do RGB to CMYK conversion, not even with a profile. I am not looking to do generic color replacement in a PDF. My customer will specify a definite CMYK target color, and I need to match it exactly; using color conversion on an RGB approximation is not okay.

I've flailed around with various things. The closest I got was to swap the colors while staying in RGB -- black becomes transparent, transparent becomes white -- and draw a filled magenta box under the image. That then draws white pixels where the text is not, and the magenta underneath shows through. Considered as a group, this ends up looking like warped magenta text on white. But anything sitting under the group does not show through. link

Any ideas?

Answer 1

Jos Vernon at websupergoo came up with a splendidly simple answer that uses ABCpdf capabilities.

using WebSupergoo.ABCpdf10;
using WebSupergoo.ABCpdf10.Objects;

namespace RecolorRenderedImage
{
    class Program
    {
        static void Main(string[] args)
        {
            using (Doc theDoc = new Doc()) {
                // this background image is there to prove that the text is not obscuring other pdf objects
                theDoc.AddImage("c:\\temp\\background-pic.jpg");

                // this is the black warped text
                string fullFilePath = "c:\\temp\\foobar.png";

                // read it into an XImage. Add to the doc, and retrieve the pixmap
                XReadOptions readOptions = new XReadOptions();
                XImage image = XImage.FromFile(fullFilePath, readOptions);
                int theID = theDoc.AddImageObject(image, true);
                int imageID = theDoc.GetInfoInt(theID, "XObject");
                PixMap thePM = (PixMap)theDoc.ObjectSoup[imageID];

                // recolor the pixmap temporary spot color space with the desired color (gold in this case)
                int spotColorID = theDoc.AddColorSpaceSpot("tempSpace", "24 44 100 2");
                ColorSpace theSP = (ColorSpace)theDoc.ObjectSoup[spotColorID];
                thePM.Recolor(theSP);

                // immediately recolor the pixmap back to CMYK
                ColorSpace theSP2 = new ColorSpace(theDoc.ObjectSoup, ColorSpaceType.DeviceCMYK);
                thePM.Recolor(theSP2);

                theDoc.Save("c:\\temp\\test.pdf");
            }
        }

    }
}

LATE EDIT: removed the link to the output file since I deleted it from GDrive at some point.