TOPIC:

BMP mBMP = new BMP();
mBMP.Create(bitmap);
// MulAlpha is needed to renormalize RGB values in presence of alpha channel !0xFF
mBMP.MulAlpha();

            // Import the first page from the test document
            String inputPath = "/sdcard/DCIM/ImportTest/Alpha_Test_Orig.pdf";
            String outputPath = "/sdcard/DCIM/Alpha_Test_Mod.pdf";
            new File(outputPath).delete();
            Document doc = new Document();
            doc.Open(inputPath, "");
            Document newDoc = new Document();
            newDoc.Create(outputPath);
            newDoc.SetCache(mActivity.getCacheDir() + "/" + new File(outputPath).getName() + ".dat");
            Document.ImportContext context = newDoc.ImportStart(doc);
            newDoc.ImportPage(context, 0, 0);
            context.Destroy();
            doc.Close();

            // Render a bitmap with the word "Test" with a white background
            // at 50% transparency
            int bpWidth = 300, bpHeight = 300;
            Bitmap bitmap = Bitmap.createBitmap(bpWidth, bpHeight, Bitmap.Config.ARGB_8888);
            Canvas canvas = new Canvas(bitmap);
            // Create a transparent background first
            canvas.drawColor(0, PorterDuff.Mode.CLEAR);
            Paint paint = new Paint();
            paint.setColor(Color.WHITE);
            paint.setAlpha(0x80); // 50% transparency
            paint.setTextSize(48);
            paint.setTextAlign(Paint.Align.CENTER);
            paint.setFlags(Paint.ANTI_ALIAS_FLAG | Paint.FILTER_BITMAP_FLAG);
            paint.setStyle(Paint.Style.FILL_AND_STROKE);
            // Render the background
            canvas.drawRect(0, 0, bpWidth, bpHeight, paint);
            // Render the red text (also at 50% transparency)
            paint.setColor(Color.RED);
            canvas.drawText("Test", bpWidth / 2, bpHeight / 3, paint);

            // Now add the bp to the PDF
            Page page = newDoc.GetPage(0);
            page.ObjsStart();
            float pageHeight = newDoc.GetPageHeight(0);
            float left = 100;
            float top = pageHeight - 200;
            // For simplicity, use bitmap annotation, but could also use PageContent for same result
            page.AddAnnotBitmap(bitmap, true, new float[]{left, top - bpHeight, left + bpWidth, top});
            newDoc.Save();
            newDoc.Close();