How to: Replace a Form Field with an Image

• Jun 20, 2019
• 4 minutes to read

This topic describes how to substitute an interactive form field in a PDF document with an image. To perform this task, remove the required form field from the document and display an image instead of the deleted field. Form field removal is necessary, because widget annotation is drawn over the page content and can overlap an image added at the form field’s position.

Follow the steps below to replace a form field with an image that maintains the same size and position.

• Iterate through the PdfInteractiveForm.Fields collection (PdfDocumentProcessor.Document.AcroForm.Fields) to obtain a form field you want to replace.
• Use the PdfInteractiveFormField.Widget property to access widget annotation associated with the form field. The PdfAnnotation.Page property returns the page where this widget annotation is located.
• Obtain the page boundaries and the widget’s size and position on the page using the PdfPage.CropBox (PdfPageTreeObject.CropBox) and PdfAnnotation.Rect properties, respectively.

• Call the PdfGraphics.DrawImage method to draw an image at the form field’s position.

  Note that PdfGraphics uses the world coordinate system, whereas the PdfAnnotation.Page and PdfPageTreeObject.CropBox properties work with the user coordinate system (refer to the Coordinate Systems article for more information on coordinate systems in the PDF Document API). The following formula is used to convert user coordinates to world coordinates.

  UnitX = ((rect.Left – cropBox.Left) / 72) * dpiX; UnitY = ((cropBox.Top – rect.Bottom) / 72 ) * dpiY; where PdfRectangle rect = field.Widget.Rect; PdfRectangle cropBox = field.Widget.Page.CropBox;

  This code example uses 72 for dpiX and dpiY values to simplify the formula above.

• Use the PdfGraphics.AddToPageForeground method to add graphics to the PDF page’s foreground. Pass 72 as DPI values to this method to transform world coordinates to page coordinates without scaling.
• Remove the form field by calling the PdfDocumentProcessor.RemoveFormField method.

Note

A complete sample project is available on GitHub at https://github.com/DevExpress-Examples/how-to-replace-form-field-with-image.

C#

using System.Drawing;
using DevExpress.Pdf;

namespace ReplaceFormFieldWithImage {
    class Program {
        static void Main(string[] args) {

            using (PdfDocumentProcessor processor = new PdfDocumentProcessor()) {

                // Load a PDF document with AcroForm data. 
                processor.LoadDocument("..\\..\\InteractiveForm.pdf");

                foreach (PdfInteractiveFormField field in processor.Document.AcroForm.Fields) {

                    if (field.Name == "Address") {

                        PdfRectangle cropBox = field.Widget.Page.CropBox;
                        PdfRectangle rect = field.Widget.Rect;

                        double x = rect.Left - cropBox.Left;
                        double y = cropBox.Top - rect.Bottom;

                        // Create graphics and draw an image instead of the form field.
                        using (PdfGraphics graphics = processor.CreateGraphics()) {
                            DrawImage(graphics, rect, x, y);
                            graphics.AddToPageForeground(processor.Document.Pages[0], 72, 72);
                        }
                    }
                }
                // Remove the form field.
                processor.RemoveFormField("Address");
                // Save the document.
                processor.SaveDocument("..\\..\\Result.pdf");
            }
        }

        static void DrawImage(PdfGraphics graphics, PdfRectangle rect, double x, double y) {

            Bitmap image = new Bitmap("..\\..\\AddressFormField.png");

            double aspectRatio = image.Width / image.Height;

            double scaleX = image.Width / rect.Width;
            double scaleY = image.Height / rect.Height;

            double width;
            double height;

            if (scaleX < scaleY) {
                width = rect.Width;
                height = width / aspectRatio;
            }

            else {
                height = rect.Height;
                width = height * aspectRatio;
            }

            RectangleF imageRect = new RectangleF((float)x, (float)(y - height), (float)width, (float)height);
            graphics.DrawImage(image, imageRect);
        }
    }
}

VB.NET

Imports System.Drawing
Imports DevExpress.Pdf

Namespace ReplaceFormFieldWithImage
    Friend Class Program
        Shared Sub Main(ByVal args() As String)

            Using processor As New PdfDocumentProcessor()

                ' Load a PDF document with AcroForm data. 
                processor.LoadDocument("..\..\InteractiveForm.pdf")

                For Each field As PdfInteractiveFormField In processor.Document.AcroForm.Fields

                    If field.Name = "Address" Then

                        Dim cropBox As PdfRectangle = field.Widget.Page.CropBox
                        Dim rect As PdfRectangle = field.Widget.Rect

                        Dim x As Double = rect.Left - cropBox.Left
                        Dim y As Double = cropBox.Top - rect.Bottom

                        ' Create graphics and draw an image instead of the form field.
                        Using graphics As PdfGraphics = processor.CreateGraphics()
                            DrawImage(graphics, rect, x, y)
                            graphics.AddToPageForeground(processor.Document.Pages(0), 72, 72)
                        End Using
                    End If
                Next field
                ' Remove the form field.
                processor.RemoveFormField("Address")
                ' Save the document.
                processor.SaveDocument("..\..\Result.pdf")
            End Using
        End Sub

        Private Shared Sub DrawImage(ByVal graphics As PdfGraphics, ByVal rect As PdfRectangle, ByVal x As Double, ByVal y As Double)

            Dim image As New Bitmap("..\..\AddressFormField.png")

            Dim aspectRatio As Double = image.Width \ image.Height

            Dim scaleX As Double = image.Width \ CInt(rect.Width)
            Dim scaleY As Double = image.Height \ CInt(rect.Height)

            Dim width As Double
            Dim height As Double

            If scaleX < scaleY Then

                width = rect.Width
                height = width / aspectRatio

            Else
                height = rect.Height
                width = height * aspectRatio
            End If

            Dim imageRect As New RectangleF(CSng(x), CSng(y - height), CSng(width), CSng(height))
            graphics.DrawImage(image, imageRect)
        End Sub
    End Class
End Namespace