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How to Export Presentations to HTML with Externally Linked Images inside .NET Apps

by sher azam   on Aug 12, 2015   Category: C#   |  Views: 609    |  Points: 25   |  Gold 


This technical tip explains how .NET developers can export presentations to HTML with externally linked images inside their .NET applications. This article describes an advanced technique that allows controlling which resources are embedded into the resulting HTML file and which are saved externally and referenced from the HTML file. The default HTML export behavior is to embed any resource into the HTML file. Such approach results in a single HTML file that is easy to view and distribute. All necessary resources are base64-encoded inside. But such approach has two drawbacks. The size of output is significantly larger because of the base64 encoding.* It is difficult to replace the images contained in the file. In this article we will see how we can change the default behavior using the Aspose.Slides for .NET to link the images externally rather than embedding in the HTML file. We will use ILinkEmbedController interface which contains three methods to control the resource embedding and saving process. We can pass this interface to HtmlOptions class constructor when preparing the export. Following is the complete code of LinkController class which implements the ILinkEmbedController interface. As mentioned before, the LinkController must implement ILinkEmbedController interface.


// Code Snippet for Exporting Presentations to HTML with Externally Linked Images

//[C# Code Sample]

/// <summary>
/// This class is responsible for making decisions about the resources saved externally.
/// It must implement the Aspose.Slides.Export.ILinkEmbedController interface.
/// </summary>
class LinkController : ILinkEmbedController
{
static LinkController()
{
s_templates.Add("image/jpeg", "image-{0}.jpg");
s_templates.Add("image/png", "image-{0}.png");
}

/// <summary>
/// Default parameterless constructor
/// </summary>
public LinkController()
{
m_externalImages = new Dictionary<int, string>();
}

/// <summary>
/// Creates a class instance and sets the path where generated resource files will be saved to.
/// </summary>
/// <param name="savePath">Path to the location where generated resource files will be stored.</param>
public LinkController(string savePath)
:this()
{
SavePath = savePath;
}

/// <summary>
/// A ILinkEmbedController member
/// </summary>
public LinkEmbedDecision GetObjectStoringLocation(int id, byte[] entityData, string semanticName,
string contentType,
string recomendedExtension)
{
// Here we make the decision about storing images externally.
// The id is unique identifier of each object during the whole export operation.

string template;

// The s_templates dictionary contains content types we are going to store externally and the corresponding file name template.
if (s_templates.TryGetValue(contentType, out template))
{
// Storing this resource to the export list
m_externalImages.Add(id, template);
return LinkEmbedDecision.Link;
}

// All other resources, if any, will be embedded
return LinkEmbedDecision.Embed;
}

/// <summary>
/// A ILinkEmbedController member
/// </summary>
public string GetUrl(int id, int referrer)
{
// Here we construct the resource reference string to form the tag: <img src="%result%">
// We need to check the dictionary to filter out unnecessary resources.
// Along with checking we extract the corresponding file name template.
string template;
if (m_externalImages.TryGetValue(id, out template))
{
// Assuming we are going to store resource files just near the HTML file.
// The image tag will look like <img src="image-1.png"> with the appropriate resource Id and extension.
var fileUrl = String.Format(template, id);
return fileUrl;
}

// null must be returned for the resources remaining embedded
return null;
}

/// <summary>
/// A ILinkEmbedController member
/// </summary>
public void SaveExternal(int id, byte[] entityData)
{
// Here we actually save the resource files to disk.
// Once again, checking the dictionary. If the id is not found here it is a sign of an error in GetObjectStoringLocation or GetUrl methods.
if (m_externalImages.ContainsKey(id))
{
// Now we use the file name stored in the dictionary and combine it with a path as required.

// Constructing the file name using the stored template and the Id.
var fileName = String.Format(m_externalImages[id], id);

// Combining with the location directory
var filePath = Path.Combine(SavePath ?? String.Empty, fileName);

using (var fs = new FileStream(filePath, FileMode.Create))
fs.Write(entityData, 0, entityData.Length);
}
else
throw new Exception("Something is wrong");
}

/// <summary>
/// Gets or sets the path where generated resource files will be saved to.
/// </summary>
public string SavePath { get; set; }

/// <summary>
/// A dictionary to store associations between resource ids and corresponding file names.
/// </summary>
private readonly Dictionary<int, string> m_externalImages;

/// <summary>
/// A dictionary to store associations between content types of resources we are going to store externally
/// and corresponding file name templates.
/// </summary>
private static readonly Dictionary<string, string> s_templates = new Dictionary<string, string>();
}

//[VB.NET Code Sample]


''' <summary>
''' This class is responsible for making decisions about the resources saved externally.
''' It must implement the Aspose.Slides.Export.ILinkEmbedController interface.
''' </summary>
Public Class LinkController
Implements ILinkEmbedController
Shared Sub New()
s_templates.Add("image/jpeg", "image-{0}.jpg")
s_templates.Add("image/png", "image-{0}.png")
End Sub

''' <summary>
''' Default parameterless constructor
''' </summary>
Public Sub New()
m_externalImages = New Dictionary(Of Integer, String)()
End Sub

''' <summary>
''' Creates a class instance and sets the path where generated resource files will be saved to.
''' </summary>
''' <param name="savePath">Path to the location where generated resource files will be stored.</param>
Public Sub New(ByVal savePath As String)
Me.New()
Me.SavePath = savePath
End Sub

''' <summary>
''' A ILinkEmbedController member
''' </summary>
Public Function GetObjectStoringLocation(ByVal id As Integer, ByVal entityData() As Byte, ByVal semanticName As String, ByVal contentType As String, ByVal recomendedExtension As String) As LinkEmbedDecision
' Here we make the decision about storing images externally.
' The id is unique identifier of each object during the whole export operation.

Dim template As String

' The s_templates dictionary contains content types we are going to store externally and the corresponding file name template.
If s_templates.TryGetValue(contentType, template) Then
' Storing this resource to the export list
m_externalImages.Add(id, template)
Return LinkEmbedDecision.Link
End If

' All other resources, if any, will be embedded
Return LinkEmbedDecision.Embed
End Function

''' <summary>
''' A ILinkEmbedController member
''' </summary>
Public Function GetUrl(ByVal id As Integer, ByVal referrer As Integer) As String
' Here we construct the resource reference string to form the tag: <img src="%result%">
' We need to check the dictionary to filter out unnecessary resources.
' Along with checking we extract the corresponding file name template.
Dim template As String
If m_externalImages.TryGetValue(id, template) Then
' Assuming we are going to store resource files just near the HTML file.
' The image tag will look like <img src="image-1.png"> with the appropriate resource Id and extension.
Dim fileUrl = String.Format(template, id)
Return fileUrl
End If

' null must be returned for the resources remaining embedded
Return Nothing
End Function

''' <summary>
''' A ILinkEmbedController member
''' </summary>
Public Sub SaveExternal(ByVal id As Integer, ByVal entityData() As Byte)
' Here we actually save the resource files to disk.
' Once again, checking the dictionary. If the id is not found here it is a sign of an error in GetObjectStoringLocation or GetUrl methods.
If m_externalImages.ContainsKey(id) Then
' Now we use the file name stored in the dictionary and combine it with a path as required.

' Constructing the file name using the stored template and the Id.
Dim fileName = String.Format(m_externalImages(id), id)

' Combining with the location directory
Dim filePath = Path.Combine(If((SavePath <> Nothing), SavePath, String.Empty), fileName)

Using fs = New FileStream(filePath, FileMode.Create)
fs.Write(entityData, 0, entityData.Length)
End Using
Else
Throw New Exception("Something is wrong")
End If
End Sub

''' <summary>
''' Gets or sets the path where generated resource files will be saved to.
''' </summary>
Private privateSavePath As String
Public Property SavePath() As String
Get
Return privateSavePath
End Get
Set(ByVal value As String)
privateSavePath = value
End Set
End Property

''' <summary>
''' A dictionary to store associations between resource ids and corresponding file names.
''' </summary>
Private ReadOnly m_externalImages As Dictionary(Of Integer, String)

''' <summary>
''' A dictionary to store associations between content types of resources we are going to store externally
''' and corresponding file name templates.
''' </summary>
Private Shared ReadOnly s_templates As Dictionary(Of String, String) = New Dictionary(Of String, String)()
End Class



After writing the LinkController class, now we will use it with HTMLOptions class to export the presentation to HTML having externally linked images using the following code.


//[C# Code Sample]

using (var pres = new Presentation(@"C:\data\input.pptx"))
{
var htmlOptions = new HtmlOptions(new LinkController(@"C:\data\out\"));
htmlOptions.SlideImageFormat = SlideImageFormat.Svg(new SVGOptions());

// This line is needed to remove the slide title display in HTML.
// Comment it out if your prefer slide title displayed.
htmlOptions.HtmlFormatter = HtmlFormatter.CreateDocumentFormatter(String.Empty, false);

Console.WriteLine("Starting export");
pres.Save(@"C:\data\out\output.html, SaveFormat.Html, htmlOptions);
}

//[VB.NET Code Sample]

Using pres = New Presentation("C:\data\input.pptx")
Dim htmlOptions = New HtmlOptions(New LinkController("C:\data\out\"))
htmlOptions.SlideImageFormat = SlideImageFormat.Svg(New SVGOptions())

' This line is needed to remove the slide title display in HTML.
' Comment it out if your prefer slide title displayed.
htmlOptions.HtmlFormatter = HtmlFormatter.CreateDocumentFormatter(String.Empty, False)

Console.WriteLine("Starting export")
pres.Save("C:\data\out\output.html, SaveFormat.Html, htmlOptions);" & ControlChars.CrLf & "}



Overview: Aspose.Slides for .NET

Aspose.Slides is a .NET component to read, write and modify a PowerPoint document without using MS PowerPoint. PowerPoint versions from 97-2007 and all three PowerPoint formats: PPT, POT, PPS are also supported. Now you can create, access, copy, clone, edit and delete slides in your presentations. Other features include PPT/PPTX printing, PPT to XPS format, saving PowerPoint slides into PDF, adding & modifying audio & video frames, using shapes like rectangles or ellipses and saving presentations in SVG format, streams or images.

- Homepage of Aspose.Slides for .NET: http://www.aspose.com/.net/powerpoint-component.aspx

- Download Aspose.Slides for .NET: http://www.aspose.com/community/files/51/.net-components/aspose.slides-for-.net/default.aspx



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