How to assign session value in javascript ? (webcam data uri not not accessible on button click in server )

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VB

ASP.NET

Javascript

i am working on a application where users can capture their photo through webcam. after capturing , data-uri (base64 string) saved in database table. i am binding the data-uri to the image control.and the captured image showing in the image control.

i am using the asp hidden field control in javascript to get the data uri , after click on the capture button, the data-uri value (base64 string) is saving in database table. (image is also showing in the asp image control)

in local machine , it is working fine. (data uri is saving in database table).
but when upload on server, it is not working . after click on capture button, the hidden field value is coming blank. (the hidden field contains the data-uri(base64 string)).

base64string:data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAfQAAAFeCAYAAABzUe0CAAAgAElEQVR4XlS9WY+lWXaetyNOnBhzqKyseeyhqieSTYmSKdOWJUgCbfjCF7YF+BcZ9oVlg5BpyxQMkWqRIiTbIgTSgAATkKAby7IsDi2JbLJpqtnNrq7qqqwcYjwnIvw+z7u/LDqrozPjnG/Ye+211rumvfbOL/7d//H29nYzdnZuxhi7Y+d2Z4yb23



how to get the hidden field value on button click ? or how to assign the hidden field value in session object in javascript. so that i can get the data-uri from the session object.

please help ..

What I have tried:

<asp:HiddenField ID="hdnVal" runat="server"  EnableViewState="true"  />

<script type="text/javascript" src="css/webcam.min.js"></script>
	
	<!-- Configure a few settings and attach camera -->
	<script language="JavaScript" type="text/javascript" >
	    Webcam.set({
	        width: 320,
	        height: 240,
	        dest_width: 500,
	        dest_height: 350,
	        image_format: 'png',
	        jpeg_quality: 90
	    });
	    Webcam.attach('#my_camera');





	    function take_snapshot() {
	        // take snapshot and get image data
	        var val = document.getElementById('<%=hdnVal.ClientID %>');
	        Webcam.snap(function (data_uri) {
	            var data = data_uri.toString();
	            val.value = data_uri.toString();
	            alert(val.value.toString());
	            document.getElementById('<%=hdnVal.ClientID %>').innerText = data_uri;
	        });
	    }
	
	
	</script>

Capture Button click event code :

img.ImageUrl = hdnVal.Value.ToString()
img.Visible = True

Try
    con.Open()
    Dim cmd As New SqlCommand()
    cmd.CommandText = "spAddImages"
    cmd.CommandType = Data.CommandType.StoredProcedure
    cmd.Connection = con
    cmd.Parameters.Add("@val", Data.SqlDbType.NVarChar).Value = hdnVal.Value.ToString()

    cmd.ExecuteNonQuery()
    con.Close()
Catch ex As Exception

End Try