Added ability to render via viewport.

src/DtronixPdf/Actions/RenderPageAction.cs

@@ -10,7 +10,7 @@ namespace DtronixPdf.Actions
     {
         public readonly FpdfPageT _pageInstance;
         private readonly float _scale;
-        private readonly Rectangle _viewport;
+        private readonly RectangleF _viewport;
         private readonly RenderFlags _flags;
         private readonly Color? _backgroundColor;
         private readonly bool _includeAlpha;
@@ -20,7 +20,7 @@ namespace DtronixPdf.Actions
             ThreadDispatcher dispatcher,
             FpdfPageT pageInstance,
             float scale,
-            Rectangle viewport,
+            RectangleF viewport,
             RenderFlags flags,
             Color? backgroundColor,
             bool includeAlpha)
@@ -36,15 +36,17 @@ namespace DtronixPdf.Actions
 
         protected override PdfBitmap OnExecute()
         {
+            
             var bitmap = fpdfview.FPDFBitmapCreateEx(
-                _viewport.Width,
-                _viewport.Height,
+                (int)_viewport.Width,
+                (int)_viewport.Height,
                 (int) (_includeAlpha ? FPDFBitmapFormat.BGRA : FPDFBitmapFormat.BGR) , 
                 IntPtr.Zero, 
                 0);
 
             if(_backgroundColor.HasValue)
-                fpdfview.FPDFBitmapFillRect(bitmap, 0, 0, _viewport.Width, _viewport.Height, (uint) _backgroundColor.Value.ToArgb());
+                fpdfview.FPDFBitmapFillRect(
+                    bitmap, 0, 0, (int)_viewport.Width, (int)_viewport.Height, (uint) _backgroundColor.Value.ToArgb());
 
             if (bitmap == null)
                 throw new Exception("failed to create a bitmap object");
@@ -73,8 +75,8 @@ namespace DtronixPdf.Actions
 
                 return new PdfBitmap(
                     bitmap,
-                    _viewport.Width,
-                    _viewport.Height,
+                    (int)_viewport.Width,
+                    (int)_viewport.Height,
                     _dispatcher,
                     _includeAlpha ? PixelFormat.Format32bppArgb : PixelFormat.Format24bppRgb,
                     _scale,

src/DtronixPdf/PdfBitmap.cs

@@ -27,7 +27,7 @@ namespace DtronixPdf
 
         public float Scale { get; }
 
-        public Rectangle Viewport { get; }
+        public RectangleF Viewport { get; }
 
         public bool IsDisposed { get; private set; }
 
@@ -38,7 +38,7 @@ namespace DtronixPdf
             ThreadDispatcher dispatcher, 
             PixelFormat format, 
             float scale, 
-            Rectangle viewport)
+            RectangleF viewport)
         {
             _pdfBitmap = pdfBitmap;
             _dispatcher = dispatcher;

src/DtronixPdf/PdfPage.cs

@@ -59,19 +59,36 @@ namespace DtronixPdf
 
         public Task<PdfBitmap> Render(RenderFlags flags, float scale)
         {
-            return Render(flags, scale, new Rectangle(0,0, (int) (Size.Width * scale),(int) (Size.Height * scale)));
+            return Render(flags, scale, new Rectangle(0, 0, (int) (Size.Width * scale), (int) (Size.Height * scale)));
         }
 
 
-        public Task<PdfBitmap> Render(RenderFlags flags, float scale, Rectangle viewport)
+        public Task<PdfBitmap> Render(RenderFlags flags, float scale, RectangleF viewport)
         {
             return Render(flags, scale, viewport, false, Color.White);
         }
 
+        public Task<PdfBitmap> Render(
+            RenderFlags flags,
+            float scale,
+            Viewport viewport,
+            bool alpha,
+            Color? backgroundColor)
+        {
+            var translatedRectangle = new RectangleF(
+                (int) ((Size.Width / 2 - viewport.Size.Width / 2 + viewport.Center.X) * scale + viewport.Size.Width / 2 * (scale - 1)),
+                (int) ((Size.Height / 2 - viewport.Size.Height / 2 - viewport.Center.Y) * scale + viewport.Size.Height / 2 * (scale - 1)),
+                viewport.Size.Width,
+                viewport.Size.Height);
+
+            return Render(flags, scale, translatedRectangle, alpha, backgroundColor);
+        }
+
+
         public async Task<PdfBitmap> Render(
             RenderFlags flags,
             float scale,
-            Rectangle viewport,
+            RectangleF viewport,
             bool alpha,
             Color? backgroundColor)
         {
@@ -79,6 +96,7 @@ namespace DtronixPdf
                 new RenderPageAction(_dispatcher, _pageInstance, scale, viewport, flags, backgroundColor, alpha));
         }
 
+
         public async ValueTask DisposeAsync()
         {
             await _dispatcher.QueueForCompletion(() =>

src/DtronixPdf/Viewport.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.Drawing;
+using System.Text;
+
+namespace DtronixPdf
+{
+    public struct Viewport
+    {
+         public readonly PointF Center;
+         public readonly SizeF Size;
+
+        public Viewport(PointF center, SizeF size)
+        {
+            Center = center;
+            Size = size;
+        }
+
+        public Viewport(int x, int y, int width, int height)
+        {
+            Center = new Point(x, y);
+            Size = new Size(width, height);
+        }
+    }
+}