Update README.md

README.md

@@ -2,10 +2,16 @@
 PNG library for reading and writing of PNG images in combination with a TBitmap32 class from the GR32 library.
 
 ## Usage
+In order to use the library ensure that you use the unit GR32_PNG. This unit uses GR32_PortableNetworkGraphics to read and write PNG files, which makes it necessary to add the file to your project as well. Or just add the directory to the library path.
+
 The simplest way to load a PNG to a TBitmap32 instance is by using:
 
     LoadBitmap32FromPNG(Bitmap: TBitmap32; const Filename: string);
 
+You can check if the file is a valid PNG file by using
+
+    function IsValidPNG(const Filename: string): Boolean;
+
 If the PNG file comes from a stream one can also use
 
     LoadBitmap32FromPNG(Bitmap: TBitmap32; Stream: TStream);
@@ -32,3 +38,35 @@ To save the content of a TBitmap32 instance to a PNG file or stream the function
     procedure SaveBitmap32ToPNG(Bitmap: TBitmap32; Stream: TStream);
 
 can be used.
+
+## Advanced Usage
+If you use the simple functions you won't be able to track the progress during loading. To do so you must create an instance of TPortableNetworkGraphic32 and load the PNG file manually. Before actually loading you can specify an OnProgress handler to track the progress. The code will then look like this:
+
+      with TPortableNetworkGraphic32.Create do
+      try
+        OnProgress := OnProgressHandler;
+        LoadFromFile(Filename);
+        AssignTo(YourBitmap); // can be Image32.Bitmap for example
+      finally
+        Free;
+      end;
+
+Where the OnProgressHandler might looks like this:
+
+    procedure TForm1.OnProgressHandler(Sender: TObject; Percent: Single);
+    begin
+      ProgressBar.Position := Round(Percent);
+    end;
+
+If you want to change some settings while saving you need to do this manually as well. The code can then look like this:
+
+      with TPortableNetworkGraphic32.Create do
+      try
+        AssignTo(YourBitmap); // can be Image32.Bitmap for example
+        Interlaced := imAdam7; // save as interlaced png file
+        SaveToFile(Filename);
+      finally
+        Free;
+      end;
+
+With the current state of implementation it's not possible to alter everything without ending in an invalid PNG file. For example you could convert the image to use a pallete and then alter or delete the palette before saving. Doing so will not give you an error, but most probably the image won't be readable by other programs (or at least different to the original image).