JPEG's lossy compression algorithm works well on photographs but performs quite poorly on images with sharp edges, such as drawings, charts and graphs. The "random art" image below was saved as JPEG and PNG. The former shows some unsightly artifacts while the latter is nice and clean.

JPEG	PNG

Unless animation is needed, PNG is a much better alternative to GIF as well. While the latter uses lossless compression, it only supports palettes of up to 256 colors and therefore is unsuitable for high-quality photographic images. PNG, on the other hand, supports both palette-based and true colors. PNG's support for transparency is far better than GIF's as well.

Starting with version 2.1, AspJpeg is capable of saving images in PNG format. All you need to do is set the property PNGOutput to True before calling Save (or SendBinary/Binary).

If the currently opened image has an alpha channel, it will be automatically preserved when the image is saved as a PNG. If the image is resized, its alpha channel is resized also.

The following code sample resizes the sample PNG image realty.png used in Chapter 6 and preserves its alpha channel.

VB Script:

Set Jpeg = Server.CreateObject("Persits.Jpeg") Jpeg.Open Server.MapPath("../images/realty.png") ' resize by 50% Jpeg.PreserveAspectRatio = True Jpeg.Width = Jpeg.OriginalWidth / 2 ' Output as PNG Jpeg.PNGOutput = True ' Save Jpeg.Save Server.MapPath("realty_small.png")

C#:

IASPJpeg objJpeg = new ASPJpeg(); objJpeg.Open( Server.MapPath("../images/realty.png") ); // resize by 50% objJpeg.PreserveAspectRatio = 1; objJpeg.Width = objJpeg.OriginalWidth / 2; // Output as PNG objJpeg.PNGOutput = 1; // Save objJpeg.Save( Server.MapPath("realty_small.png") );

Click the links below to run this code sample:

AspJpeg enables you to set a new alpha channel for the image, and also remove an existing one, via the methods SetAlpha and RemoveAlpha, respectively. It also allows you to modify the alpha channel in several ways.

10.3.1 SetAlpha and RemoveAlpha Methods

The SetAlpha method expects two arguments: an instance of the ASPJpeg object representing the alpha channel, and a Boolean flag specifying whether the alpha values should be inversed.

The image representing the alpha channel must have the same dimensions as the image to which it is assigned, and be in grayscale colorspace (i.e. 1 byte per pixel.) If the 2nd argument is False, the color value of 0 (black) of the alpha channel image corresponds to full transparency, and the color value of 255 (white) to full opacity. If the 2nd argument is True, 255 corresponds to full transparency and 0 to full opacity.

For example, the following code snippet creates a red image with a fully transparent round hole by using another image with a filled circle as the alpha channel:

VB Script:

Set Jpeg = Server.CreateObject("Persits.Jpeg") Jpeg.New 100, 100, &HFFFF0000 ' red background Set Alpha = Server.CreateObject("Persits.Jpeg") Alpha.New 100, 100, &HFFFFFFFF Alpha.Canvas.DrawCircle 50, 50, 30 Alpha.ToGrayscale (0) Jpeg.SetAlpha Alpha, False Jpeg.PNGOutput = True Jpeg.Save "c:\path\out.png"

C#:

IASPJpeg objJpeg = new ASPJpeg(); IASPJpeg objAlpha = new ASPJpeg(); objJpeg.New( 100, 100, 0xFFFF0000 ); objAlpha.New( 100, 100, 0xFFFFFFFF ); objAlpha.Canvas.DrawCircle( 50, 50, 30 ); objAlpha.ToGrayscale(0); objJpeg.SetAlpha( (ASPJpeg)objAlpha, false ); objJpeg.PNGOutput = 1; objJpeg.SaveUnique( @"c:\path\out.png" );

Here is the output image shown on a blue background to make its transparency more obvious:

If the 2nd argument to SetAlpha is changed to True, the output will be as follows:

The alpha channel image must be in the grayscale color space. That is why the script above calls the method ToGrayscale (introduced in Version 2.1 also.) Failure to call this method would result in an exception since images created with the New method are always RGB.

The SetAlpha method also allows the 1st argument to be Nothing (null.) In that case, the alpha channel will be set evenly to 255 (full opacity) for all pixels if the 2nd argument is False, and to 0 (full transparency) if True.

To remove an existing alpha channel, call RemoveAlpha. This method has no arguments. If the current image has no alpha channel, calling this method has no effect. To check if the current image has an alpha channel, use the Boolean property AlphaExists.

10.3.2 AlphaPixels Property

You can read and change the individual pixels of the current alpha channel via the parameterized property AlphaPixels, for example:

val = Jpeg.AlphaPixels(10, 20)

or

Jpeg.AlphaPixels(10, 20) = 20

The property sets and returns values in the range 0 to 255.

10.3.3 Drawing on Images with Alpha Channel

Various Canvas methods such as DrawLine or PrintTextEx can be used on an image whether it has an alpha channel or not. However, if the graphics or text being drawn falls on a fully transparent area of the image, it will not be visible on the resultant PNG.

To ensure the 100%-visibility of the text or graphics being drawn, the alpha channel has to be modified also to provide an opaque background for the drawing. The same text or graphics needs to be drawn separately on the alpha channel in white color (which designates full opacity).

The method AlphaToImage turns the alpha channel of an image into an independent image with a Canvas object of its own, which can be drawn on.

The following code snippet opens a PNG image with an alpha channel, separates its alpha channel into a separate ASPJpeg object, draws some text on the original image and the alpha channel image, and merges the modified alpha channel with the original image, thus ensuring that the text is fully visible on the resultant PNG.

VB Script:

Set Jpeg = Server.CreateObject("Persits.Jpeg") Jpeg.Open Server.MapPath("../images/realty.png") Font = Jpeg.WindowsDirectory & "\Fonts\Arial.ttf" ' Print on image Jpeg.Canvas.PrintTextEx "Some text", 50, 20, Font ' Print on alpha channel by creating a separate object Set Alpha = Server.CreateObject("Persits.Jpeg") Alpha.Open Server.MapPath("../images/realty.png") Alpha.AlphaToImage ' populate image with alpha channel Alpha.ToRGB ' make it RGB so that we can print on it Alpha.Canvas.Font.Color = &HFFFFFFFF ' white color! Alpha.Canvas.PrintTextEx "Some text", 50, 20, Font Alpha.ToGrayscale(1) ' turn back into grayscale Jpeg.SetAlpha Alpha, False ' Output as PNG Jpeg.PNGOutput = True ' Save Jpeg.Save Server.MapPath("realty_small.png")

C#:

IASPJpeg objJpeg = new ASPJpeg(); string strFont = objJpeg.WindowsDirectory + @"\Fonts\Arial.ttf"; objJpeg.Open( Server.MapPath("../images/realty.png") ); // Print on image objJpeg.Canvas.PrintTextEx( "Some text", 50, 20, strFont ); // Print on alpha channel by creating a separate object IASPJpeg objAlpha = new ASPJpeg(); objAlpha.Open( Server.MapPath("../images/realty.png") ); objAlpha.AlphaToImage(); // populate image with alpha channel objAlpha.ToRGB(); // make it RGB so that we can print on it objAlpha.Canvas.Font.Color = 0xFFFFFF; // white color! objAlpha.Canvas.PrintTextEx( "Some text", 50, 20, strFont ); objAlpha.ToGrayscale(1); // turn back into grayscale objJpeg.SetAlpha( (ASPJpeg)objAlpha, false ); // Output as PNG objJpeg.PNGOutput = 1; // Save objJpeg.Save( Server.MapPath("realty_small.png") );

Click the links below to run this code sample: