ALPHA CHANNELS AND TRANSPARENCY in Java

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ALPHA CHANNELS AND TRANSPARENCY
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Many image formats support some form of transparency, but only a few support full alpha channels. An alpha channel is an extra layer in your image that defines the amount of transparency or opacity for each pixel. Typically this is expressed as an integer value between 0 and 255, although floating point values between 0 and 1 are also fairly common (see, for example, the SVG format). Alpha channels normally express the amount of opacity, with a value of 0 denoting a fully transparent pixel, and a value of 255 (or 1) a fully opaque one. Transparency can also be specified without a full alpha channel, for image formats that use a color palette. For example, the GIF format allows you to mark one of the colors in the palette as fully transparent, and the PNG format allows associating an opacity value with each palette entry separately. Both of the main image manipulation packages for Perl, Image::Magick and GD, support transparency, but in very different ways. GD only allows you to mark a single color in its palette as transparent, while Image::Magick supports a full alpha channel model. Thus, if you want to do any serious transparency work, your only real option is Image::Magick.
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Transparency and the GD module
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As stated, the GD module only supports transparency in a very limited manner. This is due to the fact that all colors in GD are stored in an index, and only one of those index entries can be marked as transparent. This in turn is most likely inherited from the GIF format, wherein transparency is implemented in much the same way.
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ALPHA CHANNELS AND TRANSPARENCY
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transparent() is the method to call for making a color transparent, and is used in the following manner:
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use GD; my $gd = GD::Image->new(200, 100); my $white = $gd->colorAllocate(255, 255, 255); my $black = $gd->colorAllocate( 0, 0, 0); # Draw a black ellipse, centered $gd->arc(99, 49, 150, 75, 0, 360, $black); # Make the ellipse transparent $gd->transparent($black); # I changed my mind. Make the background transparent $gd->transparent($white);
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That s all there is to it. Note that this transparency will only be stored in image formats that can support it. If you save your GD object as a JPEG image, the transparency is lost.12 12.4.2
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Transparency and Image::Magick
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Image::Magick supports a full alpha channel, but it also supports simple palette-based transparency as described in the section on GD. The choice between them depends on whether you re working with an indexed image or a full color image, and in which format you are saving your results. In Image::Magick, the alpha channel is called the matte layer,13 and the opacity is generally referred to as the matte value. A matte value of 0 indicates that a pixel is fully transparent, while a matte value of 255 indicates full opacity. Whenever an image has an active matte channel, the matte attribute for the image object should have a true value. If you want to add transparency to an image, you need to set matte to a true value. Methods that can be used to directly influence the matte value of pixels are MatteFloodFill(), Draw() with the matte primitive, Transparent(), and Set() with the pixel[$x,$y] attribute. The matte layer as a whole can be inspected using the Layer() method with the argument 'Matte', and the value of individual pixels can be read with Get() and the pixel[$x,$y] argument See appendix A for an indepth discussion of these methods. Replacing the whole alpha channel The most flexible and easy way to add an alpha channel to your image is by directly inserting a grayscale image as the matte layer. The lighter the pixels in the grayscale
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GD also has trouble reading PNG images that contain more transparency information than it can handle internally. When you attempt to read a PNG image with a full alpha channel, an error message will be printed, and the alpha channel will be ignored. Although the terminology in the latest versions of Image::Magick is slowly changing towards channels, as opposed to layers.
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