Java Doc for Image.java in  » 6.0-JDK-Modules » j2me » javax » microedition » lcdui » Java Source Code / Java DocumentationJava Source Code and Java Documentation

Images are either mutable or immutable depending upon how they are created. Immutable images are generally created by loading image data from resource bundles, from files, or from the network. They may not be modified once created. Mutable images are created as blank images containing only white pixels. The application may render on a mutable image by calling Image.getGraphics on the Image to obtain a Graphics object expressly for this purpose.

Images may be placed within Alert, Choice, Form, or ImageItem objects. The high-level user interface implementation may need to update the display at any time, without notifying the application. In order to provide predictable behavior, the high-level user interface objects provide snapshot semantics for the image. That is, when a mutable image is placed within an Alert, Choice, Form, or ImageItem object, the effect is as if a snapshot is taken of its current contents. This snapshot is then used for all subsequent painting of the high-level user interface component. If the application modifies the contents of the image, the application must update the component containing the image (for example, by calling ImageItem.setImage) in order to make the modified contents visible.

An immutable image may be created from a mutable image through the use of the Image.createImage(Image) createImage method. It is possible to create a mutable copy of an immutable image using a technique similar to the following:

 Image source; // the image to be copied    
 source = Image.createImage(...);    
 Image copy = Image
 .createImage(source.getWidth(), source.getHeight());        
 Graphics g = copy.getGraphics();    
 g.drawImage(source, 0, 0, TOP|LEFT);       

Alpha Processing

Every pixel within a mutable image is always fully opaque. Immutable images may contain a combination of fully opaque pixels (alpha = 2bitdepth - 1), fully transparent pixels (alpha = 0), and semitransparent pixels (0 < alpha <  2bitdepth - 1), where bitdepth is the number of bits per sample in the source data.

Implementations must support storage, processing, and rendering of fully opaque pixels and fully transparent pixels in immutable images. When creating an image from source data (whether from a PNG file or from an array of ARGB data), a fully opaque pixel in the source data must always result in a fully opaque pixel in the new image, and a fully transparent pixel in the source data must always result in a fully transparent pixel in the new image.

The required treatment of semitransparent pixel data depends upon whether the implementation supports alpha blending at rendering time. If the implementation supports alpha blending, a semitransparent pixel in the source data must result in a semitransparent pixel in the new image. The resulting alpha value may be modified to accommodate the number of levels of semitransparency supported by the platform. (See the Display.numAlphaLevels Display.numAlphaLevels() method.) If an implementation does not support alpha blending, any semitransparent pixels in the source data must be replaced with fully transparent pixels in the new image.

PNG Image Format

Implementations are required to support images stored in the PNG format, as specified by the PNG (Portable Network Graphics) Specification, Version 1.0. All conforming MIDP implementations are also conformant to the minimum set of requirements given by the PNG Specification. MIDP implementations also must conform to additional requirements given here with respect to handling of PNG images. Note that the requirements listed here take precedence over any conflicting recommendations given in the PNG Specification.

Critical Chunks

All of the 'critical' chunks specified by PNG must be supported. The paragraphs below describe these critical chunks.

The IHDR chunk. MIDP devices must handle the following values in the IHDR chunk:

• All positive values of width and height are supported; however, a very large image may not be readable because of memory constraints. The dimensions of the resulting Image object must match the dimensions of the PNG image. That is, the values returned by Image.getWidth() getWidth() and Image.getHeight() getHeight() and the rendered width and height must equal the width and height specified in the IHDR chunk.
• All color types are supported, although the appearance of the image will be dependent on the capabilities of the device's screen. Color types that include alpha channel data are supported.
• For color types 4 & 6 (grayscale with alpha and RGB with alpha, respectively) the alpha channel must be decoded. Any pixels with an alpha value of zero must be treated as transparent. Any pixels with an alpha value of 255 (for images with 8 bits per sample) or 65535 (for images with 16 bits per sample) must be treated as opaque. If rendering with alpha blending is supported, any pixels with intermediate alpha values must be carried through to the resulting image. If alpha blending is not supported, any pixels with intermediate alpha values must be replaced with fully transparent pixels.
• All bit depth values for the given color type are supported.
• Compression method 0 (deflate) is the only supported compression method. This method utilizes the "zlib" compression scheme, which is also used for jar files; thus, the decompression (inflate) code may be shared between the jar decoding and PNG decoding implementations. As noted in the PNG specification, the compressed data stream may comprised internally of both compressed and uncompressed (raw) data.
• The filter method represents a series of encoding schemes that may be used to optimize compression. The PNG spec currently defines a single filter method (method 0) that is an adaptive filtering scheme with five basic filter types. Filtering is essential for optimal compression since it allows the deflate algorithm to exploit spatial similarities within the image. Therefore, MIDP devices must support all five filter types defined by filter method 0.
• MIDP devices are required to read PNG images that are encoded with either interlace method 0 (None) or interlace method 1 (Adam7). Image loading in MIDP is synchronous and cannot be overlapped with image rendering, and so there is no advantage for an application to use interlace method 1. Support for decoding interlaced images is required for compatibility with PNG and for the convenience of developers who may already have interlaced images available.

The PLTE chunk. Palette-based images must be supported.

The IDAT chunk. Image data may be encoded using any of the 5 filter types defined by filter method 0 (None, Sub, Up, Average, Paeth).

The IEND chunk. This chunk must be found in order for the image to be considered valid.

Ancillary Chunks

PNG defines several 'ancillary' chunks that may be present in a PNG image but are not critical for image decoding.

The tRNS chunk. All implementations must support the tRNS chunk. This chunk is used to implement transparency without providing alpha channel data for each pixel. For color types 0 and 2, a particular gray or RGB value is defined to be a transparent pixel. In this case, the implementation must treat pixels with this value as fully transparent. Pixel value comparison must be based on the actual pixel values using the original sample depth; that is, this comparison must be performed before the pixel values are resampled to reflect the display capabilities of the device. For color type 3 (indexed color), 8-bit alpha values are potentially provided for each entry in the color palette. In this case, the implementation must treat pixels with an alpha value of 0 as fully transparent, and it must treat pixels with an alpha value of 255 as fully opaque. If rendering with alpha blending is supported, any pixels with intermediate alpha values must be carried through to the resulting image. If alpha blending is not supported, any pixels with intermediate alpha values must be replaced with fully transparent pixels.

The implementation may (but is not required to) support any of the other ancillary chunks. The implementation must silently ignore any unsupported ancillary chunks that it encounters. The currently defined optional ancillary chunks are:

 cHRM gAMA hIST iCCP iTXt pHYs
 sBIT sPLT sRGB tEXt tIME zTXt 

Reference

PNG (Portable Network Graphics) Specification, Version 1.0. W3C Recommendation, October 1, 1996. http://www.w3.org/TR/REC-png.html. Also available as RFC 2083, http://www.ietf.org/rfc/rfc2083.txt.

The source image may be mutable or immutable.

This method is useful for placing the contents of mutable images into Choice objects. The application can create an off-screen image using the Image.createImage(int,int) createImage(w, h) method, draw into it using a Graphics object obtained with the Image.getGraphics() getGraphics() method, and then create an immutable copy of it with this method. The immutable copy may then be placed into Choice objects.

The imageoffset and imagelength parameters specify a range of data within the imageData byte array. The imageOffset parameter specifies the offset into the array of the first data byte to be used. It must therefore lie within the range [0..(imageData.length-1)]. The imageLength parameter specifies the number of data bytes to be used. It must be a positive integer and it must not cause the range to extend beyond the end of the array. That is, it must be true that imageOffset + imageLength < imageData.length.

This method is intended for use when loading an image from a variety of sources, such as from persistent storage or from the network.

The source image may be mutable or immutable. For immutable source images, transparency information, if any, is copied to the new image unchanged.

On some devices, pre-transformed images may render more quickly than images that are transformed on the fly using drawRegion. However, creating such images does consume additional heap space, so this technique should be applied only to images whose rendering speed is critical.

The transform function used must be one of the following, as defined in the javax.microedition.lcdui.game.Sprite Sprite class:
Sprite.TRANS_NONE - causes the specified image region to be copied unchanged
Sprite.TRANS_ROT90 - causes the specified image region to be rotated clockwise by 90 degrees.
Sprite.TRANS_ROT180 - causes the specified image region to be rotated clockwise by 180 degrees.
Sprite.TRANS_ROT270 - causes the specified image region to be rotated clockwise by 270 degrees.
Sprite.TRANS_MIRROR - causes the specified image region to be reflected about its vertical center.
Sprite.TRANS_MIRROR_ROT90 - causes the specified image region to be reflected about its vertical center and then rotated clockwise by 90 degrees.
Sprite.TRANS_MIRROR_ROT180 - causes the specified image region to be reflected about its vertical center and then rotated clockwise by 180 degrees.
Sprite.TRANS_MIRROR_ROT270 - causes the specified image region to be reflected about its vertical center and then rotated clockwise by 270 degrees.

The size of the returned image will be the size of the specified region with the transform applied. For example, if the region is 100 x 50 pixels and the transform is TRANS_ROT90, the returned image will be 50 x 100 pixels.

Note: If all of the following conditions are met, this method may simply return the source Image without creating a new one:

• the source image is immutable;
• the region represents the entire source image; and
• the transform is TRANS_NONE.

Consider P(a,b) to be the value of the pixel located at column a and row b of the Image, where rows and columns are numbered downward from the top starting at zero, and columns are numbered rightward from the left starting at zero. This operation can then be defined as:

 P(a, b) = rgb[a + b * width];    

for

 0 <= a < width
 0 <= b < height    

The newly created Graphics object has the following properties:

• the destination is this Image object;
• the clip region encompasses the entire Image;
• the current color is black;
• the font is the same as the font returned by Font.getDefaultFont Font.getDefaultFont() ;
• the stroke style is Graphics.SOLID SOLID ; and
• the origin of the coordinate system is located at the upper-left corner of the Image.

The lifetime of Graphics objects created using this method is indefinite. They may be used at any time, by any thread.