Geometric transformations of images in MIDP 1.0
MIDP 1.0 neither provides low-level access to the pixel data
of an Image object, nor any other means to even query the color
of a specific pixel. However, it is still possible to rotate,
mirror and scale images, or to transform them in any other way
which only involves copying individual pixels to new locations.
This can be achieved simply by calling
once for each destination pixel,setting the clipping rectangle
and drawing coordinates appriopriately each time.
Why would you want to do this at all?
The approach outlined above works perfectly, albeit with the
Many games graphics contain redundancies (e.g.
terrain tiles), where the same graphics appear in
multiple orientations. Putting redundant image data
into a packaged application which may already have a
tight size restriction seems a bit wasteful. Incidentally,
you also use this technique to crop individual tiles out
of an image containing an entire tileset.
Scaling allows to use image data in applications which are
intended to run with arbitry display sizes (within reasonable
limits), although the effects of scaling on image quality
needs to be considered in each case.
The performance can be increased for many transformations
by exploiting their specific properties. For example, to mirror
an image you can "draw-copy" entire rows or columns instead
of individual pixels. The same holds if you scale an image
along a single axis only. A rotation about 180 degrees can
be expressed by two mirrors, and scaling along both axes
can be decomposed by scaling one axis first. Cutting a tile
out of a tileset image involves only a single draw operation.
Even so, you will usually apply these transformations
only once during application initialization.
The lack of interpolation is not relevant for many practical
transformations. Of the transformations mentioned above, only
the scaling would benefit from it. The lack of transparency
is more serious, as it prevents using the above technique for
game foreground objects usually referred to as sprites.
Unfortunately, MIDP 1.0 (and unbelievably, also MIDP 2.0) does
not provide the means to create a mutable image with a
transparent background (like, e.g., the Nokia UI
It is not very efficient in terms of speed.
It does not allow interpolation.
It does not support transparency.
And now for something completely different
In theory, there is yet another approach for working with images
in MIDP, and that is to load the image data from the resources into
an array yourself. Since you are probably not willing to write and
package a fully fledged PNG decoder, this would mean to define a
rather simple custom image format with some basic compression
suitable for your type of graphics. Since you have the image
data in an array, you can then apply any transformation your
heart desires. Finally, you need to
create images from your arrays.
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