I want to create a utility which will seamlessly tileablize any bitmap it inputs, and output the result to another bitmap file. I have already devised an algorithm to vertically or horizontally seamlessly tilize any bitmap, pseudocode goes like this:
take bottom half of image, superimpose on top half of image, smoothly gradiating the superimposed half's translucency from 0% to 100% (top to bottom). do the same, superimposing top half on bottom half.
This will make the bitmap vertically seamlessly tileable. the same concept can be used for the right and left halves of the image to make it horizontally seamlessly tileable. Unfortunately, I haven't yet figured out how to make an image both vertically AND horizonatally tileable. I know it's possible, because one can do it in any good image editing program, such as GIMP or Photoshop, but unfortunately my pain program doesn't allow this, and besides, I want to do it to bitmaps on mass production scales for my game. Anybody out there know a way to do this?

Sorry Matt, but if it's what I think it is, it won't work to do one after the other. What you could do is instead of doing a rectangle the way you mentioned, do a triangle from the corners to the center. That might work a little better.
When Paintshop Pro does it, it requires that the source bitmap be at least 4 times the size of the destination (2x as wide, 2x as tall). Obviously it uses some of the outside areas around it but I'm not sure how...
-Goku
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well, this may get messy with some bitmaps, but i use this "algorithm" (more like a method) to make a tile when i'm using corel photo-paint or whatever:

1 - take the bitmap and shift it by half the width horizontally and half the height vertically, wrapping at the edges. this essentially moves the borders into a 'crosshairs' formation into the center of your image.

2 - now work from the centerpoint of each side, blurring or translucently merging as you which, growing wider as you reach the center of the image. do this incrementally, taking one step closer to the center from each edge at a time.

3 - repeat 1 and 2 at least once.

however, you should know that what you are attempting is quite ambitious. for instance, the method i describe above would not work for textures with sharp edges such as marble. it would just look terrible.

to do something with sharp edges, i would keep step 1, but step 2 would be much more complex. you would have to discern where the sharp edges were with a edge detection routine. apply segments to the edges, and connect the segments across the borders by shifting the endpoints. then use the morph algorithm on the segments you drew previously. sound messy? i can tell you, it certainly is!

but for blurry textures, the first algorithm i described should be somewhat effective.

good luck!

jase

Update on the tilizablizer algorithm!
It turns out I was agonizing over this for nothing. It was as easy as doing my method first vertically, then horizonatlly, as was originally suggested by MattSmith. The reason I thought it wouldn't work was that I thought I'd tried it. I hadn't though, foolishly, because what I had done was to perform the algorithm horizontally and vertically, and merge the two images together. This destroyed the ability of the algorithm to make the image both vertically and horizotally tileable. This is a very nice, very compact algorithm which tileablizes any input bitmap, without the use of messy blurring effects, and without the use of messy triangular bitmap pieces.
-Ed

The Inescapable conclusion of doing this to too many different inputs would be that they all have blurry grey edges.
ideally you want to match the 4 edges individually and then blend to them. You could define one bitmap as the 'archetype' of each edge.
If you store the edge types in the tile data you can use this to help your map-generating algorithm.