# [1.8] collisionRayTrace() help needed

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Hi, I made a block with lots of different possible shapes, which is made up of up to 8 smaller blocks, each an exact 8th of a full block. So you can build slabs, stairs or something else. I'm using DrawBlockHighlightEvent to draw the wireframe as it should be, but I also need to use collisionRayTrace so the block doesn't get selected when you look at it's 1x1x1 bounding box, but the spot where you look is empty. Like for stairs, you can "look through" the part that has no texture.

I looked at it's collisionRayTrace method, and also at the super method, but I can't figure out what's going on there. If somebody has a good understanding of this method and is willing to explain it to me, that would be nice

Reygok

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Hi

The basic idea is to shoot a 'ray' (a line) from the player's eyes.  You can see the full algorithm in action in World.rayTraceBlocks().

You can imagine a grid of cubes around the player, say like glass blocks.  The ray trace algorithm starts at the player's eyes, starts moving in the direction that the player is looking, and finds the first side of the "glass block" that the ray hits.  If it's a solid block, it stops.  If it's not solid, it continues the ray through the block to find out which side the ray exits, then checks that block.  It keeps doing that until either it hits something or the ray gets too long.

When the ray enters a more complicated blocks (say like a torch), it will check to see whether the ray actually hits the torch, or whether it misses and passes through to the next block.  That's what Block.collisionRayTrace does.  For most blocks, they just need to set their bounds correctly and the vanilla code does the rest.  In your case, you will probably need to do a "sub-block" ray trace.  BlockStairs is a good example, although it's a bit optimised so it's harder to follow.  A simpler algorithm for you would be "test every side of every sub-cube to see if the ray intersects it (similar to the super method).  It will probably be fast enough already, and if it isn't, you'll have a good test case for when you develop a faster implementation.

-TGG

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Okay, that helps, will look into it more. Thanks

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