I'm currently endeavouring to add some visual reference to an already-functional mining laser tool. The player pushes a button, it triggers a boolean in extended data, which sets the mining process in motion where they're looking. What I'm currently struggling with is using the tessellator to draw the actual laser beam for everyone nearby.

 

This occurs in RenderPlayerEvent via my client event bus and I have verified that the following code is being run and returning the correct vector positions.

			if(data.getIsMining())
		{
			MovingObjectPosition rayTrace = RMHelper.getTargetPosition(thePlayer, theWorld, 16F);
			if(rayTrace != null && rayTrace.getBlockPos() != null)
			{
				/** Origin point of the laser beam */
				Vec3 posA = Vec3Helper.getLookVecPos(theEvent.entityPlayer);
				/** Position being struck by the laser beam */
				Vec3 posB = new Vec3(rayTrace.getBlockPos().getX(), rayTrace.getBlockPos().getY(), rayTrace.getBlockPos().getZ());

	            		Tessellator tessellator = Tessellator.getInstance();
	            		WorldRenderer world = tessellator.getWorldRenderer();
	            
	            		/** Code experimentally borrowed from RenderLightningBolt */
	            		GlStateManager.disableTexture2D();
	            		GlStateManager.disableLighting();
	            		GlStateManager.enableBlend();
	            		GlStateManager.blendFunc(770, 1);
	            
	            		world.startDrawing(7);
	            		world.setColorRGBA_F(1F, 0F, 0F, 1F);
	            
	            		Vec3 dirVec = posA.subtract(posB);
	            		Vec3 perpVec = (new Vec3(dirVec.zCoord, dirVec.yCoord, dirVec.xCoord)).normalize();
	            
	            		float width = 1F / 16F;
	            
	            		Vec3 R1 = new Vec3(posA.xCoord + perpVec.xCoord * width, posA.yCoord - 0.01, posA.zCoord + perpVec.zCoord * width);
	            		Vec3 R2 = new Vec3(posA.xCoord - perpVec.xCoord * width, posA.yCoord - 0.01, posA.zCoord + perpVec.zCoord * width);
	            		Vec3 R3 = new Vec3(posB.xCoord + perpVec.xCoord * width, posB.yCoord + 0.75, posB.zCoord + perpVec.zCoord * width);
	            		Vec3 R4 = new Vec3(posB.xCoord - perpVec.xCoord * width, posB.yCoord + 0.75, posB.zCoord + perpVec.zCoord * width);
	            
	            		world.addVertex(R1.xCoord + 0.5, R1.yCoord, R1.zCoord + 0.5);
	            		world.addVertex(R3.xCoord + 0.5, R3.yCoord, R3.zCoord + 0.5);
	            		world.addVertex(R4.xCoord + 0.5, R4.yCoord, R4.zCoord + 0.5);
	            		world.addVertex(R2.xCoord + 0.5, R2.yCoord, R2.zCoord + 0.5);
	            
	            		tessellator.draw();

	            		GlStateManager.disableBlend();
	            		GlStateManager.enableLighting();
	            		GlStateManager.enableTexture2D();
			}
		}

Currently, nothing appears on screen at all. I've tried switching the direction (b -> a instead of a -> b) and changing the origin point of the vectors, but to no avail. Graphics programming is by far my least level of competence, can anyone help me work this out?

You might find this post helpful. It explains how to render a beam from one point to another. You should be able to adapt this explanation to your needs.

You might find this post helpful. It explains how to render a beam from one point to another. You should be able to adapt this explanation to your needs.

Though certainly helpful in the theory section of things, my problem seems to lie more in the precise implementation I'm afraid.

Its just vector math.

This is the closest code I have that does what you need, but it is going to be in a different language.

http://answers.unity3d.com/answers/1065843/view.html

You'll want to look at the function starting on line 108, which takes in two Vec3 positions and calculates a camera facing quad.