diff --git a/index.js b/index.js
index 1067a69..1ff44de 100644
--- a/index.js
+++ b/index.js
@@ -1,6 +1,7 @@
 import { loadTexture, makeProgram } from "./gl";
 import { blockLookup, BlockType, generateMissingChunks, makeWorld, updateWorldGeometry } from './world';
 import * as se3 from './se3';
+import { makeBufferFromFaces, makeFace } from "./geometry";
 
 const TEST_VSHADER = `
 attribute vec3 aPosition;
@@ -62,6 +63,8 @@ function draw(gl, params, objects) {
 
     gl.enable(gl.CULL_FACE);
     gl.cullFace(gl.BACK);
+    gl.enable(gl.BLEND);
+    gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
 
     gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
 
@@ -196,6 +199,34 @@ function updatePhysics(params) {
     params.isOnGround = isOnGround;
 }
 
+function tagABlock(gl, params, objects) {
+    const dir = [0, 0, -1, 1.0];
+    const camori = params.camera.orientation;
+    const ori = se3.inverse(se3.rotxyz(-camori[0], -camori[1], -camori[2]));
+    const viewDirection = se3.apply(ori, dir).slice(0, 3);
+
+    const face = markBlock(params.world, params.camera.position, viewDirection);
+    if (face === undefined) {
+        return;
+    }
+
+    if (params.tagBuffer !== undefined) {
+        gl.deleteBuffer(params.tagBuffer.glBuffer);
+        delete params.tagBuffer;
+    }
+    const buffer = makeBufferFromFaces(gl, [face]);
+    params.tagBuffer = buffer;
+
+    const obj =  {
+            position: [0.0, 0.0, 0.0],
+            orientation: [0.0, 0.0, 0.0],
+            geometry: buffer,
+            glContext: params.worldGl,
+    };
+
+    objects.push(obj);
+}
+
 function tick(time, gl, params) {
     handleKeys(params);
     updatePhysics(params);
@@ -218,6 +249,9 @@ function tick(time, gl, params) {
     }
 
     const objects = getObjects(params.world, campos[2], campos[0], params.worldGl);
+
+    tagABlock(gl, params, objects);
+
     draw(gl, params, objects);
 
     const dt = (time - stuff.lastFrameTime) * 0.001;
@@ -278,13 +312,99 @@ function checkCollision(curPos, newPos, world) {
     };
 }
 
+function minIndex(arr) {
+    return arr.reduce((min, val, i) => val >= arr[min] ? min : i, -1);
+}
+
+function movePoint(p, s, u) {
+    return [p[0] + s * u[0], p[1] + s * u[1], p[2] + s * u[2]];
+}
+
+function rayThroughGrid(origin, direction, maxDistance) {
+    const range = i => [...Array(i).keys()];
+    const length = p => Math.sqrt(p[0] * p[0] + p[1] * p[1] + p[2] * [2]);
+
+    const nextGrid = range(3).map(i => direction[i] > 0 ?
+        Math.floor(origin[i] + 0.5) + 0.5 :
+        Math.floor(origin[i] + 0.5) - 0.5);
+    const distanceToGrid = range(3).map(i => (nextGrid[i] - origin[i]) / direction[i])
+        .map(v => v === 0.0 ? Number.POSITIVE_INFINITY : v);
+    const axis = minIndex(distanceToGrid);
+    const rayLength = distanceToGrid[axis];
+
+    if (rayLength > maxDistance) {
+        return {};
+    }
+
+    const position = movePoint(origin, distanceToGrid[axis], direction);
+    const normal = range(3).map(i => i === axis ? -Math.sign(direction[i]) : 0);
+
+    return {position, normal, distance: rayLength};
+}
+
+function castRay(world, origin, direction, maxDistance) {
+    let currentPoint = origin;
+
+    while (maxDistance > 0) {
+        const {position, normal, distance} = rayThroughGrid(currentPoint, direction, maxDistance);
+
+        if (position === undefined) {
+            return;
+        }
+
+        maxDistance -= distance;
+        currentPoint = position;
+        const blockCenter = movePoint(position, -0.5, normal);
+        const block = blockLookup(world, ...blockCenter);
+
+        if (block.type === BlockType.AIR) {
+            continue;
+        }
+
+        return {
+            block,
+            normal,
+        };
+    }
+}
+
+function markBlock(world, cameraPosition, direction) {
+    const maxDistance = 10;
+    const hit = castRay(world, cameraPosition, direction, maxDistance);
+
+    if (hit === undefined) {
+        return;
+    }
+
+    const texture = [0, 14];
+    const {normal, block} = hit;
+    const faceCenter = movePoint(block.centerPosition, 0.51, normal);
+
+    if (normal[0] > 0) {
+        return makeFace('+x', texture, faceCenter);
+    } else if (normal[0] < 0) {
+        return makeFace('-x', texture, faceCenter);
+    } else if (normal[1] > 0) {
+        return makeFace('+y', texture, faceCenter);
+    } else if (normal[1] < 0) {
+        return makeFace('-y', texture, faceCenter);
+    } else if (normal[2] > 0) {
+        return makeFace('+z', texture, faceCenter);
+    } else if (normal[2] < 0) {
+        return makeFace('-z', texture, faceCenter);
+    }
+}
+
 // Mine & place
 // ------------
-// [ ] ray casting
-// [ ] block outline
+// [x] ray casting
+// [x] block outline
 // [ ] crosshair
 // [ ] dynamic terrain re-rendering
 // [ ] should use a linked list of air contact blocks
+//    --> might not be needed. Only need to re-render a single chunk,
+//        should be fast enough. We render 16 of them every time we
+//        walk 16 blocks in any direction.
 
 // Stuff I need to do:
 // [x] a skybox
diff --git a/se3.js b/se3.js
index 3b88fae..5717e9d 100644
--- a/se3.js
+++ b/se3.js
@@ -68,6 +68,16 @@ export function translation(x, y, z) {
     ];
 }
 
+export function inverse(m) {
+    // TODO: translation
+    return [
+        m[0], m[4], m[8], 0.0,
+        m[1], m[5], m[9], 0.0,
+        m[2], m[6], m[10], 0.0,
+        0, 0, 0, 1,
+    ];
+}
+
 export function product(a, b) {
     const c = (i, j) => (
         a[4 * 0 + i] * b[4 * j + 0] +
diff --git a/texture.png b/texture.png
index aab2d7b..72efbd4 100644
Binary files a/texture.png and b/texture.png differ