我一直在用纯 Python 编写一个路径跟踪器，只是为了好玩，而且由于我之前的阴影不太漂亮（兰伯特余弦定律），我正在尝试实现递归路径跟踪。
我的引擎给出了一个失败的输出：
我的路径跟踪函数是递归定义的，如下所示：
def TracePath2(ray, scene, bounce_count):
result = 100000.0
hit = False
answer = Color(0.0, 0.0, 0.0)
for object in scene.objects:
test = object.intersection(ray)
if test and test < result:
result = test
hit = object
if not hit:
return answer
if hit.emittance:
return hit.diffuse * hit.emittance
if hit.diffuse:
direction = RandomDirectionInHemisphere(hit.normal(ray.position(result)))
n = Ray(ray.position(result), direction)
dp = direction.dot(hit.normal(ray.position(result)))
answer += TracePath2(n, scene, bounce_count + 1) * hit.diffuse * dp
return answer
我的场景（我制作了自定义 XML 描述格式）是这样的：
<?xml version="1.0" ?>
<scene>
<camera name="camera">
<position x="0" y="-5" z="0" />
<direction x="0" y="1" z="0" />
<focalplane width="0.5" height="0.5" offset="1.0" pixeldensity="1600" />
</camera>
<objects>
<sphere name="sphere1" radius="1.0">
<material emittance="0.9" reflectance="0">
<diffuse r="0.5" g="0.5" b="0.5" />
</material>
<position x="1" y="0" z="0" />
</sphere>
<sphere name="sphere2" radius="1.0">
<material emittance="0.0" reflectance="0">
<diffuse r="0.8" g="0.5" b="0.5" />
</material>
<position x="-1" y="0" z="0" />
</sphere>
</objects>
</scene>
我很确定我的引擎存在一些根本缺陷，但我就是找不到它......
这是我的新跟踪功能：
def Trace(ray, scene, n):
if n > 10: # Max raydepth of 10. In my scene, the max should be around 4, since there are only a few objects to bounce off, but I agree, there should be a cap.
return Color(0.0, 0.0, 0.0)
result = 1000000.0 # It's close to infinity...
hit = False
for object in scene.objects:
test = object.intersection(ray)
if test and test < result:
result = test
hit = object
if not hit:
return Color(0.0, 0.0, 0.0)
point = ray.position(result)
normal = hit.normal(point)
direction = RandomNormalInHemisphere(normal) # I won't post that code, but rest assured, it *does* work.
if direction.dot(ray.direction) > 0.0:
point = ray.origin + ray.direction * (result + 0.0000001) # We're going inside an object (for use when tracing glass), so move a tad bit inside to prevent floating-point errors.
else:
point = ray.origin + ray.direction * (result - 0.0000001) # We're bouncing off. Move away from surface a little bit for same reason.
newray = Ray(point, direction)
return Trace(newray, scene, n + 1) * hit.diffuse + Color(hit.emittance, hit.emittance, hit.emittance) # Haven't implemented colored lights, so it's a shade of gray for now.
我很确定路径跟踪代码有效，因为我手动投射了一些光线并得到了相当合理的结果。我（现在）遇到的问题是相机不会通过图像平面中的所有像素拍摄光线。我编写了这段代码来查找与像素相交的射线，但它无法正常工作：
origin = scene.camera.pos # + 0.5 because it #
# puts the ray in the # This calculates the width of one "unit"
# *middle* of the pixel #
worldX = scene.camera.focalplane.width - (x + 0.5) * (2 * scene.camera.focalplane.width / scene.camera.focalplane.canvasWidth)
worldY = scene.camera.pos.y - scene.camera.focalplane.offset # Offset of the imaging plane is know, and it's normal to the camera's direction (directly along the Y-axis in this case).
worldZ = scene.camera.focalplane.height - (y + 0.5) * (2 * scene.camera.focalplane.height / scene.camera.focalplane.canvasHeight)
ray = Ray(origin, (scene.camera.pos + Point(worldX, worldY, worldZ)).norm())