Reflectivity in materials architectural Visualisation

Abstract 200-250

In this paper the
author will investigate Reflectivity in Architectural Visualisation. The author
will be focussing on implementing the findings into a 3D environment within
Autodesk Maya. The structure of this paper will consist of two main parts; a
qualitative and quantitative study. The qualitative element will be created
from information gathered from the literature review sourced from journal papers
and internet sources. The second element; a quantitative study, will be formed
from creating technical tests that will be shown to audiences to gather
opinions and data in order to form opinions. The research will utilise
questionnaires, surveys, and focus groups to gather evidence from the tests.  These two elements will then be compared,
evaluated to form conclusions. These conclusions will be applied to a CGI BSc
architectural visualisation project. The author will use qualitative and
quantitative processing methods in an academic triangulation method to process
the results. The author will apply the findings of the paper to create a
120-150 second 3D CGI animation.

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In this paper the
Author intends to research Reflectivity in Architectural Visualisation. The
Author will conduct a qualitative and quantitative study and triangulate the
information to find information and conclusions to the research questions. The
author will look at and analyse current literature and previous studies that
have been carried out in this field and this paper will identify key findings
and discoveries. The Author will cross reference previous and contemporary
information to high areas of interest of concern. The second section of this
paper will use quantitative research mechanisms to specifically test and
evaluate key findings from the qualitative study. The author will then
summarise the results and conclusions and areas for future research will be


Literature Review


This research as stated will investigate reflectivity of
materials within the field of 3D CGI architectural visualisation animations.
Before commencement can proceed it is important that the author provides
definitions for the terms ‘reflectivity’ and ‘materials’ in CGI. Reflectivity
is defined as “The
property of reflecting light or radiation, especially reflectance as measured
independently of the thickness of a material.” Oxford dictionary 2017. Date?
Autodesk Maya have released 2 definitions of reflectivity one for smooth
surfaces “Light bounces off the surface of a material at an angle equal
to the angle of the incoming light wave.” Maya and one for hard surfaces “Light
waves bounce off at many of angles because the surface is uneven” Autodesk Maya

Another important term ”materials’ within –
quote definition



To create a realistic computer-generated picture of a shiny
surface it is often necessary to simulate the reflections in the surface. In
the 1990’s Ray tracing could provide accurate reflections, but required a great
deal of CPU time. There were a few less time-consuming ways to simulate
reflections back then using PRMan. However None of the methods were both effective
and efficient in all situations. For the best results, it was important that
you chose the most appropriate method for your application.

The author will now describe a list
of different methods that would have been used to simulate reflections. The
first method uses a texture map and requires an additional rendering step to
create the texture map from the scene.

There were less expensive methods for
simulating reflections however they had a lower degree of realism. The sky is
often the main source of reflections, in outdoor scenes. A simple shader that
selects sky and ground colors based on the “up” component of the
reflected vector would give an impression of reflections without the
requirement of any additional rendering steps or texture files. This method
worked well for surfaces that were curved.

However the “sky and ground”
 Technique wouldn’t have worked If the
reflective surfaces weren’t flat. In this scenario, you could have simulated
reflections using environment textures. Although this method consists of additional
rendering steps to create the environment texture, and it would have taken
longer to render the final image. Reflections in curved surfaces could have been
simulated quite accurately using environment maps, especially if the objects were
a good distance away from the reflecting surface. It was far more realistic to
use the environment map technique than the simple reflection technique, however
it was a lot more expensive.

Reflections in a plane

Simulating reflections was particularly
easy when the reflecting surface was flat (planar).

Imagine that you point a camera at a
mirror and take a picture of the image reflected in the mirror. Now imagine
that the mirror is replaced with a clear glass window, and the camera is moved
to an exactly opposite position on the other side of the window. Take a second
picture from the new vantage point with the camera looking into the room
through the window. Remarkably enough, when you compare the two pictures, one
is the “mirror image” of the other, that is, the same image with left
and right reversed. This thought experiment suggests a technique for simulating
reflections in a mirror or other flat surfaces in a computer-generated picture.

Surface Type

Level of Realism Required

Best Method


All types


Sky and ground shader


A single flat surface



Curved, complex


Environment maps


In the mathematical world of computer
graphics, we could simulate a reflection exactly (including the left-right
reversal) by reflecting the camera through the mirror, instead of simply moving
it to the other side of the mirror.

Reflecting the camera

The camera used to render the reflection
image was simply the scene camera reflected through the reflection plane. An
example of this, the mirror lies in the plane ‘z=-0.05’. If the reflection
plane were ‘z=0’ in world space, the camera could be reflected by
adding the command: ‘RiScale (1., 1., -1.);’

This scale operation would do nothing
except negate all of the z coordinates of the camera coordinate system. If the
camera was positioned at (x, y, z) before the scale operation, it will be
positioned at (x, y, -z) after the scale; this is, the reflection of the original

The case of reflection in the ‘z=-0.05’ plane
is only slightly more difficult. First we would translate the ‘z=0’ plane
to the position of the actual reflection plane using an ‘RiTranslate’ call,
then do the scale operation, which reflects through ‘z=0’, and then
translate the ‘z=0’ plane back to its original position.


‘RiTranslate (0., 0, -0.5);’

‘RiScale (1., 1., -1.);’

‘RiTranslate (0., 0., 0.05);’


Notice that points which lie on the ‘z=-0.05’
plane in world space are unaffected by this sequence of transformations.

There is nothing special about z in the
above procedure. The reflection through an ‘x=k’ or ” y=k’ plane (for some
number k) is very similar, simply negating the x or y coordinates using an
appropriate RiScale call.

If the reflection plane were not aligned
with the coordinate system axes, it’s a little harder to reflect the camera
through the reflection plane. Instead of using just a ‘RiTranslate’ call
to move the ‘z=0’ (or ‘x=02’ or ‘y=0’) plane to coincide
with the reflection plane, it was necessary to use both an ‘RiRotate’ and
an ‘RiTranslate’. The axis of rotation is the cross product of the normal
vectors of the reflection plane and the ‘z=0’ plane. The direction of
translation is along the normal vector of the reflection plane. The inverse ‘RiTranslate’ and
‘RiRotate’ must be used to transform the reflection plane back to ‘z=0’ after
the ‘RiScale’ is applied. If you prefer, all of the rotations,
translations, and scales can be combined into a single transformation matrix
that can be applied using the ‘RiConcatTransform’ call.

Rendering the reflective image

If the example program was compiled with
the preprocessor symbol REFLECTION defined, it would contain the RenderMan
calls needed to render the reflection image that will appear reflected in the
mirror when the final scene image is rendered. The reflection image is rendered
into a TIFF file called ‘refl.tif’ and then used to make a texture file.
It is most efficient to make the texture from a square image whose resolution
is an integer power of 2. The scene image might not be square, so the pixel
aspect ratio of the reflection image must be adjusted so that the reflection
image covers the same screen window as the scene image. The screen window
determines how much of the scene is visible in the image.

Making the Reflection texture

Having rendered the reflection image, we
could then make it into a texture file called refl.tex using the ‘RiMakeTexture’.

The ‘RI_BLACK’ parameters specify
that the texture values outside the ‘0:1’  texture coordinate range
of the reflection map should be zero. If the reflection texture is used
properly, values outside the ‘0:1’ texture coordinate range should never
be accessed, but the ‘RiMakeTexture’ call requires the specification of a
wrap mode and ‘RI_BLACK’ is a reasonable choice. The filtering parameters ‘RiBoxFilter,
1., 1.’  are the default values for ‘RiMakeTexture’ .

The Reflection shader

The mirror or other reflecting surface
must have a surface shader that uses the texture map created in the preceding
step. Each pixel on the mirror is shaded using the texture map that was created
from the reflection image. The pixel position in the scene image is used
to look up the correct pixels from the texture map. In effect, the two images
are being composited, but only at the pixels where the mirror is visible in the
scene image.

Each point on the mirror is shaded
with a color from the texture map, multiplied by the surface color and opacity
so that colored and partially transparent mirrors are possible. The texture
coordinates used to access the texture map are simply the x and y components of
PNDC. PNDC is the point expressed in the NDC (normalized device
coordinate) system, in which the x and y coordinates range from 0 to 1 across
and down the image.

The simple shader shown here can be made
more sophisticated by combining the reflection with a plastic shading model or
a wood shading model. This would be appropriate to add reflections to a shiny
floor or tabletop that is not a pure reflector like a mirror.


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