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Transparency

Mode: All Modes

Panel: Shading/Material Context → Transparency

Materials in Blender can be set to be transparent, so that light can pass through any objects using the material. Transparency is controlled using an "alpha" channel, where each pixel has an additional value, range 0-1, in addition to its RGB color values. If alpha=0, then the pixel is transparent, and the RGB values for the surface contribute nothing to the pixel's appearance; for alpha=1, the surface is fully opaque, and the color of the surface determines the final color of the pixel.

Transparency Panel

In Blender, there are three ways in which the transparency of a material can be set: Mask, Z-Buffer and Ray-trace. Each of these is explained in more detail below. The Material Preview option with a sphere object gives a good demonstration of the capabilities of these three options.


Common Options

The following property controls are available for all transparency options:

Alpha
Sets the transparency of the material by setting all pixels in the alpha channel to the given value.
Fresnel
Sets the power of the Fresnel effect. The Fresnel effect controls how transparent the material is, depending on the angle between the surface normal and the viewing direction. Typically, the larger the angle, the more opaque a material becomes (this generally occurs on the outline of the object).
Specular -
Controls the alpha/falloff for the specular color.
Blend
Controls the blending between transparent and non-transparent areas. Only used if Fresnel is greater than 0.


Mask

This option simply masks the Background. It uses the alpha channel to mix the color of each pixel on the active object plane with the color of the corresponding background pixel, according to the alpha channel of the pixel. Thus for alpha = 1, the object color is seen - the object is completely opaque; but if alpha = 0, only the background is seen - the object is transparent (but note that any other object behind the active object disappears).

This is useful for making textures of solid or semi-transparent objects from photographic reference material - a mask is made with alpha opaque for pixels within the object, and transparent for pixels outside the object.

See Mask Transparency.


Z Buffer

This uses the alpha buffer for transparent faces. The alpha value of each pixel determines the mix of the basic color of the material, and the color of the pixel is determined from the objects/background behind it. Only basic settings are available with this option; it does not calculate refractions.


Raytraced Transparency

Uses ray tracing to calculate refractions. Ray tracing allows for complex refractions, falloff, and blurring, and is used for simulating the refraction of light rays through a transparent material, like a lens.

Note that the RayTrace option is only available in the Blender Render and Cycles render engines, but not in the Game Engine.

A ray is sent from the camera and travels through the scene until it encounters an object. If the first object hit by the ray is non-transparent, then the ray takes the color of the object.

If the object is transparent, then the ray continues its path through it to the next object, and so on, until a non-transparent object is finally encountered which gives the whole chain of rays its color. Eventually, the first transparent object inherits the colors of its background, proportional to its Alpha value (and the Alpha value of each transparent Material hit in between).

But while the ray travels through the transparent object, it can be deflected from its course according to the Index of Refraction (IOR) of the material. When you actually look through a plain sphere of glass, you will notice that the background is upside-down and distorted: this is all because of the Index of Refraction of glass.

Enable Raytracing

To get ray-traced transparency, you need to:

  1. enable ray tracing in your Render settings. This is done in the Render context  → Shading Panel. Ray tracing is enabled by default.
  2. set your Alpha value to something other than 1.0.
  3. in order for the background material to receive light passing through your transparent object, Receive Transparent must be turned on for that material in the Material → Shadow panel.


Options

The Transparency Panel.

In addition to the common options given above, the following property controls are available:

IOR
Index of Refraction. Sets how much a ray traveling through the material will be refracted, hence producing a distorted image of its background. See IOR values for Common Materials below.
Filter
Amount of filtering for transparent ray trace. The higher this value, the more the base color of the material will show. The material will still be transparent but it will start to take on the color of the material. Disabled (0.0) by default.
Falloff
How fast light is absorbed as it passes through the material. Gives 'depth' and 'thickness' to glass.
Limit
Materials thicker than this are not transparent. This is used to control the threshold after which the filter color starts to come into play.
Depth
Sets the maximum number of transparent surfaces a single ray can travel through. There is no typical value. Transparent objects outside the Depth range will be rendered pitch black if viewed through the transparent object that the Depth is set for. In other words, if you notice black areas on the surface of a transparent object, the solution is probably to increase its Depth value (this is a common issue with ray tracing transparent objects). You may also need to turn on transparent shadows on the background object.
Gloss
Settings for the glossiness of the material.
Amount
The clarity of the refraction. Set this to something lower than zero to get a blurry refraction.
Threshold
Threshold for adaptive sampling. If a sample contributes less than this amount (as a percentage), sampling is stopped.
Samples
Number of cone samples averaged for blurry refraction.

Examples

Index of Refraction

Influence of the IOR of an Object on the distortion of the background: spheres of Water, Glass and Diamond (top to bottom).

(Influence of the IOR of an Object on the distortion of the background: spheres of Water, Glass and Diamond (top to bottom).). There are different values for typical materials: Air is 1.000 (no refraction), Alcohol is 1.329, Glass is 1.517, Plastic is 1.460, Water is 1.333 and Diamond is 2.417.

Fresnel

Manual-2.5-Material-RayTraceTransp-FresnelExampel.png
Manual-2.5-Material-RayTraceTransp-FresnelExampelZTransp.png
16 pieces of glass rotated in various directions demonstrate the angle-dependent Fresnel effect with ray-traced (left) and alpha buffered transparency (right). Note that the major difference is the lack of IOR effect in the latter case. (Download .blend.)
Manual-2.5-Material-RayTraceTransp-FresnelSettings.png
Manual-2.5-Material-RayTraceTransp-FresnelSettingsZTransp.png
Settings for Fresnel using ray-traced (left) and Z transparency (right).

Note the specular highlight in the F4 glass tile (which is facing midway between the light and the camera); the Fresnel effect can be seen in row C and column 6 where the faces are turned away from the camera.

The amount of Fresnel effect can be controlled by either increasing the Blend value or decreasing the Alpha value.

Depth

A simple scene with three glasses on a surface, and three lamps. Depth was set to 4, 8, 12, and 14, resulting in render times of 24 sec, 34 sec, 6 min, and 11 min respectively. (Download .blend.)

Increasing Depth also considerably increases render time. Each time a light ray passes through a surface, the ray-tracing algorithm is called recursively. In the example above, each side of each glass has an exterior and an interior surface. Light rays thus have to pass through four surfaces for each glass.

But not only that, at every point on a surface, some of the light can be reflected, or mirrored off the surface in various directions. This results in multiple rays needing to be calculated for each point (often referred to as a tree of rays[1]). In each of the rendered images above there are 640×400=256 000 pixels. By increasing Depth, at least one tree of rays is added to each pixel.

Be kind to your computer. Carefully placing objects in a scene to avoid overlapping transparent objects is often an interesting alternative.

Hints

Transparent shadows

No transparent shadows
No transparent shadows, environment lighting enabled
Transparent shadows enabled, alpha set to 0.0
As previous, alpha set to 0.25
Transparent shadows with ambient occlusion set to multiply, distance 1 (radius of sphere)
As previous, distance increased to 2 (diameter of sphere)

By default, the shadows of transparent objects are rendered solid black, as if the object was not transparent at all. But in reality, the more transparent an object is, the lighter its shadow will be.

In Blender, transparent shadows are set on the materials that receive the shadows from the transparent object. This is enabled and disabled with the Receive Transparent button, in the Material context → Shadow panel. The shadow's brightness is dependent on the Alpha value of the shadow casting material.

Alternatives to transparent ray-traced shadows can be found in the World context, namely the Ambient Occlusion, Environment Lighting, and Gather panels. Alternatively, a texture can be used to control the Intensity value of the shadow-receiving material.

IOR values for Common Materials

The following list provides some index of refraction values to use when ray-traced transparency is used for various liquids, solids (gems), and gases:

A
Acetone 1.36
Actinolite 1.618
Agalmatolite 1.550
Agate 1.544
Agate 1.540
Air 1.000
Alcohol 1.329
Alcohol, Ethyl (grain) 1.36
Alexandrite 1.745
Alexandrite 1.750
Almandine 1.83
Aluminum 1.44
Amber 1.545
Amblygonite 1.611
Amethyst 1.540
Ammolite 1.600
Anatase 2.490
Andalusite 1.640
Anhydrite 1.571
Apatite 1.632
Apophyllite 1.536
Aquamarine 1.575
Aragonite 1.530
Argon 1.000281
Asphalt 1.635
Axinite 1.674 - 1.704
Axinite 1.675
Azurite 1.730
B
Barite 1.636
Barytocalcite 1.684
Beer 1.345
Benitoite 1.757
Benzene 1.501
Beryl 1.57 - 1.60
Beryl, Red 1.570 - 1.598
Beryllonite 1.553
Brazilianite 1.603
Bromine (liq) 1.661
Bronze 1.18
Brownite 1.567
C
Calcite 1.486
Calspar 1.486
Cancrinite 1.491
Carbon Dioxide (gas) 1.000449
Carbon Disulfide 1.628
Carbon Tetrachloride 1.460
Carbonated Beverages 1.34 - 1.356
Cassiterite 1.997
Celestite 1.622
Cerussite 1.804
Ceylonite 1.770
Chalcedony 1.544 - 1.553
Chalk 1.510
Chalybite 1.630
Chlorine (gas) 1.000768
Chlorine (liq) 1.385
Chrome Green 2.4
Chrome Red 2.42
Chrome Tourmaline 1.61 - 1.64
Chrome Yellow 2.31
Chromium 2.97
Chrysoberyl 1.745
Chrysoberyl, Cat's eye 1.746 - 1.755
Chrysocolla 1.500
Chrysoprase 1.534
Citrine 1.532 - 1.554
Citrine 1.550
Clinohumite 1.625 - 1.675
Clinozoisite 1.724
Cobalt Blue 1.74
Cobalt Green 1.97
Cobalt Violet 1.71
Colemanite 1.586
Copper 1.10
Copper Oxide 2.705
Coral 1.486
Coral 1.486 - 1.658
Cordierite 1.540
Corundum 1.766
Cranberry Juice (25%) 1.351
Crocoite 2.310
Crystal 2.000
Cuprite 2.850
D
Danburite 1.627 - 1.641
Danburite 1.633
Diamond 2.417
Diopside 1.680
Dolomite 1.503
Dumortierite 1.686
E
Ebonite 1.66
Ekanite 1.600
Elaeolite 1.532
Emerald 1.560 - 1.605
Emerald Catseye 1.560 - 1.605
Emerald, Synth flux 1.561
Emerald, Synth hydro 1.568
Enstatite 1.663
Epidote 1.733
Ethanol 1.36
Ethyl Alcohol 1.36
Euclase 1.652
F
Fabulite 2.409
Feldspar, Adventurine 1.532
Feldspar, Albite 1.525
Feldspar, Amazonite 1.525
Feldspar, Labradorite 1.565
Feldspar, Microcline 1.525
Feldspar, Oligoclase 1.539
Flourite 1.434
Formica 1.47
G
Garnet, Andradite 1.88 - 1.94
Garnet, Demantoid 1.880 - 1.9
Garnet, Demantoid 1.880
Garnet, Grossular 1.738
Garnet, Hessonite 1.745
Garnet, Mandarin 1.790 - 1.8
Garnet, Pyrope 1.73 - 1.76
Garnet, Rhodolite 1.740 - 1.770
Garnet, Rhodolite 1.760
Garnet, Spessartite 1.810
Garnet, Tsavorite 1.739 - 1.744
Garnet, Uvarovite 1.74 - 1.87
Gaylussite 1.517
Glass 1.51714
Glass, Albite 1.4890
Glass, Crown 1.520
Glass, Crown, Zinc 1.517
Glass, Flint, Dense 1.66
Glass, Flint, Heaviest 1.89
Glass, Flint, Heavy 1.65548
Glass, Flint, Lanthanum 1.80
Glass, Flint, Light 1.58038
Glass, Flint, Medium 1.62725
Glycerine 1.473
Gold 0.47
H
Hambergite 1.559
Hauyne 1.490 - 1.505
Hauynite 1.502
Helium 1.000036
Hematite 2.940
Hemimorphite 1.614
Hiddenite 1.655
Honey, 13% water content 1.504
Honey, 17% water content 1.494
Honey, 21% water content 1.484
Howlite 1.586
Hydrogen (gas) 1.000140
Hydrogen (liq) 1.0974
Hypersthene 1.670
I
Ice 1.309
Idocrase 1.713
Iodine Crystal 3.34
Iolite 1.522 - 1.578
Iron 1.51
Ivory 1.540
J
Jade, Jadeite 1.64 - 1.667
Jade, Nephrite 1.600 - 1.641
Jadeite 1.665
Jasper 1.540
Jet 1.660
K
Kornerupine 1.665
Kunzite 1.660 - 1.676
Kyanite 1.715
L
Labradorite 1.560 - 1.572
Lapis Gem 1.500
Lapis Lazuli 1.50 - 1.55
Lazulite 1.615
Lead 2.01
Leucite 1.509
M
Magnesite 1.515
Malachite 1.655
Meerschaum 1.530
Mercury (liq) 1.62
Methanol 1.329
Milk 1.35
Moldavite 1.500
Moonstone 1.518 - 1.526
Moonstone, Adularia 1.525
Moonstone, Albite 1.535
Morganite 1.585 - 1.594
N
Natrolite 1.480
Nephrite 1.600
Nitrogen (gas) 1.000297
Nitrogen (liq) 1.2053
Nylon 1.53
O
Obsidian 1.489
Oil of Wintergreen 1.536
Oil, Clove 1.535
Oil, Lemon 1.481
Oil, Neroli 1.482
Oil, Orange 1.473
Oil, Safflower 1.466
Oil, vegetable (50° C) 1.47
Olivine 1.670
Onyx 1.486
Opal, Black 1.440 - 1.460
Opal, Fire 1.430 - 1.460
Opal, White 1.440 - 1.460
Oregon Sunstone 1.560 - 1.572
Oxygen (gas) 1.000276
Oxygen (liq) 1.221
P
Padparadja 1.760 - 1.773
Painite 1.787
Pearl 1.530
Periclase 1.740
Peridot 1.635 - 1.690
Peristerite 1.525
Petalite 1.502
Phenakite 1.650
Phosgenite 2.117
Plastic 1.460
Plexiglas 1.50
Polystyrene 1.55
Prase 1.540
Prasiolite 1.540
Prehnite 1.610
Proustite 2.790
Purpurite 1.840
Pyrite 1.810
Pyrope 1.740
Q
Quartz 1.544 - 1.553
Quartz, Fused 1.45843
R
Rhodizite 1.690
Rhodochrisite 1.600
Rhodonite 1.735
Rock Salt 1.544
Rubber, Natural 1.5191
Ruby 1.757 - 1.779
Rum, White 1.361
Rutile 2.62
S
Sanidine 1.522
Sapphire 1.757 - 1.779
Sapphire, Star 1.760 - 1.773
Scapolite 1.540
Scapolite, Yellow 1.555
Scheelite 1.920
Selenium, Amorphous 2.92
Serpentine 1.560
Shampoo 1.362
Shell 1.530
Silicon 4.24
Sillimanite 1.658
Silver 0.18
Sinhalite 1.699
Smaragdite 1.608
Smithsonite 1.621
Sodalite 1.483
Sodium Chloride 1.544
Spessartite 1.79 - 1.81
Sphalerite 2.368
Sphene 1.885
Spinel 1.712 - 1.717
Spinel, Blue 1.712 - 1.747
Spinel, Red 1.708 - 1.735
Spodumene 1.650
Star Ruby 1.76 - 1.773
Staurolite 1.739
Steatite 1.539
Steel 2.50
Stichtite 1.520
Strontium Titanate 2.410
Styrofoam 1.595
Sugar Solution 30% 1.38
Sugar Solution 80% 1.49
Sulphur 1.960
Synthetic Spinel 1.730
T
Taaffeite 1.720
Tantalite 2.240
Tanzanite 1.690-1.7
Teflon 1.35
Thomsonite 1.530
Tiger eye 1.544
Topaz 1.607 - 1.627
Topaz, Blue 1.610
Topaz, Imperial 1.605 - 1.640
Topaz, Pink 1.620
Topaz, White 1.630
Topaz, Yellow 1.620
Tourmaline 1.603 - 1.655
Tourmaline 1.624
Tourmaline, Blue 1.61 - 1.64
Tourmaline, Catseye 1.61 - 1.64
Tourmaline, Green 1.61 - 1.64
Tourmaline, Paraiba 1.61 - 1.65
Tourmaline, Red 1.61 - 1.64
Tremolite 1.600
Tugtupite 1.496
Turpentine 1.472
Turquoise 1.610
U
Ulexite 1.490
Uvarovite 1.870
V-W
Wardite 1.590
Variscite 1.550
Water (0° C) 1.33346
Water (100° C) 1.31766
Water (20° C) 1.33283
Water (gas) 1.000261
Water (35° C, room temp) 1.33157
Whisky 1.356
Willemite 1.690
Witherite 1.532
Vivianite 1.580
Vodka 1.363
Wulfenite 2.300
Z
Zincite 2.010
Zircon 1.777 - 1.987
Zircon, High 1.960
Zircon, Low 1.800
Zirconia, Cubic 2.173 - 2.21



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