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Wednesday, 4 May 2016

Gamma 2.2 setup / Linear workflow in 3ds Max and VRay

A computer monitor is a nonlinear device. The difference in actual luminance values does not change with value stored in the monitor. The gamma correction compensates for this non-linearity of computer monitors and image file formats. In other words, Gamma is the degree of non-linearity of the color gradient from dark to bright values.

If a render image calculated using linear data is displayed directly on a computer monitor, the image will not match what it would look like under real-world conditions (lit by real light, viewed by real eyes). This mismatch is compensated using the gamma correction.

The gamma value 2.2 is the standard gamma value which is theoretically correct value specified by the sRGB standard. Most of the modern monitors are calibrated to this standard. The 2.2 gamma value makes the linear color space stored in an image appear to be linear when displayed on a computer monitor. Figures F1 and F2 show the images without gamma 2.2 setup (gamma=1) and with 2.2 gamma setup.

Gamma 2.2 setup / Linear workflow in 3ds Max and VRay  
Figure F1
Gamma 2.2 setup / Linear workflow in 3ds Max and VRay
Figure F2
To enable gamma correction:

1. Choose Preferences from the Customize menu to open the Preferences dialog.

2. In this dialog, go to the Gamma and LUT tab and then ensure Gamma is chosen in the Display group and the Gamma control is set to 2.2. Also, make ensure the Affect Color Selectors and Affect Material Editor checkboxes are checked.

Tip: The default gamma value
You can adjust the gamma using the Gamma parameter. However, it is recommended that you leave the value of the Gamma parameter to 2.2 and calibrate your monitor to compensate.

3.  Click OK.

To set the gamma in the VRay renderer:

1. Click Render Setup from the main toolbar to open the Render Setup dialog. Go to the V-Ray tab.

2. On the Color mapping rollout, ensure Gamma is set to 2.2 (Figure F3).
Gamma 2.2 setup / Linear workflow in 3ds Max and VRay
Figure F3
Some image file formats such as bmp, gif, jpg, and avi are gamma encoded. In other words, the values stored in the image are non-linear. Generally, all 8-bit and 16-bit images are gamma encoded. However, the floating point image formats such as tif and tga have the ability to save the gamma correction. Below is the list (Figure F4) showing image formats and their ability to store the gamma correction:
Gamma 2.2 setup / Linear workflow in 3ds Max and VRay
Figure F4

When you load an image file, you can override the gamma value stored in the image by using the controls available in Gamma group of the Select Bitmap Image File dialog (Figure F5). By default, Automatic is selected in the Gamma group. You can set the explicit gamma value for the image if image was created using a nonstandard gamma, or bitmap does not has an embedded gamma. You can also override the gamma value to 1, when you are loading a bitmap that will be used for computing displacement mapping, normal mapping, or bump mapping.
Gamma 2.2 setup / Linear workflow in 3ds Max and VRay
Figure F5
You can also view the information about the gamma value from the Select Bitmap Image File dialog. Click Info to open the Image Information dialog. If there is no gamma field in the image, or if the field is not set, the dialog shows N/A (Figure F6).
Gamma 2.2 setup / Linear workflow in 3ds Max and VRay
Figure F6
Tip: Image formats to be used?
For an efficient workflow, you can use only those image formats that embed the gamma such as PNG. If you have a high configuration computer, you can use floating-point formats such as EXR. These formats are always linear and do not need gamma encoding.

The linear workflow (gamma 2.2) is the default workflow for VRay and 3ds Max. Almost all modern renderers use this approach.  To have greater control on the rendered output, save the rendered images in the exr format.


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