Linear Workflow - braverabbit · in the past linear workflow was impossible to maintain because of technical ... Linear Workflow Settings Maya/VRay for Maya Example 2011 Ingo Clemens

Linear Workflow

Linear WorkflowVersion 1.0 - 2011-10-11

2011 Ingo Clemens | brave rabbit www.braverabbit.de

Linear Workflow

Linear Workflow

● method to calculate (3d) or process (compositing) images while respecting image and output device color profiles in order to display mathematical and physical correct brightness and appearance

● gaining importance through developments in "physical correct rendering" over the last years

● in the past linear workflow was impossible to maintain because of technical restrictions (i.e. missing 16/32 bit float image formats)


Linear Workflow

Current Situation and Advantages

● linear workflow is currently present in most software packages

● imperative, if high quality renderings with convincing photorealism are needed

● eases the process of creating photo real renderings


Linear Workflow

CGI pre Linear Workflow

● lighting

● compositing


Linear Workflow

Lighting

● the Inverse Square Law describes physically correct light attenuation

„The light intensity is inversely proportional to the square of the distance from the light.

Doubling the distance reduces the light intensity to a quarter.“


Linear Workflow

Standard 3D-Lighting

● has initially no decay - the scene is evenly lit


Linear Workflow

● a quadratic decay appears visually with too much contrast

the bottom image has been color corrected to reveal the lighting information that is present even in the background


Linear Workflow

● Rendering without decay (without texture adjustments)


Linear Workflow

Workaround Lighting

● utilizing linear decay, to gain the advantage of distance-based lighting

● does not appear as visually harsh as quadratic decay

● physical inaccuracy is tolerated - the visual result is more important


Linear Workflow

Compositing

● rendering of separate passes for diffuse color and reflections for a better control during compositing

● the process is similar to the one in 3D

● a renderer calculates shader/material information similar to:


Linear Workflow

● reflections can be layered via the add or screen operation

● using add leads to over bright and unnatural highlights


Linear Workflow

● using the screen operation produces a better result


Linear Workflow

● comparison between add, screen and the original rendering


Linear Workflow

Physical Sun & Sky

● calculation of "physical correct" outdoor lighting

● lens-shaders are used to account for the high-level brightness of the sun

● textures appear washed out

● requires linear workflow


Linear Workflow

Linear, Physical Brightness

● light is always linear

● doubled light intensity = doubled brightness

● linear color space graphs light intensities as a straight line between black and white


Linear Workflow

Human Perception

● not linear, comparable to the human hearing

● stands in contrast to how brightness appears physically

● very sensitive in the low brightness range➔ contours can still be seen under very low lighting conditions

● less sensitive in the high brightness range➔ small differences in very bright intensities are hard to perceive

● the perceived brightness curve is steep in the range of shadow values and flatter towards highlight values (Gamma between 0.3 - 0.5)


Linear Workflow

left image: linear light distribution and histogram; center image: brightness distribution after gamma correction and histogram of the 8 bit image; the resulting banding effects in the dark colors are prominent; the comparison between the two histograms shows how much information is stored in the dark portions of the image; right image: visible degradation of quality with 8 bit color information after a broad color correction


Linear Workflow

Gamma Correction

„A function required to correct a physically proportional, linear growing value (brightness) to compensate for the non-linear properties of human vision and technical shortcomings.“

● gamma alters the brightness for any value that is not black or white

● almost every part in the image processing chain (cameras, digital images, screens, monitors, etc.) is gamma corrected

● sRGB is almost identical to a gamma correction of 2.2


Linear Workflow

Gamma Curve (sRGB)


Linear Workflow

Reasons for Gamma Correction

● most common image formats offer only a limited dynamic range

● usually 8 bit with 256 discrete values per color channel

● without any correction:

➔ too many values would be stored in the non-sensitive range (highlights)

➔ too few would be stored in the sensitive range (shadows)

● with gamma correction the dynamic range of a media is used more efficiently


Linear Workflow

Practical Application of Gamma Correction

Starting Point

● brightness is physically linear (linear curve)

➔ a direct transfer makes poor use of the dynamic range

Solution

● the image information is gamma corrected

➔ efficient use of the dynamic range➔ reciprocal value of the monitor gamma: 1/2.2 = 0.4545

● linear display of the image on screen through compensating the gamma

correction encoded in the image

➔ monitor gamma 2.2


Linear Workflow

Linear Images (16/32 bit float)

● are not limited to a small dynamic range

● all image information is stored as float values

● i.e. openEXR, Radiance, float TIFF

● gamma correction is not required


Linear Workflow

Main Problem

● 3D applications work and render scene information in a linear way

● monitors, cameras, images work in a non-linear color space

● gamma correction is necessary for displaying images the way the human vision perceives colors


Linear Workflow

Impact on 3D

Linear Workflow concerns

● Input (textures, colors)

● Rendering

● Output (images)


Linear Workflow

Input

● all color spaces must be linear

● non-linear images have to be corrected

➔ a gamma correction reverses the gamma encoding of an image

➔ the image will be processed correctly in linear color space

● color pickers and swatches usually display sRGB colors and have to be corrected as well (depending on the software)

● linear images require no correction


Linear Workflow

● rendering without gamma corrected colors/textures


Linear Workflow

● rendering with gamma corrected colors/textures


Linear Workflow

Rendering

● always linear

● render output is stored linear (float format)

● the render output is viewed with a sRGB correction

➔ visually linear display (through the color profile of the monitor)


Linear Workflow


Linear Workflow

Compositing

● compositing applications are working in linear color space

● linear footage material requires no gamma processing

● sRGB footage has to be linearized (usually all integer formats - 8 bit/10 bit/12 bit/16 bit integer)

● no gamma correction for linear output formats

● non-linear output formats (non-float) have to be processed with a gamma correction (either automatic or manual)


Linear Workflow


Linear Workflow

Settings 3ds max Example


Linear Workflow

Settings Maya/Mental Ray Example


Linear Workflow

Settings Maya/VRay for Maya Example


Linear Workflow

Settings Nuke Example


Linear Workflow

Happy Rendering!


Documents

Linear Workflow - braverabbit · in the past linear workflow was impossible to maintain because of technical ... Linear Workflow Settings Maya/VRay for Maya Example 2011 Ingo Clemens