A Digital Craftsperson's Introduction to Color Science

In association with the Entertainment Technology Center

Description

In digital movie-making, craftspeople with many specialties are involved with color image data representing elements of the movie. Model makers, materials specialists, animators, rotoscope artists, lighters, shaders, and compositors all have a hand in manipulating the color values that ultimately produce the colors in the movie. However, several different color systems are used to represent image data at various points in the pipeline. HSL and HSI are typically used for color selection; RGB is used for image generation; nonlinear R'G'B' is often used for storage, and SDTV Y'C_BC_R is used for preview. Different color spaces are used for measurement and calibration - CIE XYZ, xyY, and LAB. Finally, Cineon/DPX or HDTV Y'C_BC_R coding systems are likely to be used for mastering and film recording.

In this seminar, we help the film craftsperson understand the perceptual and technical reasons why all of these representations are necessary, and how they relate to each other. We introduce the critical importance of perceptual coding, and explain why mid gray - 50% of the way up Photoshop's RGB scale - is only 18% intensity. We explain the fundamentals of the transforms between various color representations. We outline color management systems, and the associated ICC profiles, that have been deployed in film production to maintain accurate color through the pipeline.

Colors are altered depending upon their context in the image; we describe aspects of visual perception that are important in the selection of colors. For example, yellow and brown have identical CIE XYZ values - the difference is whether the surrounding area is dark or light. Reproduction of accurate color depends upon technical parameters of display equipment. We introduce display primaries, white point, and contrast ratio; we examine the effect of these parameters upon reproduced color, and describe methods of compensating the differences. Color appearance depends upon the ambient environment at the display; we explain why colors look different at different displayed intensities.

Finally, although color science provides an essential technical foundation for creating and maintaining accurate color, we discuss some weaknesses of color science, and identify the areas where craft and art will always rule.

Outline

Fundamentals

• Lightness terminology - brightness, intensity, luminance, lightness;
• Contrast, contrast ratio, viewing conditions, and image quality;
• The relative nature of lightness sensitivity;
• The logarithmic optical density measure.

Tone Reproduction

• Perceptual uniformity, and why it leads to nonlinear coding;
• How image data is altered to compensate for viewing conditions;
• Gamma in video, PCs, Macs, CGI, and film;
• Setting the brightness and contrast controls.

Color Reproduction

• Concepts of color - lightness, hue, and saturation;
• Classical CIE colorimetry; spectral and tristimulus reproduction;
• The concept of metamerism, whereby 3 components suffice;
• Additive reproduction, gamut; and the necessity of matrixing;
• Subtractive reproduction, and its unwanted absorptions;
• Limitations of classical color science; shortcomings of LAB and LUV.

Image Coding

• Perceptual uniformity and codeword utilization issues;
• Coding schemes: XYZ, LAB, R'G'B', Y'C_BC_R , log RGB, DPX/Cineon;
• Chroma subsampling - 4:4:4, 4:2:2, 4:2:0, etc.

Film

• Sensitometry (D-log E curves), film gamma;
• Emulsion sensitivities and dye spectral characteristics;
• Film gamut, compared to additive gamut (e.g., HDTV);
• Film timing.

Color Appearance

• Subjective effects: chromatic adaptation, induction;
• Rendering in film, video, HDTV, and CGI;
• Introduction to color appearance models.

Color Management Theory And Practice

• Color transform techniques, and gamut mapping;
• ICC architecture, profiles, and profile connection space;
• Device characterization and calibration.

Charles Poynton - Courses & seminars
www.poynton.com/notes/events/
2003-09-25