4.3 Image Techniques that Take Photosensitivity into Account

4.3.1 About Photosensitivity and These Guidelines

Unlike films and television programs, which produce only one sequence of images each time they are played, one video game can produce an infinite sequence of images. This is because video games are interactive, so that each time a game is played, a different sequence of pictures and images is displayed, depending on the choices and inputs made by the game’s user or (in the case of multiplayer games) users. In addition, the luminance of images displayed in three-dimensional games are not simply those of the video game artist’s original image, but are the result of the game’s programming processes, which render the image in a three-dimensional form in a three-dimensional space, with variations of light, shadow, distance, orientation, and player perspective. These variables are also affected by choices made by the individual user.

Because of these infinite variations that are possible within a single game, it may be possible with many games that certain user inputs cause screen imagery that exceeds the suggested limits described in this document. Try to design games that comply with the limits when the games are played with normal gaming strategies and inputs, understanding that it may still be possible for user inputs to cause sequences of images that may exceed the suggested limits, particularly if the gameplay is idiosyncratic or counterintuitive. Compliance with this document, or any other guidelines that have been or will be developed, may reduce the incidence of photosensitivity seizures, but it will not eliminate them or eliminate seizures that occur during video gameplay from causes other than the visual content of the games.

This document attempts to take what medical science has learned about the images that can trigger photosensitive seizures in susceptible individuals and, in a few paragraphs, apply it to the infinite variety of imagery produced by modern video game technology. Medical research in this field is still developing, and the particular susceptibilities of photosensitive persons vary widely from individual to individual.

As the developers of other guidelines have recognized, it is impossible to craft guidelines that eliminate all risk of seizures, and the measures taken should be proportionate to the risks involved and should not stifle developers’ creativity, imagination, or freedom of expression. It may be possible that a game, even though complying with the guidelines, may produce a problematic sequence. Alternatively, a sequence out of compliance with the guidelines may not be a problematic in its context. It is therefore recommended that all games, before final release, be reviewed by one or more persons knowledgeable about photosensitivity, who can check for potentially problematic sequences. It is also recommended that such persons review decisions to deviate from the restrictions listed in this chapter when that may be desirable for the artistic or creative imperatives of a game.

This chapter uses the following lighting technology terms.

Table 4-2 Lighting Technology Terms
Term Meaning

Luminance

A quantifiable measure of the observed brightness of an object—in this case, of a Nintendo 3DS system's screens.

Nits

A shorthand name for candelas per square meter, the metric system's measurement unit for luminance.
(A candela is a measure of the candle power or angular density of light from a source.)

Photometer

A device that measures the luminance of an object. A photometer with CIEEE characteristics is calibrated to match the response to various color spectra of the average human eye.

RGB Value

The RGB value of a color on a Nintendo 3DS system's screens is a three-number representation of the intensities of, respectively, the red, green, and blue elements of the display that combine to form the color. Each value is a number from 0 to 255. Consequently, an RGB value of (0,0,0) is black; an RGB value of (255,255,255) is white; and an RGB value of (255,0,0) is pure red.

No required guideline items.

4.3.2 Restrictions on Flashing Images and Light

Do not use a sequence of images that does all of the following.

  1. Flashes so that the change in luminance of the flash exceeds 20 nits (candela/square meter).
  2. Occupies more than one-fourth of either the upper or lower screen or more than one-eighth of the combined areas of both the upper and lower screens.
  3. Has more than three flashes occurring in any 1-second period.
Note:

A "flash" is a pair of opposing changes in luminance: that is, an increase in luminance followed by a decrease or a decrease followed by an increase. If the luminance measurements of successive flashes over time are plotted using x- and y-coordinates (x=time; y=luminance), the shape of the resulting plot appears in profile as alternating peaks (frames of localized maximum brightness) and valleys (frames of localized minimum brightness). Flashes should be evaluated for the change in luminance between adjacent peaks and valleys. No more than three of these peaks (or, alternatively, no more than three valleys) should occur in any 60 consecutive frames.

Screen luminance can be measured or calculated as described in section 4.3.2 Restrictions on Flashing Images and Light.

No required guideline items.

4.3.3 Restrictions on Flashing Saturated Red Colors

Do not use a sequence of images that does all of the following.

  1. The images produce flashes (regardless of the change in luminance of the flashes).
  2. One of the images contains saturated red.
  3. The saturated red occupies more than one-eighth of either the upper or lower screen or more than one-sixteenth of the combined areas of both the upper and lower screens.
  4. Has more than three flashes occurring in any 1-second period.
Note:

The RGB value for the red in saturated red is greater than 85 percent of the sum of the color’s RGB values.

No required guideline items.

4.3.4 Restrictions on Image Reversals

If the luminance of an image's components are switched or interchanged (for example, switching between the negative and positive of an image or black and white images in which the black turns white and the white turns black, as in Figure 4-1 Changes in Screen Contrast and Luminance), and the image occupies more than one-fourth of either the upper or lower screen, or more than one-eighth of the combined areas of both the upper and lower screens, the changes in luminance should not exceed 20 nits or occur at a rate faster than that allowed for flashing in section 4.3.2 Restrictions on Flashing Images and Light.

Figure 4-1 Changes in Screen Contrast and Luminance

No required guideline items.

4.3.5 Restrictions on Regular Patterns

Do not use an image that does all of the following.

  1. Consists of striped patterns composed of parallel lines or dots or other regular elements with distinct edges, such as the examples shown in Figure 4-2 Regular Patterns below.
  2. Has high contrast between the bright and dark elements of the pattern, as defined below.
  3. Occupies more than one-fourth of either the upper or lower screen or more than one-eighth of the combined areas of both the upper and lower screens.
  4. Has more than five light–dark pairs of stripes in any orientation.
Figure 4-2 Regular Patterns

An image has high contrast when it meets either of the following conditions.

  • The luminance of the brighter element of the pattern is 30 nits or more, and its contrast is greater than 40 percent.
    Contrast is calculated by (L1-L2)/(L1+L2), where L1 is the luminance of the brighter element of the pattern and L2 is the luminance of the darker.
  • The luminance of the brighter element of the pattern is less than 30 nits and the difference in luminance between the brighter and darker elements (L1-L2) is 17 nits or more.
Note:

The stripes may be parallel or radial, curved or straight, black and white, or a combination of colors. Avoid especially stripes that oscillate or flash and moving stripes that change direction. Do not switch the luminance of the lighter and darker stripes (so that the dark becomes light and vice versa). However, striped patterns that flow smoothly across, into, or out of the screen in one direction may be used. Checkerboard patterns and plaids are also acceptable.

No required guideline items.

4.3.6 Area Brightness Calculations

Screen luminance can be measured directly from the screen of the device using a hand-held spot photometer with a CIE characteristic designed for making measurements from a television screen.

The formula for the screen luminance of a system in the Nintendo 3DS family differs depending on which type of system it is. The screen luminance of a CTR device can be calculated from the RGB values input to the LCD screens, as indicated in the equation below.

The T value represents the screen luminance (in candelas/m2) when the CTR device is set to maximum brightness. The R value represents the gradation of red, G represents green, and B represents blue. Gradations are indicated as an integer ranging from 0 to 255.

Use this CTR formula as the standard method to calculate the screen luminance of Nintendo 3DS applications.

No required guideline items.

 


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