[Public WebGL] gamma/color profile and sRGB

Kenneth Russell [email protected]
Thu Nov 20 14:36:37 PST 2014

On Wed, Nov 19, 2014 at 10:40 PM, Florian Bösch <[email protected]> wrote:
> A frequent problem I encounter when rendering things in webgl, is that it's
> difficult to produce a scene that looks approximately the same on a wide
> variety of displays.
> I identify several reasons for this:
> The users brightness and contrast setting
> The displays color response curves
> The operating systems color profile (if any)
> I don't think there's much to do about brightness and contrast settings
> other than to make sure to test it at medium settings. Likewise for the
> operating systems color profile specifically, there isn't much to do,
> because the assumption is that whatever it does, it combines with the
> monitors response curve to provide a better picture.
> However this leads us to the displays color response curve, which is a
> product of the hardware itself, manufacturing deviations and the operating
> systems color profile setting. Some operating systems allow users to change
> the color profile from a default (but most don't) and other systems (like
> mobiles) usually come with a color calibration profile that's been factory
> set and cannot be changed, it's intended to account for manufacturing
> differences, but it often isn't perfect as it's done manually by far and
> large.
> Ideally it would be sufficient to apply a simple gamma correction for output
> (gl_FragColor.rgb = pow(color, 1.0/2.2), a method advocated by this GPU gem:
> http://http.developer.nvidia.com/GPUGems3/gpugems3_ch24.html
> But this isn't exactly sRGB, and most consumer display hardware is intended
> to display sRGB/Rec.709. I'm currently using an alternative function, but
> I'm not convinced it's any better. This is based on the sRGB standard
> (http://www.color.org/srgb.pdf),
> A convenient way to sidestep the problem of producing output that conforms
> to sRGB would be to render to a sRGB output target. This is currently not
> possible. WebGL 1.0 does not contain any sRGB support. EXT_sRGB allows to
> render to an sRGB render target, but it's my understanding that if you'd get
> to the final blit to screen, the shader would read out the linear values
> from the sRGB texture, and put them on screen, so no improvement there.
> WebGL 2.0 decouples the context and drawing buffer somewhat, however neither
> its context attributes nor its drawing buffer interface allow for specifying
> the drawing buffer as a sRGB surface. On any account, it's unclear that even
> if you would render into an sRGB drawing buffer in WebGL, the right thing
> would happen, as the browser compositor is likely to just read out that
> texture and composit the linear value with the rest of the page.
> I have recently done some work with GLFW, and it has a function called
> getGammaRamp (for a given display). It returns an array of values for each
> channel, that represent the monitors color response curves. These can be
> used in a shader as a texture for conversion.
> This way of handling color response curves strikes me as rather flexible,
> because it's not tied to a particular standard, but can represent any
> conceivable standard.
> My question is, do you think something like getGammaRamp would make sense
> for inclusion into WebGL 2.0?

How would an API like this work in the browser? The browser's window
can be dragged from screen to screen, changing the answer this API

The browser is supposed to transform colors specified in the sRGB
color space ( http://www.w3.org/TR/css3-color/ ) to the display's
color profile. In practice, only Safari and IE handle this correctly
today, and they rely on the OS's colorspace support to do so. There's
an ongoing effort in Chrome to handle multiple displays' color
profiles correctly, and to do so on all operating systems. Not sure
about work in this area in Firefox.

For WebGL 2.0 I think a new context creation attribute is needed to
let the application allocate an sRGB back buffer. The browser's
compositor would handle that differently when drawing it to the
screen. More guidance from experts in colorspace handling is needed.


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