'Re: [Flightgear-devel] Where does Linear->sRGB conversion take place?'

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List:       flightgear-devel
Subject:    Re: [Flightgear-devel] Where does Linear->sRGB conversion take place?
From:       James Turner <james () flightgear ! org>
Date:       2022-02-17 6:53:24
Message-ID: 0643ADEE-ED2F-4508-AD7D-ACF5933555E9 () flightgear ! org
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> On 15 Feb 2022, at 20:46, James Hogan <james@albanarts.com> wrote:
> 
> Given image formats are normally encoded non-linear sRGB, and linear
> textures would need gamma correcting to something similar before
> display, is there any reason not to just continue using non-linear
> srgb textures?
> 

Probably, we should indeed continue that way : I'm just aware there are changes in \
the medium term where we might need to be better defined about all this. HDR / PBR is \
one, but my memory is also that if we ever want to do explicit texture compression, \
some of the formats might expect one or other encoding.

> I've recently had a play with making a new aircraft and hit the issue
> of a part textured model, where the colour in the (non-linear SRGB)
> texture matches the diffuse colour of a neighbouring material in
> blender, but the untextured material is much darker than the texture
> in flightgear. It appears to be because the diffuse colour is a linear
> colour value, so technically flightgear should gamma correct it to
> match the textures (if the texture had an SRGB format opengl would
> convert to linear, then both texture and material could be gamma
> corrected together by GL_FRAMEBUFFER_SRGB).

‘Logically' (in the Mr Spock sense of it) doing all our computations in linear \
seems the right thing to aim for, but we're some way from being able to require SRGB \
formats for our textures and framebuffers, so I think that gets cumbersome fast. We \
could make the texture-cache convert textures to linear when caching, if the system \
supports GL_FRAMEBUFFER_SRGB : then we could run in ‘linear mode' where supported, \
without undue changes, and accept some colour matching weirdness on systems without \
SRGB framebuffers?

(We could even set a uniform for this, or select a different technique, to have \
versions of an Effect run in non-linear vs linear colourspace)

I'm not saying that's a perfect solution either, but it would allow us to keep our \
existing input textures, but work in a well-defined colour-space in the shaders.

Kind regards,
James


[Attachment #5 (unknown)]

<html><head><meta http-equiv="Content-Type" content="text/html; \
charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; \
line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote \
type="cite" class=""><div class="">On 15 Feb 2022, at 20:46, James Hogan &lt;<a \
href="mailto:james@albanarts.com" class="">james@albanarts.com</a>&gt; \
wrote:</div><br class="Apple-interchange-newline"><div class=""><span \
style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; \
font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: \
normal; text-align: start; text-indent: 0px; text-transform: none; white-space: \
normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; \
float: none; display: inline !important;" class="">Given image formats are normally \
encoded non-linear sRGB, and linear</span><br style="caret-color: rgb(0, 0, 0); \
font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: \
normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: \
0px; text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none;" class=""><span \
style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; \
font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: \
normal; text-align: start; text-indent: 0px; text-transform: none; white-space: \
normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; \
float: none; display: inline !important;" class="">textures would need gamma \
correcting to something similar before</span><br style="caret-color: rgb(0, 0, 0); \
font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: \
normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: \
0px; text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none;" class=""><span \
style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; \
font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: \
normal; text-align: start; text-indent: 0px; text-transform: none; white-space: \
normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; \
float: none; display: inline !important;" class="">display, is there any reason not \
to just continue using non-linear</span><br style="caret-color: rgb(0, 0, 0); \
font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: \
normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: \
0px; text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none;" class=""><span \
style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; \
font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: \
normal; text-align: start; text-indent: 0px; text-transform: none; white-space: \
normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; \
float: none; display: inline !important;" class="">srgb textures?</span><br \
style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; \
font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: \
normal; text-align: start; text-indent: 0px; text-transform: none; white-space: \
normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" \
class=""><br class=""></div></blockquote><div><br class=""></div>Probably, we should \
indeed continue that way : I'm just aware there are changes in the medium term where \
we might need to be better defined about all this. HDR / PBR is one, but my memory is \
also that if we ever want to do explicit texture compression, some of the formats \
might expect one or other encoding.<br class=""><br class=""><blockquote type="cite" \
class=""><div class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">I've recently had a play with making a new aircraft and hit the \
issue</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: \
12px; font-style: normal; font-variant-caps: normal; font-weight: normal; \
letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; \
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; \
text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">of a part textured model, where the colour in the (non-linear \
SRGB)</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: \
12px; font-style: normal; font-variant-caps: normal; font-weight: normal; \
letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; \
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; \
text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">texture matches the diffuse colour of a neighbouring material \
in</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: \
12px; font-style: normal; font-variant-caps: normal; font-weight: normal; \
letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; \
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; \
text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">blender, but the untextured material is much darker than the \
texture</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; \
font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; \
letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; \
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; \
text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">in flightgear. It appears to be because the diffuse colour is a \
linear</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: \
12px; font-style: normal; font-variant-caps: normal; font-weight: normal; \
letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; \
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; \
text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">colour value, so technically flightgear should gamma correct it \
to</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: \
12px; font-style: normal; font-variant-caps: normal; font-weight: normal; \
letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; \
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; \
text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">match the textures (if the texture had an SRGB format opengl \
would</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: \
12px; font-style: normal; font-variant-caps: normal; font-weight: normal; \
letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; \
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; \
text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">convert to linear, then both texture and material could be \
gamma</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: \
12px; font-style: normal; font-variant-caps: normal; font-weight: normal; \
letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; \
white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; \
text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: \
Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; \
font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; \
text-transform: none; white-space: normal; word-spacing: 0px; \
-webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline \
!important;" class="">corrected together by \
GL_FRAMEBUFFER_SRGB).</span></div></blockquote></div><br class=""><div \
class="">‘Logically' (in the Mr Spock sense of it) doing all our computations in \
linear seems the right thing to aim for, but we're some way from being able to \
require SRGB formats for our textures and framebuffers, so I think that gets \
cumbersome fast. We could make the texture-cache convert textures to linear when \
caching, if the system supports GL_FRAMEBUFFER_SRGB : then we could run in ‘linear \
mode' where supported, without undue changes, and accept some colour matching \
weirdness on systems without SRGB framebuffers?</div><div class=""><br \
class=""></div><div class="">(We could even set a uniform for this, or select a \
different technique, to have versions of an Effect run in non-linear vs linear \
colourspace)</div><div class=""><br class=""></div><div class="">I'm not saying \
that's a perfect solution either, but it would allow us to keep our existing input \
textures, but work in a well-defined colour-space in the shaders.</div><div \
class=""><br class=""></div><div class="">Kind regards,</div><div \
class="">James</div><div class=""><br class=""></div></body></html>





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