I've been under the assumption that my gamma correction pipeline should be as follows:
Use sRGB format for all textures loaded in (GL_SRGB8_ALPHA8) as all art programs pre-gamma correct their files. When sampling from a GL_SRGB8_ALPHA8 texture in a shader OpenGL will automatically convert to linear space.
Do all lighting calculations, post processing, etc. in linear space.
Convert back to sRGB space when writing final color that will be displayed on the screen.
Note that in my case the final color write involves me writing from a FBO (which is a linear RGB texture) to the back buffer.
My assumption has been challenged as if I gamma correct in the final stage my colors are brighter than they should be. I set up for a solid color to be drawn by my lights of value { 255, 106, 0 }, but when I render I get { 255, 171, 0 } (as determined by print-screening and color picking). Instead of orange I get yellow. If I don't gamma correct at the final step I get exactly the right value of { 255, 106, 0 }.
According to some resources modern LCD screens mimic CRT gamma. Do they always? If not, how can I tell if I should gamma correct? Am I going wrong somewhere else?
Edit 1
I've now noticed that even though the color I write with the light is correct, places where I use colors from textures are not correct (but rather far darker as I would expect without gamma correction). I don't know where this disparity is coming from.
Edit 2
After trying GL_RGBA8 for my textures instead of GL_SRGB8_ALPHA8, everything looks perfect, even when using the texture values in lighting computations (if I half the intensity of the light, the output color values are halfed).
My code is no longer taking gamma correction into account anywhere, and my output looks correct.
This confuses me even more, is gamma correction no longer needed/used?
Edit 3 - In response to datenwolf's answer
After some more experimenting I'm confused on a couple points here.
1 - Most image formats are stored non-linearly (in sRGB space)
I've loaded a few images (in my case both .png and .bmp images) and examined the raw binary data. It appears to me as though the images are actually in the RGB color space, as if I compare the values of pixels with an image editing program with the byte array I get in my program they match up perfectly. Since my image editor is giving me RGB values, this would indicate the image stored in RGB.
I'm using stb_image.h/.c to load my images and followed it all the way through loading a .png and did not see anywhere that it gamma corrected the image while loading. I also examined the .bmps in a hex editor and the values on disk matched up for them.
If these images are actually stored on disk in linear RGB space, how am I supposed to (programatically) know when to specify an image is in sRGB space? Is there some way to query for this that a more featured image loader might provide? Or is it up to the image creators to save their image as gamma corrected (or not) - meaning establishing a convention and following it for a given project. I've asked a couple artists and neither of them knew what gamma correction is.
If I specify my images are sRGB, they are too dark unless I gamma correct in the end (which would be understandable if the monitor output using sRGB, but see point #2).
2 - "On most computers the effective scanout LUT is linear! What does this mean though?"
I'm not sure I can find where this thought is finished in your response.
From what I can tell, having experimented, all monitors I've tested on output linear values. If I draw a full screen quad and color it with a hard-coded value in a shader with no gamma correction the monitor displays the correct value that I specified.
What the sentence I quoted above from your answer and my results would lead me to believe is that modern monitors output linear values (i.e. do not emulate CRT gamma).
The target platform for our application is the PC. For this platform (excluding people with CRTs or really old monitors), would it be reasonable to do whatever your response to #1 is, then for #2 to not gamma correct (i.e. not perform the final RGB->sRGB transformation - either manually or using GL_FRAMEBUFFER_SRGB)?
If this is so, what are the platforms on which GL_FRAMEBUFFER_SRGB is meant for (or where it would be valid to use it today), or are monitors that use linear RGB really that new (given that GL_FRAMEBUFFER_SRGB was introduced 2008)?
--
I've talked to a few other graphics devs at my school and from the sounds of it, none of them have taken gamma correction into account and they have not noticed anything incorrect (some were not even aware of it). One dev in particular said that he got incorrect results when taking gamma into account so he then decided to not worry about gamma. I'm unsure what to do in my project for my target platform given the conflicting information I'm getting online/seeing with my project.
Edit 4 - In response to datenwolf's updated answer
Yes, indeed. If somewhere in the signal chain a nonlinear transform is applied, but all the pixel values go unmodified from the image to the display, then that nonlinearity has already been pre-applied on the image's pixel values. Which means, that the image is already in a nonlinear color space.
Your response would make sense to me if I was examining the image on my display. To be sure I was clear, when I said I was examining the byte array for the image I mean I was examining the numerical value in memory for the texture, not the image output on the screen (which I did do for point #2). To me the only way I could see what you're saying to be true then is if the image editor was giving me values in sRGB space.
Also note that I did try examining the output on monitor, as well as modifying the texture color (for example, dividing by half or doubling it) and the output appeared correct (measured using the method I describe below).
How did you measure the signal response?
Unfortunately my methods of measurement are far cruder than yours. When I said I experimented on my monitors what I meant was that I output solid color full screen quad whose color was hard coded in a shader to a plain OpenGL framebuffer (which does not do any color space conversion when written to). When I output white, 75% gray, 50% gray, 25% gray and black the correct colors are displayed. Now here my interpretation of correct colors could most certainly be wrong. I take a screenshot and then use an image editing program to see what the values of the pixels are (as well as a visual appraisal to make sure the values make sense). If I understand correctly, if my monitors were non-linear I would need to perform a RGB->sRGB transformation before presenting them to the display device for them to be correct.
I'm not going to lie, I feel I'm getting a bit out of my depth here. I'm thinking the solution I might persue for my second point of confusion (the final RGB->sRGB transformation) will be a tweakable brightness setting and default it to what looks correct on my devices (no gamma correction).
First of all you must understand that the nonlinear mapping applied to the color channels is often more than just a simple power function. sRGB nonlinearity can be approximated by about x^2.4, but that's not really the real deal. Anyway your primary assumptions are more or less correct.
If your textures are stored in the more common image file formats, they will contain the values as they are presented to the graphics scanout. Now there are two common hardware scenarios:
The scanout interface outputs a linear signal and the display device will then internally apply a nonlinear mapping. Old CRT monitors were nonlinear due to their physics: The amplifiers could put only so much current into the electron beam, the phosphor saturating and so on – that's why the whole gamma thing was introduced in the first place, to model the nonlinearities of CRT displays.
Modern LCD and OLED displays either use resistor ladders in their driver amplifiers, or they have gamma ramp lookup tables in their image processors.
Some devices however are linear, and ask the image producing device to supply a proper matching LUT for the desired output color profile on the scanout.
On most computers the effective scanout LUT is linear! What does this mean though? A little detour:
For illustration I quickly hooked up my laptop's analogue display output (VGA connector) to my analogue oscilloscope: Blue channel onto scope channel 1, green channel to scope channel 2, external triggering on line synchronization signal (HSync). A quick and dirty OpenGL program, deliberately written with immediate mode was used to generate a linear color ramp:
#include <GL/glut.h>
void display()
{
GLuint win_width = glutGet(GLUT_WINDOW_WIDTH);
GLuint win_height = glutGet(GLUT_WINDOW_HEIGHT);
glViewport(0,0, win_width, win_height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, 1, 0, 1, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glBegin(GL_QUAD_STRIP);
glColor3f(0., 0., 0.);
glVertex2f(0., 0.);
glVertex2f(0., 1.);
glColor3f(1., 1., 1.);
glVertex2f(1., 0.);
glVertex2f(1., 1.);
glEnd();
glutSwapBuffers();
}
int main(int argc, char *argv[])
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE);
glutCreateWindow("linear");
glutFullScreen();
glutDisplayFunc(display);
glutMainLoop();
return 0;
}
The graphics output was configured with the Modeline
"1440x900_60.00" 106.50 1440 1528 1672 1904 900 903 909 934 -HSync +VSync
(because that's the same mode the flat panel runs in, and I was using cloning mode)
gamma=2 LUT on the green channel.
linear (gamma=1) LUT on the blue channel
This is how the signals of a single scanout line look like (upper curve: Ch2 = green, lower curve: Ch1 = blue):
You can clearly see the x⟼x² and x⟼x mappings (parabola and linear shapes of the curves).
Now after this little detour we know, that the pixel values that go to the main framebuffer, go there as they are: The OpenGL linear ramp underwent no further changes and only when a nonlinear scanout LUT was applied it altered the signal sent to the display.
Either way the values you present to the scanout (which means the on-screen framebuffers) will undergo a nonlinear mapping at some point in the signal chain. And for all standard consumer devices this mapping will be according to the sRGB standard, because it's the smallest common factor (i.e. images represented in the sRGB color space can be reproduced on most output devices).
Since most programs, like webbrowsers assume the output to undergo a sRGB to display color space mapping, they simply copy the pixel values of the standard image file formats to the on-screen frame as they are, without performing a color space conversion, thereby implying that the color values within those images are in sRGB color space (or they will often merely convert to sRGB, if the image color profile is not sRGB); the correct thing to do (if, and only if the color values written to the framebuffer are scanned out to the display unaltered; assuming that scanout LUT is part of the display), would be conversion to the specified color profile the display expects.
But this implies, that the on-screen framebuffer itself is in sRGB color space (I don't want to split hairs about how idiotic that is, lets just accept this fact).
How to bring this together with OpenGL? First of all, OpenGL does all it's color operations linearly. However since the scanout is expected to be in some nonlinear color space, this means, that the end result of the rendering operations of OpenGL somehow must be brougt into the on-screen framebuffer color space.
This is where the ARB_framebuffer_sRGB extension (which went core with OpenGL-3) enters the picture, which introduced new flags used for the configuration of window pixelformats:
New Tokens
Accepted by the <attribList> parameter of glXChooseVisual, and by
the <attrib> parameter of glXGetConfig:
GLX_FRAMEBUFFER_SRGB_CAPABLE_ARB 0x20B2
Accepted by the <piAttributes> parameter of
wglGetPixelFormatAttribivEXT, wglGetPixelFormatAttribfvEXT, and
the <piAttribIList> and <pfAttribIList> of wglChoosePixelFormatEXT:
WGL_FRAMEBUFFER_SRGB_CAPABLE_ARB 0x20A9
Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
and by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv,
and GetDoublev:
FRAMEBUFFER_SRGB 0x8DB9
So if you have a window configured with such a sRGB pixelformat and enable sRGB rasterization mode in OpenGL with glEnable(GL_FRAMEBUFFER_SRGB); the result of the linear colorspace rendering operations will be transformed in sRGB color space.
Another way would be to render everything into an off-screen FBO and to the color conversion in a postprocessing shader.
But that's only the output side of rendering signal chain. You also got input signals, in the form of textures. And those are usually images, with their pixel values stored nonlinearly. So before those can be used in linear image operations, such images must be brought into a linear color space first. Lets just ignore for the time being, that mapping nonlinear color spaces into linear color spaces opens several of cans of worms upon itself – which is why the sRGB color space is so ridiculously small, namely to avoid those problems.
So to address this an extension EXT_texture_sRGB was introduced, which turned out to be so vital, that it never went through being ARB, but went straight into the OpenGL specification itself: Behold the GL_SRGB… internal texture formats.
A texture loaded with this format undergoes a sRGB to linear RGB colorspace transformation, before being used to source samples. This gives linear pixel values, suitable for linear rendering operations, and the result can then be validly transformed to sRGB when going to the main on-screen framebuffer.
A personal note on the whole issue: Presenting images on the on-screen framebuffer in the target device color space IMHO is a huge design flaw. There's no way to do everything right in such a setup without going insane.
What one really wants is to have the on-screen framebuffer in a linear, contact color space; the natural choice would be CIEXYZ. Rendering operations would naturally take place in the same contact color space. Doing all graphics operations in contact color spaces, avoids the opening of the aforementioned cans-of-worms involved with trying to push a square peg named linear RGB through a nonlinear, round hole named sRGB.
And although I don't like the design of Weston/Wayland very much, at least it offers the opportunity to actually implement such a display system, by having the clients render and the compositor operate in contact color space and apply the output device's color profiles in a last postprocessing step.
The only drawback of contact color spaces is, that there it's imperative to use deep color (i.e. > 12 bits per color channel). In fact 8 bits are completely insufficient, even with nonlinear RGB (the nonlinearity helps a bit to cover up the lack of perceptible resolution).
Update
I've loaded a few images (in my case both .png and .bmp images) and examined the raw binary data. It appears to me as though the images are actually in the RGB color space, as if I compare the values of pixels with an image editing program with the byte array I get in my program they match up perfectly. Since my image editor is giving me RGB values, this would indicate the image stored in RGB.
Yes, indeed. If somewhere in the signal chain a nonlinear transform is applied, but all the pixel values go unmodified from the image to the display, then that nonlinearity has already been pre-applied on the image's pixel values. Which means, that the image is already in a nonlinear color space.
2 - "On most computers the effective scanout LUT is linear! What does this mean though?
I'm not sure I can find where this thought is finished in your response.
This thought is elaborated in the section that immediately follows, where I show how the values you put into a plain (OpenGL) framebuffer go directly to the monitor, unmodified. The idea of sRGB is "put the values into the images exactly as they are sent to the monitor and build consumer displays to follow that sRGB color space".
From what I can tell, having experimented, all monitors I've tested on output linear values.
How did you measure the signal response? Did you use a calibrated power meter or similar device to measure the light intensity emitted from the monitor in response to the signal? You can't trust your eyes with that, because like all our senses our eyes have a logarithmic signal response.
Update 2
To me the only way I could see what you're saying to be true then is if the image editor was giving me values in sRGB space.
That's indeed the case. Because color management was added to all the widespread graphics systems as an afterthought, most image editors edit pixel values in their destination color space. Note that one particular design parameter of sRGB was, that it should merely retroactively specify the unmanaged, direct value transfer color operations as they were (and mostly still are done) done on consumer devices. Since there happens no color management at all, the values contained in the images and manipulated in editors must be in sRGB already. This works for so long, as long images are not synthetically created in a linear rendering process; in case of the later the render system has to take into account the destination color space.
I take a screenshot and then use an image editing program to see what the values of the pixels are
Which gives you of course only the raw values in the scanout buffer without the gamma LUT and the display nonlinearity applied.
I wanted to give a simple explanation of what went wrong in the initial attempt, because although the accepted answer goes in-depth on colorspace theory, it doesn't really answer that.
The setup of the pipeline was exactly right: use GL_SRGB8_ALPHA8 for textures, GL_FRAMEBUFFER_SRGB (or custom shader code) to convert back to sRGB at the end, and all your intermediate calculations will be using linear light.
The last bit is where you ran into trouble. You wanted a light with a color of (255, 106, 0) - but that's an sRGB color, and you're working with linear light. To get the color you want, you need to convert that color to the linear space, the same way that GL_SRGB8_ALPHA8 is doing for your textures. For your case, this would be a vec3 light with intensity (1, .1441, 0) - this is the value after applying gamma-compression.
Given an HDC, I would like to figure out not only how many bits of Red, Green, Blue and Alpha there are (using DescribePixelFormat), but even their layout (BGR(A) or RGB(A)) so that I can perform pixel and color component level manipulation.
Should I always assume they're in BGR(A) order? Is that a reasonable assumption?
http://msdn.microsoft.com/en-us/library/dd183449(VS.85).aspx
That's the docs for COLORREF which is stored as 0x00BBGGRR.
According to that which is used by most of WINAPI, I'd have to assume that everything on Windows is actually stored in BGRA format.. AFAIK, DIBs are in BGRA format.
You can try painting a window red. Create a DIB section and copy the pixels into a buffer. If the first byte is 0xFF, it is RGBA otherwise it is BGRA.
GDI32 images always stores as BGRA on my machine. GDI24 images are always BGR. I have released an Image handling API and of all the users, I haven't met a single one that has not had the same format. I have yet to see a DC or default backbuffer format in RGBA instead of BGRA.
What is the reason you really need to know the format of a DC? I never found myself wanting to know.
When reading an image, you figure out the bit count from the BitmapInfoHeader.biBitCount field.
According to the docs for the PIXELFORMATDESCRIPTOR:
iPixelType
Specifies the type of pixel data. The following types are defined:
PFD_TYPE_RGBA RGBA pixels. Each pixel has four components in this order: red, green, blue, and alpha.
PFD_TYPE_COLORINDEX Color-index pixels. Each pixel uses a color-index value.
So it seems like they're either RGBA or indexed.
I'm interested in how bit blit works in gdi. I know that that it creates a resulting bitmap based on source and destination bitmaps based on dwROP parametar, but I'm interested how? I saw some example in which it is used for masking that is done with monochrome mask and SetBkColor() function, I am really confused how is BkColor related to these bitmaps... And in the other one, SetTextColor() is used, for removing the background... How are these DC attributes (bkColor and textColor) related? Thanks
You are wrong BitBlt never uses text for background color.
BitBlt raster operations use a pattern (that's the current selected brush), the source and destination bitmap.
The dwRop code defines a calculation between this 3 data sources.
You find a good explanation how this rop codes work in the book of Charles Petzold. Here is a corresponding chapter of the book. Read the part "The Raster Operations".
Background and text color don't play a role, only the current brush that's selected in the destination device context.
BitBlt() iterates through the pixels in the source rectangle and copies the pixels after applying a mathematical operation on the pixel data. The dwRop value determines that operation. There are three pixel values that are combined by the operation to calculate the pixel value of the destination bitmap:
the pixel from the source bitmap. The ROP code identifier contains "SRC".
the pixel from the brush. The ROP code identifier contains "PAT" if used.
the pixel from the destination bitmap before it is written, you'll see "DST" if used.
The mathematical operation applied to the pixel value are very simple. They can be
none, dwROP = SRCCOPY or PATCOPY.
always set to 0, dwROP = BLACKNESS
always set to 1, dwROP = WHITENESS
NOT operator, same as ~ in a C program
AND operator, same as & in a C program
OR operator, same as | in a C program
XOR operator, same as ^ in a C program
These operations are so simple because that's what a processor can easily do. And they are very simple to accelerate in hardware. The most important thing to keep in mind is that they are bit operators, the operation is applied to each individual bit in the pixel value. That makes ROPs an historical artifact, they only have a useful outcome on monochrome bitmaps (1 pixel = 1 bit) or indexed bitmap formats, like 4bpp or 8bpp, with a carefully chosen palette. That mattered on machines in the 1980s.
The kind of video adapters you use today, as well as bitmap formats, are at least 16bpp, almost always 24bpp or 32bpp. An operation like NOT on a pixel of such a bitmap just produces a wildly different color that the human eye isn't going to recognize as in any way related to the original color. Today you only use SRCCOPY. Maybe PATCOPY to apply a texture brush, you'd use PatBlt() instead. There's hackorama with multiple BitBlts to create transparency effects, you'd use TransBlt() or GDI+ instead.
For reasons detailed
here
I need to texture a quad using a bitmap (as in, 1 bit per pixel, not an 8-bit pixmap).
Right now I have a bitmap stored in an on-device buffer, and am mounting it like so:
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, BFR.G[(T+1)%2]);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, W, H, 0, GL_COLOR_INDEX, GL_BITMAP, 0);
The OpenGL spec has this to say about glTexImage2D:
"If type is GL_BITMAP, the data is considered as a string of unsigned bytes (and format must be GL_COLOR_INDEX). Each data byte is treated as eight 1-bit elements..."
Judging by the spec, each bit in my buffer should correspond to a single pixel. However, the following experiments show that, for whatever reason, it doesn't work as advertised:
1) When I build my texture, I write to the buffer in 32-bit chunks. From the wording of the spec, it is reasonable to assume that writing 0x00000001 for each value would result in a texture with 1-px-wide vertical bars with 31-wide spaces between them. However, it appears blank.
2) Next, I write with 0x000000FF. By my apparently flawed understanding of the bitmap mode, I would expect that this should produce 8-wide bars with 24-wide spaces between them. Instead, it produces a white 1-px-wide bar.
3) 0x55555555 = 1010101010101010101010101010101, therefore writing this value ought to create 1-wide vertical stripes with 1 pixel spacing. However, it creates a solid gray color.
4) Using my original 8-bit pixmap in GL_BITMAP mode produces the correct animation.
I have reached the conclusion that, even in GL_BITMAP mode, the texturer is still interpreting 8-bits as 1 element, despite what the spec seems to suggest. The fact that I can generate a gray color (while I was expecting that I was working in two-tone), as well as the fact that my original 8-bit pixmap generates the correct picture, support this conclusion.
Questions:
1) Am I missing some kind of prerequisite call (perhaps for setting a stride length or pack alignment or something) that will signal to the texturer to treat each byte as 8-elements, as it suggests in the spec?
2) Or does it simply not work because modern hardware does not support it? (I have read that GL_BITMAP mode was deprecated in 3.3, I am however forcing a 3.0 context.)
3) Am I better off unpacking the bitmap into a pixmap using a shader? This is a far more roundabout solution than I was hoping for but I suppose there is no such thing as a free lunch.
I would like to create a fake "explosion" effect in SDL. For this, I would like the screen to go from what it is currently, and fade to white.
Originally, I thought about using SDL_FillRect like so (where explosionTick is the current alpha value):
SDL_FillRect(screen , NULL , SDL_MapRGBA(screen->format , 255, 255 , 255, explosionTick ));
But instead of a reverse fading rectangle, it shows up completely white with no alpha. The other method I tried involved using a fullscreen bitmap filled with a transparent white (with an alpha value of 1), and blit it once for each explosionTick like so:
for(int a=0; a<explosionTick; a++){
SDL_BlitSurface(boom, NULL, screen, NULL);
}
But, this ended up being to slow to run in real time.
Is there any easy way to achieve this effect without losing performance? Thank you for your time.
Well, you need blending and AFAIK the only way SDL does it is with SDL_Blitsurface. So you just need to optimize that blit. I suggest benchmarking those:
try to use SDL_SetAlpha to use per-surface alpha instead of per-pixel alpha. In theory, it's less work for SDL, so you may hope some speed gain. But I never compared it and had some problem with this in the past.
you don't really need a fullscreen bitmap, just repeat a thick row. It should be less memory intensive and maybe there is a cache gain. Also you can probably fake some smoothness by doing half the lines at each pass (less pixels to blit and should still look like a global screen effect).
for optimal performance, verify that your bitmap is at the display format. Check SDL_DisplayFormatAlpha or possibly SDL_DisplayFormat if you use per-surface alpha