I did search and read stuff about this but couldn't understand it.
What's the difference between a texture internal format and format in a call like
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 32, 32, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
?
Let's assume that data is an array of 32 x 32 pixel values where there are four bytes per each pixel (unsigned char data 0-255) for red, green, blue and alpha.
What's the difference between the first GL_RGBA and the second one? Why is GL_RGBA_INTEGER invalid in this context?
The format (7th argument), together with the type argument, describes the data you pass in as the last argument. So the format/type combination defines the memory layout of the data you pass in.
internalFormat (2nd argument) defines the format that OpenGL should use to store the data internally.
Often times, the two will be very similar. And in fact, it is beneficial to make the two formats directly compatible. Otherwise there will be a conversion while loading the data, which can hurt performance. Full OpenGL allows combinations that require conversions, while OpenGL ES limits the supported combinations so that conversions are not needed in most cases.
The reason GL_RGBA_INTEGER is not legal in this case that there are rules about which conversions between format and internalFormat are supported. In this case, GL_RGBA for the internalFormat specifies a normalized format, while GL_RGBA_INTEGER for format specifies that the input consists of values that should be used as integers. There is no conversion defined between these two.
While GL_RGBA for internalFormat is still supported for backwards compatibility, sized types are generally used for internalFormat in modern versions of OpenGL. For example, if you want to store the data as an 8-bit per component RGBA image, the value for internalFormat is GL_RGBA8.
Frankly, I think there would be cleaner ways of defining these APIs. But this is just the way it works. Partly it evolved this way to maintain backwards compatibility to OpenGL versions where features were much more limited. Newer versions of OpenGL add the glTexStorage*() entry points, which make some of this nicer because it separates the internal data allocation and the specification of the data.
The internal format describes how the texture shall be stored in the GPU. The format describes how the format of your pixel data in client memory (together with the type parameter).
Note that the internal format does specify both the number of channels (1 to 4) as well as the data type, while for the pixel data in client memory, both are specified via two separate parameters.
The GL will convert your pixel data to the internal format. If you want efficient texture uploads, you should use matching formats so that there is no conversion needed. But be aware that most GPUs store the texture data in BGRA order, this still is represented by the internal format GL_RBGA - the internal format only describes the number of channels and the data type, the internal layout is totally GPU-specific. However, that means that it is often recommended for maximum performance to use GL_BGRA as the format of your pixel data in client memory.
Let's assume that data is an array of 32 x 32 pixel values where there
are four bytes per each pixel (unsigned char data 0-255) for red,
green, blue and alpha.
What's the difference between the first GL_RGBA and the second one?
The first, internalFormat tells the GL that it should store the texture as 4 channel (RGBA) with normalized integer in the preferred precision (8 bit per channel). The second one, format tells the Gl that you are providing 4 channels per pixel in the R,G,B,A order.
You could for example supply the data as 3-channel RGB data and the GL would automatically extend this to RGBA (with setting A to 1) if the internal format is left at RGBA. You also could supply only the Red channel.
The other way around, if you use GL_RED as internalFormat, the GL would ignore the GB and A channel in your input data.
Also note that the data types will be converted. If you provide a pixel RGB with 32 bit float per channel, you could use GL_FLOAT. However, when you still use the GL_RGBA internal format, the GL will convert these to normalized integers with 8 bpit per channel, so the extra precision is lost. If you want the GL to use the floating point precision, you would also have to use a floating point texture format like GL_RGBA32F.
Why is GL_RGBA_INTEGER invalid in this context?
the _INTEGER formats are for unnormalized integer textures. There is no automatic conversion for integer textures in the GL. You have to use an integer internal format, AND you have to specify your pixel data with some _INTEGER format, otherwise it will result in an error.
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.
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.
In my OpenGL program, I'm loading a 24BPP image with the width of 501. The GL_UNPACK_ALINGMENT parameter is set to 4. They write it shouldn't work because the size of each of the rows which are being uploaded (501*3 = 1503) cannot be divided by 4. However, I can see a normal texture without artifacs when displaying it.
So my code works. I'm considering why to understand this fully and prevent the whole project from getting bugged.
Maybe (?) it works because I'm not just calling glTexImage2D. Instead, at first I'm creating a proper (with dimensions which are powers of two) blank texture, then uploading pixels with glTexSubImage2D.
EDIT:
But do you think it does a sense to write some code like that?
// w - the width of the image
// depth - the depth of the image
bool change_alignment = false;
if (depth != 4 && !is_divisible(w*depth)) // *
{
change_alignment = true;
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
}
// ... now use glTexImage2D
if (change_alingment) glPixelStorei(GL_UNPACK_ALIGNMENT, 4); // set to default
// * - of course we don't even need such a function
// but I wanted to make the code as clear as possible
Hope it should prevent the application from crashing or malfunction?
It depends on where your image data is coming from.
The Windows BMP format, for example, enforces a 4-byte row alignment. Indeed, formats like this are exactly why OpenGL has a row-alignment field: because some image formats enforce a row alignment.
So how correct it is to use a 4-byte row alignment on your data depends entirely on how your data is aligned in memory. Some image loaders will automatically align to 4 bytes. And some will not.
Just getting started with OpenFrameworks and I'm trying to do something that should be simple : test the colour of the pixel at a particular point on the screen.
I find there's no nice way to do this in openFrameworks, but I can drop down into openGL and glReadPixels. However, I'm having a lot of trouble with it.
Based on http://www.opengl.org/sdk/docs/man/xhtml/glReadPixels.xml I started off trying to do this:
glReadPixels(x,y, 1,1, GL_RGB, GL_INT, &an_int);
I figured that as I was checking the value of a single pixel (width and height are 1) and giving it a GL_INT as type GL_RGB as format, a single pixel should take up a single int (4 bytes) Hence I passed a pointer to an int as the data argument.
However, the first thing I noticed was that glReadPixels seemed to be clobbering some other local variables in my function, so I changed to making an array of 10 ints and now pass that. This has stopped any weird side-effect, but I still have no idea how to interpret what it's returning.
So ... what's the right combination of format and type arguments that I should be passing to safely get something that can easily be unpacked into its RGB values? (Note that I'm doing this through openFrameworks so I'm not explicitly setting up openGL myself. I guess I'm just getting the openFramework / openGL defaults. The only bit of configuration I know I'm doing is NOT setting up alpha-blending, which I believe means that pixels are represented by 3 bytes (R,G,B but no alpha)). I assume that GL_RGB is the format that corresponds to this.
If you do so, you need three int: one for R, one for G, one for B. If think you should use:
unsigned char RGB[3];
glReadPixels(x,y, 1,1, GL_RGB, GL_UNSIGNED_BYTE, rgb);