I'm finding a way to understand why glActiveTexture is needed. I have the code below:
glGenTextures(1, &textureId);
glBindTexture(GL_TEXTURE_2D, textureId);
If I imagine that the GL_TEXTURE_2D is a picture's frame hanging on the wall and textureId is the real picture, the glBindTexture is the only command for assigning a real picture to the frame, so, what are GL_TEXTURE0 and glActiveTexture?
Will my code below work fine?
glActiveTexture(GL_TEXTURE0);
glGenTextures(1, &textureId);
glBindTexture(GL_TEXTURE_2D, textureId);
glActiveTexture(GL_TEXTURE1);
glTexImage2D(GL_TEXTURE_2D, .....)
glTexParameteri(GL_TEXTURE_2D, ...)
I'm currently working with OpenGL2.1.
If I imagine that the GL_TEXTURE_2D is a picture's frame hangging on the wall and textureId is the real picture,
Actually a very good analogy :)
so, what GL_TEXTURE0 and glActiveTexture are?
Think about a wall with multiple picture frames, the first frame being labeled GL_TEXTURE0, the second GL_TEXTURE1 and so on.
OpenGL introduced multitexturing at some point (which is basically using more than one texture on one geometry pass); in order to keep older code working, they couldn't just add additional parameter to all the functions. That's why you have to explicitly specify which texture unit you are addressing.
Related
I was finding the relationship between glActiveTexture(...) and glBindTexture(...) and I found an awesome answer here, the very top answer(the author/user Alfonse) gives us a pseudocode for how the both functions behave, and I understood most of it. But, in it he mentions in calls such as this:
glActiveTexture(GL_TEXTURE0 + 5);
glBindTexture(GL_TEXTURE_2D, object);
glTexImage2D(GL_TEXTURE_2D, ...);
but one often binds a texture to the context just to upload some data or to modify it. It doesn’t matter at that point which texture unit you bind it to, so there’s no need to set the current texture unit. glTexImage2D doesn’t care if the current active texture is 0, 1, 40, or whatever.
So my problem is:
when generating two texture we do something like this:
glGenTextures(1, &texture1);
glBindTexture(GL_TEXTURE_2D, texture1);
glTextureParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glGenTextures(1, &texture2);
glBindTexture(GL_TEXTURE_2D, texture2);
glTextureParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
//some more code
//inside the render loop
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture1);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, texture2);
glDrawElements(...); glSwapBuffers();
//end of render loop
Notice, in the code before the render loop, I have used two texture glBindTexture(...) without calling glActiveTexture(...). Since, the default active texture unit is: GL_TEXTURE0, does this mean the parameters set for texture1 is overwritten by texture2?
No, texture parameters (set by glTexParameter) are set for a specific texture (the one that's currently bound to the active texture unit), not for a texture unit (not for one of GL_TEXTUREi, that is).
Note that your use of glTextureParameteri incorrect. Rather than GL_TEXTURE_2D, it expects a handle of a texture, as returned by glGenTextures1. You're confusing it with glTexParameteri, which indeed can be called with GL_TEXTURE_2D.
1 As noted by #derhass, glGenTextures (unlike glCreateTextures) merely reserves the handle. A texture with this handle is created only when you pass it to glBindTexture. It doesn't matter if you use glTexParameter, but if you want to use glTextureParameteri and other functions that operate directly on texture handles, it might be important.
While learning opengl, one bit of confusion is whether it is necessary to bind buffers like VAO
or texture IDs multiple times without unbinding them?
With reference to the above link and the full source code , I have come across 2 scenerios a bit distinct from each other.
First, the author creates a VAO(outside the maindraw loop) and binds the VAO using the glBindVertexArray(VAO) then he doesn't unbind it using glBindVertexArray(0); , but he binds it again in the maindraw loop. My confusion, Is this really necessary? Isn't the currently bound VAO already ... bound? Why bind it when it is already the active VAO?
And,
Second, the author creates texture ID using:
glGenTextures(1, &texture1);
glBindTexture(GL_TEXTURE_2D, texture1);
//some more code
glGenTextures(1, &texture2);
glBindTexture(GL_TEXTURE_2D, texture2);
outside the main draw loop , but does this again *inside the main draw loop: *
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture1);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, texture2);
Two things that I don't understand in what he does is:
Why does he bind the texture ID again in the mainloop when he has
done it outside of the mainloop?(This one is similar to VAO but I
don't know yet if unbinding is possible in here)
Why does he select the active texture unit (using glActiveTexture)
inside the loop? Couldn't it be done outside the main draw loop?
glBindTexture binds a texture to specified target and the current texture unit. The current texture unit is set by glActiveTexture. The current texture unit is a global state, and glBindTexture considers that state.
That meas when you do
glBindTexture(GL_TEXTURE_2D, texture1);
glBindTexture(GL_TEXTURE_2D, texture2);
glActiveTexture(GL_TEXTURE0);
glActiveTexture(GL_TEXTURE1);
after that texture2 is bound to the current texture unit (GL_TEXTURE0 by default) and the current texture unit is GL_TEXTURE1. Note texture1
is just bound for a short moment, but the binding is lost, when texture2 is bound.
In compare, when you do
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture1);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, texture2);
The current texture unit is explicitly set to GL_TEXTURE0 and texture1 is bound to texture texture unit 0. After that the current texture unit is switched to GL_TEXTURE1 and texture2 is bound to texture unit 1 (because it is current at this moment).
Note, with the use of Direct State Access and glBindTextureUnit (since OpenGL 4.5) a texture can be bound to a texture unit directly. e.g.:
glBindTextureUnit(GL_TEXTURE0, texture1);
glBindTextureUnit(GL_TEXTURE1, texture2);
I have recently implemented Awesomium into a OpenGL application.
When I load Awesomium in to a texture OpenGL includes it in its shading process regardless of whether I draw the texture onto a surface or not.
I am trying to trace down the line of code that is processing the texture into the shaders, is there a specific function OpenGL uses to access all textures or a way to tell OpenGL to ignore the texture?
Update texture block
glBindTexture(GL_TEXTURE_2D, SkypeHUD);
glTexImage2D(GL_TEXTURE_2D, 0, 4, AwesomiumW, AwesomiumH, 0, GL_BGRA, GL_UNSIGNED_BYTE, surface->buffer());
Create texture block
glBindTexture(GL_TEXTURE_2D, SkypeHUD);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT );
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT );
glBindTexture(GL_TEXTURE_2D, 0);
Drawing the scene without the texture being loaded: http://puu.sh/2bVTV
Drawing the scene after I have loaded the texture: http://puu.sh/2bVUb
You can see it blending the google texture in over the others.
Texture enable/disable should be controlled by the shader code, not some client binding state. Anyway, you most likely use several texture units (glActiveTexture); the texture binding is individual to each unit, so you'll have to do some leg work and unbind textures from each unit if you want to go this route.
I'm trying to send two textures to a fragment shader but it doesn't seem to work. Only one texture can be accessed from the sampler in the shader.
Here is what I'm doing in my code
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, fbo_texture);
glUniform1i(glGetUniformLocation(p,"fbo_texture"), 0);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, tex_back);
glUniform1i(glGetUniformLocation(p,"back"), 1);
render stuff
However if I add glActiveTexture(GL_TEXTURE0); after the last uniform, it works. Also if I specify GL_TEXTURE1 then GL_TEXTURE0, in that order, it's also good. What am I missing?
Edit *
glActiveTexture(GL_TEXTURE2);
glBindTexture(GL_TEXTURE_2D, fbo_texture);
glUniform1i(glGetUniformLocation(p,"fbo_texture"), 2);
glActiveTexture(GL_TEXTURE3);
glBindTexture(GL_TEXTURE_2D, tex_back);
glUniform1i(glGetUniformLocation(p,"back"), 3);
Works...
A problem with GL_TEXTURE0 apparntly
I see that you called one of your textures fbo_texture. I assume, that this is a render-to-texture color attachment of a FBO then.
In that case make sure that you bind the texture only after you unbound the FBO for rendering, and vice versa.
I have got a very, very strange problem in my C++ OpenGL application.
I simply load a texture and apply it to a quadric:
glGenTextures(1, &tex);
glBindTexture(GL_TEXTURE_2D, tex);
glTexImage2D(GL_TEXTURE_2D, 0, 3, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, image);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
Then
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, tex);
gluQuadricDrawStyle(quad,GLU_FILL);
gluQuadricTexture(quad,GL_TRUE);
gluCylinder(quad,1,0,2,20,1);
glDisable(GL_TEXTURE_2D);
Now: it works perfectly 9 times out of ten, but sometimes the texture isn't shown (the quadric stays white).
The image is correctly loaded, so the problem should be with OpenGL. I have tried with several different images too. Always GL_NO_ERROR.
Any idea ? It is driving me crazy...
Found :) It was the GLint texture member that wasn't correctly reallocated in the copy constructor.
However, i still don't understand why it worked sometimes...
The code you are using seems valid. Have you ...
tried to use a simple quad instead of the quadric
assured that image is filled correctly
verified that tex is not altered somewhere else
assured that no other programs are using opengl at the same time
restarted your computer ;)