Sign in to follow this  

OpenGL Simple multiple textures

Recommended Posts

Wh0p    407

I did D3D for a "long" time now and am somewhat new to OpenGL (still at the basics)


What i am trying to do is:

... Initialize

> Load texture1, Load texture2

... Render

> Bind texture1 to texture unit 0, render some geometry

> Bind texture2 to texture unit 0, render some geometry


In short it doenst work as expected. The result is, that both geometry is rendered with texture2 on it. So i looked up the docs and they were stating:

"glTexSubImage2D specifies a two-dimensional subtexture for the current texture unit, specified with glActiveTexture."


So I spent some time on trial and erroring around and finally I activated texture unit 1 before loading texture 2 which yields the correct result if I bind at rendering texture1 to tex unit 0 and texture2 to tex uint 1. Binding both textures to tex unit 0 will render the geometry only with texture1.

Actually this feels like the instand I call glTexImage* when having bound a texture to a certain tex uint (texture2 at 0) the the data I loaded before that (texture1 at 0) gets discarded?


I'm not quite sure what that would mean. Is it ture, that I need to spend a tex uint for each texture??? I cannot believe that, this would suggest that my graphics card has a 96 (GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS) textures max limit (no way). 


so here some snippets on how I (initially inteded to) do the loading and rendereing (both geometry gets rendered with texture2 on it):

glGenTextures (1, &tex);
glBindTexture (cfg.GetBindFlag (), tex);
glTexStorage2D(tex, cfg.GetMipLevel (), cfg.GetTextureFormat (), cfg.GetWidth (), cfg.GetHeight ());
glTexImage2D (cfg.GetBindFlag (), 
	cfg.GetMipLevel (), 
	cfg.GetTextureFormat (), 
	cfg.GetWidth (), cfg.GetHeight (), 0, 
	cfg.GetDataFormat (), 
	cfg.GetDataType (), initdata);
glActiveTexture (_TexUnit); // _TexUnit is asserted as GL_TEXTURE0 + _TexUnitIndex
glBindTexture (resource->GetBindFlag (), resource->GetGLID ()); // resource::_GLID is corresponds to names of texture1 and texture 2
glUniform1i (_Location, _TexUnitIndex); // _Location is the uniform sampler bindpoint in the shader, _TexUnitIndex is 0 in both rendering events
_Sampler->Bind (_TexUnitIndex);

I actually conclude that I have to have missed something, the question is what? Hopefully some of you can point me a diretion.

Share this post

Link to post
Share on other sites
Wh0p    407

Ok, so I publically denounced myself... it was a "variable not initialized error" which took me 1 and a half day to track down and of cource I posted here 2 hours befor i finally found the error....


But anyways, i would be glad of some explanation to the documentation. What does that "specifies a two-dimensional subtexture for the current texture unit" mean? Why is the texture unit relevant in the context of uploading texture data?

Share this post

Link to post
Share on other sites
Brother Bob    10344

But anyways, i would be glad of some explanation to the documentation. What does that "specifies a two-dimensional subtexture for the current texture unit" mean?

glTexSubImage, and equivalent for the other Sub-variants, modifies as sub-rectangle of an existing texture. Say you have a 128x128-pixel texture consisting of a 4x4 grid of 32x32-pixel tiles. You can use it to update/replace a single tile without having to upload an entirely new texture image.

Why is the texture unit relevant in the context of uploading texture data?

OpenGL has a bind-to-edit model which means you have to bind the texture object to make it the currently active object. Subsequent calls to glTexImage and such then operates on the currently bound texture. As you may have noticed, you never tell glTexImage which texture object to populate with texture data; that is given from the previous call to glBindTexture.


Since multitexturing allows for multiple textures to be bound at the same time, the texture bound to the currently active texture unit is the target of glTexImage.

Edited by Brother Bob

Share this post

Link to post
Share on other sites
Wh0p    407

I'm sorry to say that but you just pointed out the obvisous for me :P


you never tell glTexImage which texture object to populate with texture data


Yes of course. What I was actually wondering, is why the docs refer to the texture unit and especially glActiveTexture.

I thought glActiveTexture, should have nothing to do with what is stored in the texture but instead is a mechanism to access the texture via glsl.

Share this post

Link to post
Share on other sites
richardurich    1352

The glTexImage function doesn't have a parameter specifying which texture unit to store the data to, so the documentation tells you it will store the data to the current texture unit. The way to specify the current texture unit is glActiveTexture.


To get the texture in GLSL, you need to use a sampler and make it available to your shader program.


Edit: this is the same thing Brother Bob said, but hopefully if a few people tell you the same thing in different ways it will click.

Edited by richardurich

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Similar Content

    • By Arulbabu Donbosco
      There are studios selling applications which is just copying any 3Dgraphic content and regenerating into another new window. especially for CAVE Virtual reality experience. so that the user opens REvite or CAD or any other 3D applications and opens a model. then when the user selects the rendered window the VR application copies the 3D model information from the OpenGL window. 
      I got the clue that the VR application replaces the windows opengl32.dll file. how this is possible ... how can we copy the 3d content from the current OpenGL window.
      anyone, please help me .. how to go further... to create an application like VR CAVE. 
    • By cebugdev
      hi all,

      i am trying to build an OpenGL 2D GUI system, (yeah yeah, i know i should not be re inventing the wheel, but this is for educational and some other purpose only),
      i have built GUI system before using 2D systems such as that of HTML/JS canvas, but in 2D system, i can directly match a mouse coordinates to the actual graphic coordinates with additional computation for screen size/ratio/scale ofcourse.
      now i want to port it to OpenGL, i know that to render a 2D object in OpenGL we specify coordiantes in Clip space or use the orthographic projection, now heres what i need help about.
      1. what is the right way of rendering the GUI? is it thru drawing in clip space or switching to ortho projection?
      2. from screen coordinates (top left is 0,0 nd bottom right is width height), how can i map the mouse coordinates to OpenGL 2D so that mouse events such as button click works? In consideration ofcourse to the current screen/size dimension.
      3. when let say if the screen size/dimension is different, how to handle this? in my previous javascript 2D engine using canvas, i just have my working coordinates and then just perform the bitblk or copying my working canvas to screen canvas and scale the mouse coordinates from there, in OpenGL how to work on a multiple screen sizes (more like an OpenGL ES question).
      lastly, if you guys know any books, resources, links or tutorials that handle or discuss this, i found one with marekknows opengl game engine website but its not free,
      Just let me know. Did not have any luck finding resource in google for writing our own OpenGL GUI framework.
      IF there are no any available online, just let me know, what things do i need to look into for OpenGL and i will study them one by one to make it work.
      thank you, and looking forward to positive replies.
    • By fllwr0491
      I have a few beginner questions about tesselation that I really have no clue.
      The opengl wiki doesn't seem to talk anything about the details.
      What is the relationship between TCS layout out and TES layout in?
      How does the tesselator know how control points are organized?
          e.g. If TES input requests triangles, but TCS can output N vertices.
             What happens in this case?
      In this article,
      the isoline example TCS out=4, but TES in=isoline.
      And gl_TessCoord is only a single one.
      So which ones are the control points?
      How are tesselator building primitives?
    • By Orella
      I've been developing a 2D Engine using SFML + ImGui.
      Here you can see an image
      The editor is rendered using ImGui and the scene window is a sf::RenderTexture where I draw the GameObjects and then is converted to ImGui::Image to render it in the editor.
      Now I need to create a 3D Engine during this year in my Bachelor Degree but using SDL2 + ImGui and I want to recreate what I did with the 2D Engine. 
      I've managed to render the editor like I did in the 2D Engine using this example that comes with ImGui. 
      3D Editor preview
      But I don't know how to create an equivalent of sf::RenderTexture in SDL2, so I can draw the 3D scene there and convert it to ImGui::Image to show it in the editor.
      If you can provide code will be better. And if you want me to provide any specific code tell me.
    • By Picpenguin
      I'm new to learning OpenGL and still learning C. I'm using SDL2, glew, OpenGL 3.3, linmath and stb_image.
      I started following through and got through it until I had to load models. The problem is, it uses Assimp for loading models. Assimp is C++ and uses things I don't want in my program (boost for example) and C support doesn't seem that good.
      Things like glVertexAttribPointer and shaders are still confusing to me, but I have to start somewhere right?
      I can't seem to find any good loading/rendering tutorials or source code that is simple to use and easy to understand.
      I have tried this for over a week by myself, searching for solutions but so far no luck. With tinyobjloader-c and project that uses it, FantasyGolfSimulator, I was able to actually load the model with plain color (always the same color no matter what I do) on screen and move it around, but cannot figure out how to use textures or use its multiple textures with it.
      I don't ask much: I just want to load models with textures in them, maybe have lights affect them (directional spotlight etc). Also, some models have multiple parts and multiple textures in them, how can I handle those?
      Are there solutions anywhere?
      Thank you for your time. Sorry if this is a bit confusing, English isn't my native language
  • Popular Now