Difference between revisions of "Texture Mapping"

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First, let's talk about texture types available.<br>
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#REDIRECT [[Texture]]
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<br>
 
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GL has GL_TEXTURE_1D. You can ignore this and use GL_TEXTURE_2D instead.<br>
 
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This has been part of GL since 1.0<br>
 
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Texture coordinates are normalized. That means if you have a dimension like 256, texcoords are from 0.0 to 1.0.<br>
 
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Of course, if you go beyond that, such as -1.0 to 5.0, then the texture will repeat over your polygon.<br>
 
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<br>
 
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GL_TEXTURE_2D has width and height and usually the GPU stores this in memory in a format that is quick to access.<br>
 
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For example, small blocks of the texture are stored in sequence so that cache memory works better.
 
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This has been part of GL since 1.0<br>
 
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Texture coordinates are normalized. That means if you have a dimension like 256x256, texcoords are from 0.0 to 1.0.<br>
 
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Of course, if you go beyond that, such as -1.0 to 5.0, then the texture will repeat over your polygon.<br>
 
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<br>
 
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GL_TEXTURE_3D has width and height and depth and usually the GPU stores this in memory in a format that is quick to access.<br>
 
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Just like 2D, small blocks of the texture are stored in sequence so that cache memory works better but other techniques exist as well.<br>
 
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This has been part of GL since 1.2<br>
 
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Texture coordinates are normalized. That means if you have a dimension like 256x256x256, texcoords are from 0.0 to 1.0.<br>
 
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Of course, if you go beyond that, such as -1.0 to 5.0, then the texture will repeat over your polygon.<br>
 
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<br>
 
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GL_TEXTURE_CUBE_MAP has width and height and 6 faces. Kind of like 2D except it has 6 faces and texcoords work in a special way.<br>
 
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This has been part of GL since 1.3<br>
 
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Texture coordinates behave in a special way. You should use str coordinates and these behave as normal vectors. A certain algorithm is used to know which
 
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face has been selected, and then which texel is sampled.<br>
 
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<br>
 
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GL_TEXTURE_RECTANGLE_EXT, GL_TEXTURE_RECTANGLE_NV, GL_TEXTURE_RECTANGLE_ARB are supported as extensions. For having non power
 
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of 2 dimension 2D textures. Texcoords worked in a unusual way. From 0 to width for S. From 0 to height for the T.<br>
 
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There are certain limitations such as anisotropy might not work, mipmaps are not allowed, only certain wrap modes are supported such as GL_REPEAT, GL_CLAMP_TO_EDGE.<br>
 
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On certain GPUs, the driver will pad your texture with black pixels in order to make it power of 2. This is for getting better performance.<br>
 
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You won't ever see those black pixels.<br>
 
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<br>
 
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With GL 2.0, GL_TEXTURE_RECTANGLE becomes obsolete. You can make textures with any dimension, mipmap it, use any anisotropy supported,
 
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use any texture wrap mode.<br>
 
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<br>
 
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<br>
 
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=== How to create a texture ===
 
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<br>
 
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Create a single texture ID<br>
 
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  uint textureID;
 
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  glGenTextures(1, &textureID);
 
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<br>
 
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You have to bind the texture before doing anything to it.<br>
 
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  glBindTexture(GL_TEXTURE_2D, textureID);
 
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<br>
 
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Now you should define your texture properties. Any call sequence will work since GL is a state machine.<br>
 
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  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, TextureWrapS);
 
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  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, TextureWrapT);
 
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  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, MagFilter);
 
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  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, MinFilter);
 
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<br>
 
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Some people make the mistake of not calling glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, MinFilter); and
 
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they get the default state GL_LINEAR_MIPMAP_NEAREST and they don't define the mipmaps, so <b>the texture is considered
 
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incomplete</b> and you just get a white texture.<br>
 
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<br>
 
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If you will use mipmapping, you can either define them yourself by making many calls to glTexImage2D or let the GPU generate the mipmaps.<br>
 
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Since current GPUs can generate it automatically with a box filter technique, you can call<br>
 
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Mipmapping is usually good and increases performance.<br>
 
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<br>
 
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  //Use this if GL 1.4 is supported
 
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  glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE);
 
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  //Since the above is considered deprecated in GL 3.0, it is recommended that you use glGenerateMipmap(GL_TEXTURE_2D)
 
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  //or if GL_EXT_framebuffer_object is supported, use glGenerateMipmapEXT(GL_TEXTURE_2D)
 
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  //but call it after your call to glTexImage2D
 
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<br>
 
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<font color="#ff0000">It has been reported that on some ATI drivers, glGenerateMipmapEXT(GL_TEXTURE_2D) has no effect except if you proceede it with a call to glEnable(GL_TEXTURE_2D). Once again, to be clear, call glTexImage2D, then glEnable, then glGenerateMipmapEXT.</font><br>
 
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We recommend that you use glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE) for standard textures since this will work on both ATI and nVidia (For GL 2.1 only. For GL 3.0, it is deprecated and you should use glGenerateMipmap).<br>
 
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<br>
 
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If you need anisotropy, call<br>
 
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  glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_ANISOTROPY_EXT, Anisotropy);
 
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<br>
 
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The minimum value is 0.0 and max is glGetIntegerv(GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT, &MaxAnisotropy).<br>
 
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Anisotropy is an extension. It can drag down performance greatly and make your results look better. Use as less as possible.<br>
 
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You need to check if GL_EXT_texture_filter_anisotropic is present.<br>
 
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The spec is here http://www.opengl.org/registry/specs/EXT/texture_filter_anisotropic.txt<br>
 
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<br>
 
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Define the texture with<br>
 
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  glTexImage2D(GL_TEXTURE_2D, 0, internalFormat, width, height, border, format, type, ptexels);
 
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<br>
 
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<br>
 
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You need to make sure that your width and height are supported by the GPU.<br>
 
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  int Max2DTextureWidth, Max2DTextureHeight;
 
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  glGetIntegerv(GL_MAX_TEXTURE_SIZE, &Max2DTextureWidth);
 
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  Max2DTextureWidth=Max2DTextureHeight;
 
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  int MaxTexture3DWidth, MaxTexture3DHeight, MaxTexture3DDepth;
 
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  glGetIntegerv(GL_MAX_3D_TEXTURE_SIZE, &MaxTexture3DWidth);
 
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  MaxTexture3DHeight=MaxTexture3DDepth=MaxTexture3DWidth;
 
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  int MaxTextureCubemapWidth, MaxTextureCubemapHeight;
 
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  glGetIntegerv(GL_MAX_CUBE_MAP_TEXTURE_SIZE, &MaxTextureCubemapWidth);
 
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  MaxTextureCubemapHeight=MaxTextureCubemapWidth;
 
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  int MaxTextureRECTWidth, MaxTextureRECTHeight;
 
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  glGetIntegerv(GL_MAX_RECTANGLE_TEXTURE_SIZE_ARB, &MaxTextureRECTWidth);
 
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  MaxTextureRECTHeight=MaxTextureRECTWidth;
 
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  int MaxRenderbufferWidth, MaxRenderbufferHeight;
 
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  glGetIntegerv(GL_MAX_RENDERBUFFER_SIZE_EXT, &MaxRenderbufferWidth);
 
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  MaxRenderbufferHeight=MaxRenderbufferWidth;
 
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<br>
 
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<br>
 
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Very old GPUs don't support border texels.<br>
 
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Make sure the format is supported by the GPU else the drivers will convert into a proper format.<br>
 
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Make sure the internal format is supported by the GPU (example : GL_RGBA8) else the driver will convert the texture for you.<br>
 
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There is no way to query what formats the GPU supports but IHVs (nVidia, AMD/ATI) publish documents on what is supported.<br>
 
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For example, it is very common for GL_RGBA8 to be supported but GL_RGB8 is not.<br>
 
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You should also call glGetError to make you don't get an error message like running out of memory (GL_OUT_OF_MEMORY).<br>
 
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<br>
 
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The only thing left is calling glTexEnv but this isn't part of the texture state. It is part of the texture_environment, in other
 
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words the texture unit.<br>
 
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<br>
 
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To use the texture, bind it to a texture unit with glBindTexture and don't forget to enable texturing with
 
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glEnable(GL_TEXTURE_2D) and disable with glDisable(GL_TEXTURE_2D)
 
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<br>
 
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That's the basics of creating a texture and using it.<br>
 
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<br>
 
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=== Just allocate memory for a texture ===
 
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If you want to just allocate memory for the texture but you don't want to initialize the texels, then just give a NULL pointer to glTexImageXD. The GL specification doesn't say what values the texels will have.<br>
 
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  uint textureID;
 
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  glGenTextures(1, &textureID);
 
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  glBindTexture(GL_TEXTURE_2D, textureID);
 
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  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
 
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  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
 
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  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
 
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  glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
 
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  //GL_BGRA and GL_UNSIGNED_BYTE imply the GL_BGRA8 format.
 
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  //The driver might store it as GL_BGRA8 or it might flip red and blue to create GL_RGBA8
 
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  //Most GPUs support the Microsoft standard of GL_BGRA8
 
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  //The following call is garanteed to be very fast on Windows, nVidia, ATI/AMD
 
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  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 512, 512, 0, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
 
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=== Update some pixels ===
 
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If you want to update some pixels and your pixels are in RAM, use glTexSubImage2D.<br>
 
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Some people use glTexImage2D which causes the previous texture to be deleted and reallocated, which causes a slow down.<br>
 
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  //Never forget to bind!
 
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  glBindTexture(GL_TEXTURE_2D, textureID);
 
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  //GL_BGRA and GL_UNSIGNED_BYTE imply the GL_BGRA8 format.
 
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  //Most GPUs support the Microsoft standard of GL_BGRA8
 
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  //The following call is garanteed to be very fast on Windows, nVidia, ATI/AMD
 
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  glTexSubImage2D(GL_TEXTURE_2D, level, xoffset, yoffset, width, height, GL_BGRA, GL_UNSIGNED_BYTE, pixels);
 
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=== Copy the frame buffer to the texture ===
 
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Don't use glCopyTexImage2D. This functions deletes the previous texture and reallocates therefore it is slow.<br>
 
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Use glCopyTexSubImage2D instead, which just updates the texels.<br>
 
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So, you need to render the scene to the backbuffer, don't call SwapBuffers, bind the texture and call glCopyTexSubImage2D
 
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  RenderScene();
 
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  glBindTexture(GL_TEXTURE_2D, textureID);
 
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  glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 0, 0, 512, 512);
 
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  SwapBuffers();
 
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=== glGenTextures ===
 
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Always use this to generate texture IDs.<br>
 
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Also, glGenTextures does not allocate any texture. It just reserves an ID. It doesn't consume much RAM.<br>
 
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You must call glTexImage2D to really allocate texture memory.<br>
 
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Furthermore, it is possible that the driver won't upload your texture to VRAM when you call glTexImage2D.<br>
 
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It will wait until you use it when rendering.<br>
 
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glGenTextures generates unsigned int (32 bit) values. You can create 2^32-1 texture IDs.<br>
 
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Texture IDs returned start from 1 and go upwards. If you delete a texture with glDeleteTextures, and then you call glGenTextures, the driver
 
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may or may not return that same ID. This isn't something you need to worry about.
 
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When you shut down your program, it is recommended to call
 
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  glDeleteTextures(1, &textureID)
 
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but you are not obligated. The driver can delete it for you.
 
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=== glBindTexture ===
 
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Sometimes, you might run into a code that calls glBindTexture(GL_TEXTURE_2D, 0). It might look something like this
 
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  glEnable(GL_TEXTURE_2D);  //Enable
 
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  glBindTexture(GL_TEXTURE_2D, textureID);
 
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  DrawTheThing();
 
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  glBindTexture(GL_TEXTURE_2D, 0);
 
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glBindTexture(GL_TEXTURE_2D, 0) binds the default texture, which effectively disables texturing.<br>
 
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The advantages of that are not clear.<br>
 
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You are better off really disabling the tex unit with glDisable(GL_TEXTURE_2D).
 
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<br>
 
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You could also use some other invalid ID to disable texturing such as 1,000,000 and that will have the same effect as 0.<br>
 
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=== Texture Storage ===
 
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Once you upload your texture to GL, you don't need to keep a copy on your side. You can delete it just after calling glTexImage2D or glTexSubImage2D.
 
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=== Windows and other OSes ===
 
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Once you upload your texture to GL, where is it stored exactly?<br>
 
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The driver will most likely keep it in RAM. Keep in mind that the driver has its own memory manager.<br>
 
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When you want to render something with this texture, the driver will then upload the texture to VRAM (if you have a system with dedicated video memory). It will likely upload 100% of the texture with all the mipmaps.<br>
 
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If there isn't enough VRAM, the driver will delete another texture or another VBO. That is the driver's choice and there is nothing you can do about it.<br>
 
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The driver will always keep a copy in RAM, even when a copy is made in VRAM. RAM is considered a permanent storage. VRAM is considered volatile. Windows can destroy a texture and take over a part of VRAM if it wants to. That's why drivers always keep a copy in VRAM.<br>
 
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It is possible this will change in the future or perhaps it has already changed with Windows Vista.<br>
 
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Functions such as <b>glPrioritizeTextures</b> and <b>glAreTexturesResident</b> are useless as has been explained on another page.<br>
 
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http://www.opengl.org/wiki/Common_Mistakes#glAreTexturesResident_and_Video_Memory<br>
 
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<br>
 
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Games should not use these functions and should not rely on them. Gaming video cards such as nVidia Geforce 9800 and ATI/AMD Radeon HD drivers don't care for these functions. The driver is always the boss and not the programmer.<br>
 
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<br>
 
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For other OSes (Linux and Mac and FreeBSD), no comment.<br>
 
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For other video cards such as the workstation video card nVidia Quadro FX, no comment.
 
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[[Category:Textures]]
 

Latest revision as of 21:05, 9 June 2011

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