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Texturing

Texturing. A picture is worth a thousand words. Texturing. Texturing is the art of doing this with any picture to any model. (This is Opus the Penguin wrapped around a sphere.). Texturing. First, a few words about what a picture is:

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Texturing

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  1. Texturing A picture is worth a thousand words

  2. Texturing • Texturing is the art of doing this with any picture to any model. • (This is Opus the Penguin wrapped around a sphere.)

  3. Texturing • First, a few words about what a picture is: • A picture is a 2D array of color values (RGB or RGBA). • Each element in the array is one pixel. • The size of an image in pixels is its width times its height. • A picture stored in memory is called a bitmap. • Next, a few words about images in Windows: • Windows BMPs store DIBs, or Device Independent Bitmaps. • Loading a DIB is a really huge hassle. • Next, a few words about images in OpenGL: • The height and width of any bitmaps in OpenGL must be powers of two (32x32, 512x512, but not 31x31 or 42x27). • The GLAUX library simplifies loading DIBs for you.

  4. Texturing • A bitmap in memory: • ...a two-dimensional array of color values

  5. Texture Co-ordinates • We use texture co-ordinates to identify locations in the texture itself. • (0,0) is the lower left-hand corner of the texture. • (0.5, 0.5) is the middle of the texture. • (1,1) is the upper right-hand corner. • Caveat: OpenGL draws bottom-to-top, not top-to-bottom, so in your own rendering you may have to invert Y. (1,1) (0,0)

  6. Texture Mapping • Texture-mapping is the mapping from 3D vertices in your model to 2D texture co-ordinates. • This associates a particular point on your polygon with a particular point on your picture. • As the polygon is rendered, the inside of the polygon is filled in with the space of the picture that runs between the texture co-ordinates of the polygon’s vertices. (0,1) (1,0) (0,0)

  7. (0.25,1.25) (1.25,0.25) (0.25,0.25) Texture Mapping • Shifting texture co-ordinates • If the values you map to your texture co-ordinates shift, the image will appear to shift on the surface. • The image at right is drawnwith texture co-ordinatesshifted by (0.25, 0.25).

  8. Texture Mapping in OpenGL • OpenGL works on the idea that you bind the current texture before you glBegin(), and then you specify texture co-ordinates for each glVertex() you draw. • Fortunately, the GLAUX library makes file loading easy. • Remember, your image dimensions MUST be multiples of 2. • To compile an app which uses glaux functions, you’ll need to link glaux.lib. To link a .lib library, go to Project / Settings... / Link tab, and add ‘glaux.lib’ to the list of Library Modules.

  9. Texture Mapping in OpenGL • Loading BMPs with GLAUX: Gluint arrTextures[NUM_TEXTURES]; char *arrNames[NUM_TEXTURES] = { “filename1.bmp”, “filename2.bmp”, ... }; glEnable(GL_TEXTURE_2D); glGenTextures(NUM_TEXTURES, arrTextures); for (i = 0; i<NUM_TEXTURES; i++) { AUX_RGBImageRec *pAuxImgLoader = auxDIBImageLoad(arrNames[i]); if (pAuxImgLoader && pAuxImgLoader->data) { glBindTexture(GL_TEXTURE_2D, arrTextures[i]); glTexImage2D(GL_TEXTURE_2D, 0, 3, pAuxImgLoader->sizeX, pAuxImgLoader->sizeY, 0, GL_RGB, GL_UNSIGNED_BYTE, pAuxImgLoader->data); glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR); free(pAuxImgLoader->data); } }

  10. Texture Mapping in OpenGL • Displaying a texture-mapped square glBindTexture(GL_TEXTURE_2D, arrTextures[whichTexture]); glColor3f(1,1,1); glNormal3f(0,0,1); glBegin(GL_QUADS); glTexCoord2f(0,1); glVertex3f(-5,5,0); glTexCoord2f(0,0); glVertex3f(-5,-5,0); glTexCoord2f(1,0); glVertex3f(5,-5,0); glTexCoord2f(1,1); glVertex3f(5,5,0); glEnd(); glBindTexture(GL_TEXTURE_2D, 0);

  11. Texture Mapping in OpenGL • Texturing a parametric surface • Texture-mapping a parametric surface is very easy. If u and v range from 0 to 1, then they also cover the range of the texture co-ordinates. • You can set the current texture co-ordinate as (u,v) to map your image to any parametric surface.

  12. Texture Mapping in OpenGL • Texturing a parametric surface • This should look familiar... .......................... A = evaluateFunction(which, u, v); B = evaluateFunction(which, u, v+step); C = evaluateFunction(which, u+step, v+step); D = evaluateFunction(which, u+step, v); N = (((C-A).normalized()) ^ ((B-A).normalized())).normalized(); glColor3f(1,1,1); glNormal3f(N.x(),N.y(),N.z()); glBegin(GL_QUADS); { glTexCoord2f(u, v); glVertex3f(A.x(),A.y(),A.z()); glTexCoord2f(u, v+step); glVertex3f(B.x(),B.y(),B.z()); glTexCoord2f(u+step, v+step); glVertex3f(C.x(),C.y(),C.z()); glTexCoord2f(u+step, v); glVertex3f(D.x(),D.y(),D.z()); } glEnd(); ..........................

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