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CS293 Graphics with Java and OpenGL

Textures

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• Material from the Redbook is extensively used throughout these slides.

• Code has been translated from C++ to JOGL based on examples from the book

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Overview

• Understand what texture mapping can add to your scene

• Specify a texture image• Control how a texture image is filtered as it's applied

to a fragment• Create and manage texture images in texture objects

• Supply texture coordinates to indicate how the

texture image should be aligned to the objects in your scene

• Use automatic texture coordinate generation

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Texture mapping

• To draw a large brick wall without texture mapping, each brick must be drawn as a separate polygon.

• Without texturing, a large flat wall might need thousands of individual bricks

• Texture mapping allows you to glue an image of a brick wall (obtained, perhaps, by scanning in a photograph of a real wall) to a polygon draw the wall as a single polygon.

• Texture mapping ensures that all the right things happen as the polygon is transformed and rendered.

• E.g. when viewed in perspective, the bricks appear smaller as the wall gets farther from the viewpoint.

• Textures can also be used in flight simulation textures to create the illusion of natural.

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Texture mapping

• Textures can be mapped to surfaces made of a set of polygons or to curved surfaces, and you can repeat a texture in one or both directions to cover the surface.

• A texture can be one-dimensional. • You can automatically map a texture onto an object

in such a way that the texture indicates contours or other properties of the item being viewed.

• Shiny objects can be textured so that they appear to be in the centre of a room or other environment, reflecting the surroundings off their surfaces.

• A texture can be applied to a surface in different ways. It can be painted on directly, used to modulate the colour the surface would have been painted otherwise, or used to blend a texture colour with the surface colour.

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Texture mapping

• Textures are simply rectangular arrays of data - for example, colour data, luminance data, or colour and alpha data.

• The individual values in a texture array are often called texels.

• What makes texture mapping tricky is that a rectangular texture can be mapped to nonrectangular regions, and this must be done in a reasonable way.

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Texture mapping

• A quadrilateral displayed on the screen might be distorted by applying various transformations - rotations, translations, scaling, and projections.

• The figure shows how the texture-mapped quadrilateral might appear on your screen after these transformations.

• The quadrilaterals are only distorted by perpesctive transformations and rotations in 3D.

• Three of the quads are squares the front one is arbitrary.

• Note how the texture image of rectangular text is distorted to fit into a non-rectangular region.

• Note we have also used blending to show all the quads at the same time.

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Steps in Texture Mapping

• To use texture mapping, you perform these steps.1. Create a texture object and specify a texture for

that object.2. Indicate how the texture is to be applied to each

pixel.3. Enable texture mapping.4. Draw the scene, supplying both texture and

geometric coordinates.

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Create a Texture Object and Specify a Texture for That Object

• A texture is usually thought of as being two-dimensional, like most images, but it can also be one-dimensional.

• The data describing a texture may consist of one, two, three, or four elements per texel, representing anything from a modulation constant to an (R, G, B, A) quadruple.

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Indicate How the Texture Is to Be Applied to Each Pixel

• Four possible functions for computing the final RGBA value from the fragment color and the texture-image data. 1. Use the texture color as the final color;

the decal mode, the texture is painted on top of the fragment

2. The replace mode, a variant of the decal mode.3. Use the texture to modulate, or scale, the

fragment's color; this technique is useful for combining the effects of lighting with texturing.

4. Finally, a constant color can be blended with that of the fragment, based on the texture value.

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Enable Texture Mapping

gl.glEnable(GL.GL_TEXTURE_2D);

gl.glDisable(GL.GL_TEXTURE_2D);

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Texture and Geometric Coordinates

• Define how the texture is aligned relative to the fragments its applied to before it's "glued on.“

• For a two-dimensional texture map the texture coordinates range from 0.0 to 1.0 in both directions, but the coordinates of the items being textured can be anything.

• Define how texture coordinates outside the range [0.0,1.0] should be treated. Do textures repeat, or are they clamped to a boundary value?

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Jpeg images for textures

• It is not part of this course to cover the conversion of Jpg, or other image format, into texture maps.

• In order to have interesting examples we use the library NeHe Java ports from: http://pepijn.fab4.be/?page_id=34

• The NeHe tutorials are written for OpenGL, not JOGL. The web site ishttp://nehe.gamedev.net/

• Lab material contains details of how to use these ports for coding purposes.

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Coding conventions

String[ ] textureNames = new String[ ] { "demos/data/images/mona-lisa.png", "demos/data/images/monkey.jpg", "demos/data/images/newton2.jpg", "demos/data/images/marilyn.jpg", "demos/data/images/SEPS_Computing.png"

};

The coding convention for all the lab material is to store image names in the textureNames array, and manipulate this field directly to generate texture maps.

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Coding conventions

• Textures in JOGL are given identifiers that are integer values.

• Thus our list of images is converted into textures who’s identifiers can be stored in an integer array.

• Our convention is to define this as a field:

private int textures[] = new int[textureNames.length];

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NeHe library methods

• TextureReader• This is a NeHe class that permits us to load a

Jpg image directly into a Texture object.• Texture is and internal class to

TextureReader that is used as a temporary place holder for textures.

• Following code shows how these classes are used in a NeHe routine to attach textures to an integer array of texture identifiers.

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loadGLTextures method, version 1 public void loadGLTextures(GL gl) throws IOException { gl.glGenTextures(textureNames.length, textures, 0); gl.glTexEnvf(GL.GL_TEXTURE_ENV, GL.GL_TEXTURE_ENV_MODE, GL.GL_DECAL); for (int textureIndex = 0; textureIndex < textureNames.length; textureIndex++) { String textureName = textureNames[textureIndex]; TextureReader.Texture texture = TextureReader.readTexture(textureName); imageW[textureIndex] = texture.getWidth(); imageH[textureIndex] = texture.getHeight(); gl.glBindTexture(GL.GL_TEXTURE_2D, textures[textureIndex]); gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MAG_FILTER, GL.GL_LINEAR); gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MIN_FILTER, GL.GL_LINEAR); gl.glTexImage2D(GL.GL_TEXTURE_2D, 0, GL.GL_RGB8, texture.getWidth(), texture.getHeight(), 0, GL.GL_RGB, GL.GL_UNSIGNED_BYTE, texture.getPixels()); }

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glTexEnvf

• gl.glTexEnvf( GL.GL_TEXTURE_ENV, GL.GL_TEXTURE_ENV_MODE, GL.GL_DECAL);

• Defines the mode for combining texture map with geometric model

• In this case GL_DECAL specifies that the texture will be attached to the surface of the polygon and replace the material characteristics of the polygon.

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Loading Texture

String textureName = textureNames[textureIndex];

TextureReader.Texture texture = TextureReader.readTexture( textureName);

imageW[textureIndex]=texture.getWidth();

imageH[textureIndex]=texture.getHeight();

These lines of code use the TextureReader.readTexture method to load in a particular Jpeg and convert it to a texture. This is stored in the internal variable texture.

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Binding the texture

gl.glBindTexture(GL.GL_TEXTURE_2D, textures[textureIndex]);

gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MAG_FILTER, GL.GL_LINEAR);gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MIN_FILTER, GL.GL_LINEAR);gl.glTexImage2D( GL.GL_TEXTURE_2D, 0, GL.GL_RGB8, texture.getWidth(), texture.getHeight(), 0, GL.GL_RGB, GL.GL_UNSIGNED_BYTE, texture.getPixels());

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Binding the texture

• glGenTextures() and glBindTexture() name and create a texture object for a texture image.

• The single, full-resolution texture map is specified by glTexImage2D(), whose parameters indicate – size of the image, – type of the image, – location of the image, – and other properties of it.

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Filtering

• Texture maps are square or rectangular• After being mapped to a polygon or surface and transformed into

screen coordinates, the individual texels rarely correspond to individual pixels.

• Depending on the transformations used and the texture mapping applied, a single pixel on the screen can correspond to anything from a tiny portion of a texel (magnification) to a large collection of texels (minification)

TexturePolygon Texture

Polygon

texel portiontexel grouppixel

pixel

Magnification Minification

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Filtering

gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MAG_FILTER, GL.GL_LINEAR);gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MIN_FILTER, GL.GL_LINEAR);

These lines define how to compute texel magnification and minification.

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Texture coordinates (in display method)

gl.glPushMatrix(); gl.glBindTexture(GL.GL_TEXTURE_2D, textures[filter1]); float text_crd = 8f; gl.glTranslatef(0.0f, 0.0f, -2.0f * square_x); gl.glScalef(((float)imageW[filter1])/((float)imageH[filter1]), 1.0f, 1.0f); gl.glBegin(GL.GL_QUADS); gl.glNormal3f(0.0f, 0.0f, 1.0f);

gl.glTexCoord2f(0.0f, 0.0f); gl.glVertex3f(-15.0f, -15.0f, 15.0f); gl.glTexCoord2f(text_crd, 0.0f); gl.glVertex3f(15.0f, -15.0f, 15.0f); gl.glTexCoord2f(text_crd,text_crd); gl.glVertex3f(15.0f, 15.0f, 15.0f); gl.glTexCoord2f(0.0f, text_crd); gl.glVertex3f(-15.0f, 15.0f, 15.0f);

gl.glEnd();gl.glPopMatrix();

Define a mapping that describes howthe texture is pasted on top of anypolygon

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Repeating textures

gl.glTexCoord2f(0.0f, 0.0f); gl.glVertex3f(-15.0f, -15.0f, 15.0f); gl.glTexCoord2f(text_crd, 0.0f); gl.glVertex3f(15.0f, -15.0f, 15.0f); gl.glTexCoord2f(text_crd,text_crd); gl.glVertex3f(15.0f, 15.0f, 15.0f); gl.glTexCoord2f(0.0f, text_crd); gl.glVertex3f(-15.0f, 15.0f, 15.0f);

When texture coordinates are larger than 1.0f, then thetexture can either be cropped, or repeated.

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Repeating textures

• If we use the following lines within the loadGLTextures method, then the texture will be repeated when its texture coordinates exceed 1.0f

• gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_WRAP_S, GL.GL_REPEAT);

• gl.glTexParameteri(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_WRAP_T, GL.GL_REPEAT);

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Repeating textures

In this case taking text_crd = 8.0f results in the polygon containingan 8x8 grid of the texture.

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Modulating textures

• Use the commandgl.glTexEnvf(GL.GL_TEXTURE_ENV, GL.GL_TEXTURE_ENV_MODE, GL.GL_MODULATE);

• Instead of gl.glTexEnvf(GL.GL_TEXTURE_ENV, GL.GL_TEXTURE_ENV_MODE, GL.GL_DECAL);

• Then the colour properties of the material we are texture mapping will modulate how the final image appear.

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Modulating textures

Here the material has purered diffuse material set.

Note since we have blending enabled we also have transparency