CSE (Notre Dame) Computer Graphics Lecture 16 A Simple Draw Image Example More Texture Mapping Simple OpenGL Image Library (SOIL) Transparency & Blending Masking with Bitmaps

Simple Example Load and Draw an Image Load image unsigned char *pixels; int height, width, channels; pixels = SOIL_load_image(“dome.jpg”, &width, &height, &channels, SOIL_LOAD_AUTO); Set Position in Raster glRasterPos2i(0,0); // center of raster Flip to see image upright glScalePixels(1.0,-1.0); // flip vertically here Draw the pixels glDrawPixels(width,height,pixels, GL_RGB, GL_UNSIGNED_BYTE, pixels); Free memory when done SOIL_free_image_data(pixels);

3 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Raster Position (reminder) The raster position is part of the OpenGL state Set by glRasterPos*() glRasterPos3f(x, y, z); The raster position is a geometric entity Passes through geometric pipeline Eventually yields a 2D position in screen coordinates This position in the frame buffer is where the next raster primitive is drawn This also LOCKS the raster (drawing) color which affects use of BIT MAPS as MASKS

4 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Bitmaps are MASKS OpenGL treats 1-bit pixels (bitmaps) differently from multi-bit pixels (pixelmaps) Bitmaps are masks that determine if the corresponding pixel in the frame buffer is drawn with the present raster color 0  color unchanged 1  color changed based on writing mode Bitmaps are useful for raster text GLUT font: GLUT_BIT_MAP_8_BY_13

Same as drawing color set by glColor*() Fixed by last call to glRasterPos*() Geometry drawn in blue Ones in bitmap use a drawing color of red 5 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Raster Color glColor3f(1.0, 0.0, 0.0); // set color RED glRasterPos3f(x, y, z); // fix as raster color glColor3f(0.0, 0.0, 1.0); // set color BLUE glBitmap(……. // set MASK glBegin(GL_LINES); glVertex3f(…..) // draw/create something

6 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Drawing Bitmaps (reminder) glBitmap(width, height, x0, y0, xi, yi, bitmap) first raster position. Set prior to calling glBitmap() second raster position offset from raster position increments in raster position after bitmap drawn

7 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Example: Checker Board GLubyte wb[2] = {0x00, 0xff}; GLubyte check[512]; int i, j; for(i=0; i<64; i++) for (j=0; j<8, j++) // 8 bytes (64 bits) per row check[i*8+j] = wb[(i/8+j)%2]; glBitmap( 64, 64, 0.0, 0.0, 0.0, 0.0, check);

Another Example BitMap My Own Lettering Create a bitmap of text in Paint program Save as Hi_128.jpg & Lo_128.jpg Load as image using SOIL Convert bytes to bits (and flip) if needed SOIL_load_image() will not do this for you Set color and raster position Draw the bitmap glDrawBitmap(height,width,0,0,0,0,bits);

My Lettering Original Bitmap Shown on Screen

My Lettering (display code) glColor3ub(255,0,0); // set color RED glRasterPos2i(-100,0); // lock color & go to -100,0 glBitmap(128,128,0.0,0.0,0.0,0.0,bitmapHI.pixels); glColor3ub(255,255,0); // set color YELLOW glRasterPos2i(0,40); // lock color & go to -100,0 glBitmap(128,128,0.0,0.0,0.0,0.0,bitmapLO.pixels);

See example (hilo) Shows the following Loading and converting image to bitmap Setting color and position Drawing bitmap Background and images show through the bitmap mask

12 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Pixel Maps OpenGL works with rectangular arrays of pixels called pixel maps or images Pixels are in one byte ( 8 bit) chunks Luminance (gray scale) images 1 byte/pixel RGB 3 bytes/pixel Three functions Draw pixels: processor memory to frame buffer Read pixels: frame buffer to processor memory Copy pixels: frame buffer to frame buffer

13 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 OpenGL Pixel Functions glReadPixels(x,y,width,height,format,type,myimage) start pixel in frame buffer size type of image type of pixels pointer to processor memory GLubyte myimage[512][512][3]; glReadPixels(0,0, 512, 512, GL_RGB, GL_UNSIGNED_BYTE, myimage); glDrawPixels(width,height,format,type,myimage) starts at current raster position

Look at Example (showPic) Shows the following... Loading images using SOIL_load_image() Flipping vertically Image is NOT scaled and is always anchored at given raster position

15 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Texture Mapping in OpenGL -- Basic Strategy Three steps to applying a texture 1. specify the texture read or generate image assign to texture enable texturing 2. assign texture coordinates to vertices Proper mapping function is left to application 3. specify texture parameters wrapping, filtering

16 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Texture Mapping s t x y z image geometry display

17 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Define a texture image from an array of texels (texture elements) in CPU memory Glubyte my_texels[512][512]; Define as any other pixel map Scanned image Generate by application code Enable texture mapping glEnable(GL_TEXTURE_2D) OpenGL supports 1-4 dimensional texture maps Specifying a Texture Image

18 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Define Image as a Texture glTexImage2D( target, level, components, w, h, border, format, type, texels ); target: type of texture, e.g. GL_TEXTURE_2D level: used for mipmapping (discussed later) components: elements per texel w, h: width and height of texels in pixels border: used for smoothing (discussed later) format and type: describe texels texels: pointer to texel array glTexImage2D(GL_TEXTURE_2D, 0, 3, 512, 512, 0, GL_RGB, GL_UNSIGNED_BYTE, my_texels);

19 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Converting A Texture Image OpenGL requires texture dimensions to be powers of 2 If dimensions of image are not powers of 2 gluScaleImage( format, w_in, h_in, type_in, *data_in, w_out, h_out, type_out, *data_out ); data_in is source image data_out is for destination image Image interpolated and filtered during scaling

20 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Based on parametric texture coordinates glTexCoord*() specified at each vertex s t 1, 1 0, 1 0, 01, 0 (s, t) = (0.2, 0.8) (0.4, 0.2) (0.8, 0.4) A BC a b c Texture SpaceObject Space Mapping a Texture

21 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Typical Code glBegin(GL_POLYGON); glColor3f(r0, g0, b0); //if no shading used glNormal3f(u0, v0, w0); // if shading used glTexCoord2f(s0, t0); glVertex3f(x0, y0, z0); glColor3f(r1, g1, b1); glNormal3f(u1, v1, w1); glTexCoord2f(s1, t1); glVertex3f(x1, y1, z1);. glEnd(); Note that we can use vertex arrays to increase efficiency

22 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Interpolation OpenGL uses interpolation to find proper texels from specified texture coordinates Can be distortions good selection of tex coordinates poor selection of tex coordinates texture stretched over trapezoid showing effects of bilinear interpolation

23 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Texture Parameters OpenGL has a variety of parameters that determine how texture is applied Wrapping parameters determine what happens if s and t are outside the (0,1) range Filter modes allow us to use area averaging instead of point samples Mipmapping allows us to use textures at multiple resolutions Environment parameters determine how texture mapping interacts with shading

24 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Wrapping Mode Clamping: if s,t > 1 use 1, if s,t <0 use 0 Wrapping: use s,t modulo 1 glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP ) glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT ) texture s t GL_CLAMP wrapping GL_REPEAT wrapping

25 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Magnification and Minification TexturePolygon MagnificationMinification PolygonTexture More than one texel can cover a pixel (minification) or more than one pixel can cover a texel (magnification) Can use point sampling (nearest texel) or linear filtering ( 2 x 2 filter) to obtain texture values

26 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Filter Modes Modes determined by glTexParameteri( target, type, mode ) glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER, GL_LINEAR); Note that linear filtering requires a border of an extra texel for filtering at edges (border = 1)

27 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Mipmapped Textures Mipmapping allows for prefiltered texture maps of decreasing resolutions Lessens interpolation errors for smaller textured objects Declare mipmap level during texture definition glTexImage2D( GL_TEXTURE_*D, level, … ) GLU mipmap builder routines will build all the textures from a given image gluBuild*DMipmaps( … )

28 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Example point sampling mipmapped point sampling mipmapped linear filtering linear filtering

29 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Texture Functions Controls how texture is applied glTexEnv{fi}[v]( GL_TEXTURE_ENV, prop, param ) GL_TEXTURE_ENV_MODE modes GL_MODULATE: modulates with computed shade GL_BLEND: blends with an environmental color GL_REPLACE: use only texture color GL(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE); Set blend color with GL_TEXTURE_ENV_COLOR

30 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Perspective Correction Hint Texture coordinate and color interpolation either linearly in screen space or using depth/perspective values (slower) Noticeable for polygons “on edge” glHint( GL_PERSPECTIVE_CORRECTION_HINT, hint ) where hint is one of GL_DONT_CARE GL_NICEST GL_FASTEST

31 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Generating Texture Coordinates OpenGL can generate texture coordinates automatically glTexGen{ifd}[v]() specify a plane generate texture coordinates based upon distance from the plane generation modes GL_OBJECT_LINEAR GL_EYE_LINEAR GL_SPHERE_MAP (used for environmental maps)

Using Textures and Solids gluQuadrics Provide texture coordinates Are easily wrapped with a texture glutSolid* Require you to establish your own texture coords Mappings are either plane based or eye based See Earth Globe example

33 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Texture Objects Texture is part of the OpenGL state If we have different textures for different objects, OpenGL will be moving large amounts data from processor memory to texture memory Recent versions of OpenGL have texture objects one image per texture object Texture memory can hold multiple texture objects

34 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Applying Textures II 1. specify textures in texture objects 2. set texture filter 3. set texture function 4. set texture wrap mode 5. set optional perspective correction hint 6. bind texture object 7. enable texturing 8. supply texture coordinates for vertex coordinates can also be generated

35 Angel: Interactive Computer Graphics 45E © Addison-Wesley 2009 Other Texture Features Environment Maps Start with image of environment through a wide angle lens Can be either a real scanned image or an image created in OpenGL Use this texture to generate a spherical map Use automatic texture coordinate generation Multitexturing Apply a sequence of textures through cascaded texture units

SOIL – Load image as texture GLuint loadTex (string filename) { // use SOIL to load a texture from an image file GLuint tex_ID = SOIL_load_OGL_texture ( filename.c_str(), // from this image file SOIL_LOAD_AUTO, // auto load image SOIL_CREATE_NEW_ID, // & create new texture (SOIL_FLAG_MIPMAPS | SOIL_FLAG_INVERT_Y | SOIL_FLAG_NTSC_SAFE_RGB | SOIL_FLAG_COMPRESS_TO_DXT) ); if( tex_ID > 0 ) { // if texture got created glEnable( GL_TEXTURE_2D ); // enable texture mapping glBindTexture( GL_TEXTURE_2D, tex_ID ); // & bind this one return tex_ID; // return texture id } return 0; // return 0 if no texture created }

SOIL usage Loading two images as textures GLuint tex01_2d, tex02_2d; tex01_2d = loadTex("dome1.jpg"); tex02_2d = loadTex("dome2.jpg"); Setting active texture (binding) glBindTexture(GL_TEXTURE_2D, tex02_2d);

SOIL usage Specifying texture coordinates void drawSquare (point3 a, point3 b, point3 c, point3 d) { glBegin(GL_POLYGON); glTexCoord2f(0.0,0.0); // lower left glVertex3fv(a); glTexCoord2f(1.0,0.0); // lower right glVertex3fv(b); glTexCoord2f(1.0,1.0); // upper right glVertex3fv(c); glTexCoord2f(0.0,1.0); // upper left glVertex3fv(d); glEnd(); }

39 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Blending & Compositing Objectives Learn to use the A component in RGBA color for Blending for translucent surfaces Compositing images Antialiasing

40 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Opacity and Transparency Opaque surfaces permit no light to pass through Transparent surfaces permit all light to pass Translucent surfaces pass some light translucency = 1 – opacity (  ) opaque surface  =1

41 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Physical Models Dealing with translucency in a physically correct manner is difficult due to the complexity of the internal interactions of light and matter Using a pipeline renderer

42 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Writing Model Use A component of RGBA (or RGB  ) color to store opacity During rendering we can expand our writing model to use RGBA values Color Buffer destination component blend destination blending factor source blending factor source component

43 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Blending Equation We can define source and destination blending factors for each RGBA component s = [s r, s g, s b, s  ] d = [d r, d g, d b, d  ] Suppose that the source and destination colors are b = [b r, b g, b b, b  ] c = [c r, c g, c b, c  ] Blend as c’ = [b r s r + c r d r, b g s g + c g d g, b b s b + c b d b, b  s  + c  d  ]

44 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 OpenGL Blending and Compositing Must enable blending and pick source and destination factors glEnable(GL_BLEND) glBlendFunc(source_factor, destination_factor) Only certain factors supported GL_ZERO, GL_ONE GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA See Redbook for complete list

45 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Example Suppose that we start with the opaque background color (R 0,G 0,B 0,1) This color becomes the initial destination color We now want to blend in a translucent polygon with color (R 1,G 1,B 1,  1 ) Select GL_SRC_ALPHA and GL_ONE_MINUS_SRC_ALPHA as the source and destination blending factors R ’ 1 =  1 R 1 +(1-  1 ) R 0, …… Note this formula is correct if polygon is either opaque or transparent

46 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Clamping and Accuracy All the components (RGBA) are clamped and stay in the range (0,1) However, in a typical system, RGBA values are only stored to 8 bits Can easily loose accuracy if we add many components together Example: add together n images Divide all color components by n to avoid clamping Blend with source factor = 1, destination factor = 1 But division by n loses bits

47 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Order Dependency Is this image correct? Probably not Polygons are rendered in the order they pass down the pipeline Blending functions are order dependent

48 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Opaque and Translucent Polygons Suppose that we have a group of polygons some of which are opaque and some translucent How do we use hidden-surface removal? Opaque polygons block all polygons behind them and affect the depth buffer Translucent polygons should not affect depth buffer Render with glDepthMask(GL_FALSE) which makes depth buffer read-only Sort polygons first to remove order dependency

49 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Fog We can composite with a fixed color and have the blending factors depend on depth Simulates a fog effect Blend source color C s and fog color C f by C s ’=f C s + (1-f) C f f is the fog factor Exponential Gaussian Linear (depth cueing)

50 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Fog Functions

51 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 OpenGL Fog Functions GLfloat fcolor[4] = {……}: glEnable(GL_FOG); glFogf(GL_FOG_MODE, GL_EXP); glFogf(GL_FOG_DENSITY, 0.5); glFogv(GL_FOG, fcolor);

52 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Line Aliasing Ideal raster line is one pixel wide All line segments, other than vertical and horizontal segments, partially cover pixels Simple algorithms color only whole pixels Lead to the “jaggies” or aliasing Similar issue for polygons

53 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Antialiasing Can try to color a pixel by adding a fraction of its color to the frame buffer Fraction depends on percentage of pixel covered by fragment Fraction depends on whether there is overlap no overlapoverlap

54 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Area Averaging Use average area  1 +  2 -  1  2 as blending factor

55 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 OpenGL Antialiasing Can enable separately for points, lines, or polygons glEnable(GL_POINT_SMOOTH); glEnable(GL_LINE_SMOOTH); glEnable(GL_POLYGON_SMOOTH); glEnable(GL_BLEND); glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

56 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Accumulation Buffer Compositing and blending are limited by resolution of the frame buffer Typically 8 bits per color component The accumulation buffer is a high resolution buffer (16 or more bits per component) that avoids this problem Write into it or read from it with a scale factor Slower than direct compositing into the frame buffer

57 Angel: Interactive Computer Graphics 5E © Addison-Wesley 2009 Applications Compositing Image Filtering (convolution) Whole scene antialiasing Motion effects

Thanks to … Many slides in this presentation have been used from Edward Angel’s PowerPoint lectures at UMN. In some cases slides have been modified to suit this class Any errors or omissions may be attributed to J H Stewman