CSCI 4620/8626. Computer Graphics Attributes of Graphics Primitives (Chapter 5)

Transcription

1 CSCI 4620/8626 Computer Graphics Attributes of Graphics Primitives (Chapter 5) Last update: Non-Fill Methods While polygons can be filled (with patterns or colors), they can also be displayed without fill (that is, as a wire frame consisting of edges only) or even without edges (points only). The manner in which polygons are processed is determined by calling glpolygonmode. 2 1

2 glpolygonmode(face, displaymode) The first parameter for this function is GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK. This indicates whether the call applies to the front, back, or both sides of a polygon face. The second parameter is GL_POINT, GL_LINE, or GL_FILL and indicates whether only points are to be displayed, lines are to be displayed, or the polygon is to be filled (with color, pattern, or texture). 3 Caveat If both sides of the polygon are visible (which is the default case), then only by using GL_FRONT_AND_BACK for face (the first parameter) will the call to glpolygonmode be effective. This is because the other side (the one not specified by the first parameter) will still be processed as usual, and it will be visible. 4 2

3 Stitching (1) To draw a polygon with one color for the fill, and a different color for the border, we might use code like this: glcolor3f( color for the filled polygon ); glbegin(gl_polygon); vertices glend(); glcolor3f( color for the border ); glpolygonmode(gl_front,gl_line); glbegin(gl_polygon); vertices glend(); Two different algorithms are used: one to fill the polygon, and another to construct the lines for the border. 5 Stitching (2) When a polygon that does not lie entirely in the XY plane is used, the graphics package must compute the screen coordinates of the corresponding 3D position. If the two algorithms yield different positions for a point on the border, it is possible that the fill color could appear to be in front of the border (which is drawn last), resulting in the border being invaded by the fill color. This effect is called stitching. 6 3

4 Stitching Example Stitching Assume edges of a polygon are being drawn between points that have differing Z-components (depth). The potential exists for the fill-line and the border line rasterization algorithms to yield conflicting values at some points at the border, resulting in the fill being closer to the viewer than the border, causing it to be displayed instead of the border. 7 glpolygonoffset (1) One technique for dealing with stitching involves shifting the depth values computed by the fill algorithm so they do not overlap with the edge depth values. In OpenGL this is accomplished by (a) enabling GL_POLYGON_OFFSET_FILL, and (b) invoking the glpolygonoffset function to specify two factors that control how the depth offsets should be computed. The two factors are used as follows: depthoffset = factor1 * maxslope + factor2 * constant Typical values for the two factors are between 0.75 and

5 glpolygonoffset (2) Instead of using positive values for the offsets and moving the fill area depth back in the polygon, we can enable GL_POLYGON_OFFSET_LINE (instead of GL_POLYGON_OFFSET_FILL) to cause the border of the polygon to have its depth altered. In this case the factors for glpolygonoffset should be negative, which moves the border line closer to the viewer. 9 Polygon Edges Each edge of a primitive generated from a polygon, triangle strip, triangle fan, separate triangle set, quadrilateral strip, or separate quadrilateral strip, is flagged as either boundary or non-boundary. When drawing a polygon from separate primitives (as would be the case if we split a concave polygon into triangles and displayed each triangle separately), some of the edges that are flagged as boundaries by default should not be so considered. 10 5

6 Example A convex wire-frame polygon drawn with three triangles. Note that each triangle edge is visible. Edges that should not be visible are shown here as dashed lines. 11 gledgeflag When drawing individual edges of a polygon using various primitives, we can control which lines should be treated as boundary edges. To indicate that the next vertex does not precede (begin) a boundary edge, call gledgeflag (GL_FALSE); Naturally, edge flags default to true. Remember to turn edge flags on again for edges that should be treated as boundaries: gledgeflag (GL_TRUE); Edge flags can be specified in an array parallel to a vertex array by enabling GL_EDGE_FLAG_ARRAY and specifying the pointer (and offset) to the array with gledgeflagpointer. 12 6

7 Polygon Face Specification Remember that polygons have two faces, front and back. The order in which a polygon s vertices are specified determines which face is considered the front. The default algorithm assumes counter-clockwise specification of the vertices. To change this, use glfrontface(gl_cw); Naturally, glfrontface(gl_ccw) can be used to switch back to the default ordering. 13 Why are Front and Back Important? Although it s not clear (yet), each face of a polygon is given a (potentially) different color (or texture, or pattern). Identifying which face receives a particular treatment requires some method of precisely identifying the front and back faces of the polygon. Individual faces can be culled or not displayed by (a) enabling GL_CULL_FACE and (b) calling glcullface(gl_front), glcullface(gl_back), or glcullface(gl_front_and_back). 14 7

8 Character Styles Text can be displayed in a variety of styles: As noted, different fonts (typefaces) can be used: Roman, Helvetica, Courier, Underlining can be varied: none, single, double, etc. Text can be normal, bold, italic, bold and italic. Text can be given outline or shadow treatment. Of course, text can be assigned colors. As expected, text facilities in graphics packages operate by setting certain pixels in the frame buffer. 15 Kern Each character in a proportionally-spaced font has a rectangular box called the character body. For most characters, this box completely encloses the character. Some characters, like f, j, l, and i especially in italic or slanted treatments may have portions of the character outside the character body. These portions are called kerns. 16 8

9 Kerning The purpose of kerning is to allow closer spacing of characters with kerns and other characters. For example, consider the rendering of the word filling in an italic font: 17 Ligatures Ligatures are special two-character combinations (treated as a single character by the graphics software) sometimes used in place of two individual characters. For example, ligatures might be used for character combinations like ff, fl, and fi. 18 9

10 Text Orientation and Alignment Text orientation is controlled by the direction of the character up vector, which indicates the direction characters should be placed relative to the baseline. This text is left justified. This text is right justified, so the right margin is even. This text is centered, so the text occupies an equal amount of space to the left and the right of the center of each line. Some fonts, like those for Hebrew and Arabic, are displayed right to left. V e r t i c a l T e x t 19 AntiAliasing Nyquist (1924) proved that an arbitrary periodic signal can be recreated from samples taken at twice the maximum frequency of the signal. For example, suppose we have a signal with a maximum frequency of 200 Hz (or cycles per second). Then we would need to take 400 samples (of the amplitude of the signal) per second if we wish to faithfully reproduce the signal. For example, telephone lines usually have a bandwidth (maximum frequency) of 3000 Hz. A T1 line carries 24 telephone signals, with 8-bits per sample and 8000 samples per second per telephone signal, with one additional bit for control purposes. So a T1 line s total bandwidth is ( ) 8000 = Mbps

11 So What Has This Got To Do With Graphics? The pixels that are illuminated to draw a line represent samples of the actual line. To draw a line more accurately (that is, without the jaggies ), we can employ any of several techniques. The most obvious technique is to increase the display resolution. This yields more samples of the line or curve per unit, yielding a more accurate rendering of the line. Of course there is a limit to the resolution for a display, but this is limited by how rapidly we can paint the frame buffer on the screen (since we usually must do this at least 60 times per second). 21 Supersampling Another approach is to assume we have more resolution, and then combine the intensity of the individual (virtual) pixels at the higher resolution to obtain a composite intensity for the pixels we can actually display. For example, assume we divide each pixel s rectangle into 9 smaller squares (that is, 3 3). Now compute which of these smaller squares would be illuminated (e.g. by Bresenham s algorithm), count them, and set the intensity of the single pixel to the appropriate value. With 3 intensity levels, we d map 1-3 to the lowest intensity, 4-6 to the next, and 7-9 to the highest intensity

12 Subpixel Weighting Since a pixel is displayed by aiming the electron beam at a specific location on the screen, we expect that the center of the pixel is illuminated more than the fringe areas. We can use this observation to assign weights to the subpixels in the lower-resolution cell corresponding to a single pixel. Pixels near the center are given higher weights, while those further away from the center are given lower weights. Additionally, weights from the subpixels in neighboring cells can be used to compute the intensity of the pixel in a given lower-resolution cell. 23 Filtering Techniques Even more accurate approaches, called filtering techniques, are based on the observation that the displayed pixel intensity doesn t jump from one intensity level to the next (as modeled by the subpixel weighting approach). Instead, the intensity decreases mathematically as we get further from the center of the pixel. Using this observation, we can integrate the contributions of the line or curve we re drawing to each pixel to determine its ideal intensity

13 Pixel Phasing Some display systems may be able to adjust the position within the pixel cell to which the electron beam is directed on a pixel-by-pixel basis. Using this technique, called pixel phasing, the electron beam can be shifted closer to the actual position of the line or curve being displayed. Some display systems may also be able to adjust the size of individual pixels. 25 Line Intensity Differences Recall that a horizontal or vertical line with N pixels will, without adjustment of pixel intensities, have a higher visible intensity that a diagonal line with N pixels. This is because in the diagonal line the pixels are spaced further apart (a factor of 2 at 45 degrees). To compensate, a graphics package may adjust the intensity of pixels in a line depending on the slope of the line

14 Antialiasing Area Boundaries The boundaries of filled areas can also be antialiased using similar techniques. An obvious approach is to supersample the pixels at the boundary of the area, and use the results to adjust the intensity of the boundary pixels. A slight modification of the midpoint line algorithm (see the text) can be used to yield a number between 0 and 1 that indicates how much of the cell occupied by a pixel overlaps the filled area; this can be used to set the pixel s intensity. 27 OpenGL Antialiasing Functions The OpenGL anitaliasing routines are activated by enabling GL_POINT_SMOOTH GL_LINE_SMOOTH GL_POLYGON_SMOOTH We must also enable color blending by enabling GL_BLEND. Then apply the color-blending function glblendfunc (GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA); 28 14

15 OpenGL Query Functions OpenGL state parameters can be retrieved using one of several functions: glgetbooleanv ( ) glgetintegerv ( ) glgetfloatv ( ) glgetdoublev ( ) Each of these functions takes two arguments: the name of a parameter to be retrieved, and a pointer to an array of the appropriate type which will receive the parameter value(s). 29 Example To get the current color (last specified with glcolor*) we might use: float fa[4]; glgetfloatv(gl_current_color,&fa); To determine if GL_BLEND is enabled, we might use: boolean isblendset; glgetbooleanv(gl_blend,&isblendset); or alternately (for booleans only) we could use: isblendset = glisenabled(gl_blend); 30 15

16 OpenGL Attribute Groups Attributes and other state parameters are arranged in groups (e.g. the point attribute group, the line attribute group, and the polygon attribute group). A complete set of attributes can be saved by calling: glpushattrib (groupname); where groupname is something like GL_POINT_BIT or GL_CURRENT_BIT (for color attributes). To restore the attributes, call: glpopattrib ( ); Multiple attribute groups can be selected in a single call to glpushattrib by bitwise or ing the group names