Object space algorithm: Back-Face removal. · No faces on the back of the object are displayed. · In general – about half of objects faces are back faces. Computer Graphics Lecture 8 Hidden Surface Removal Taku Komura 1 1 Or if N.V > 0 we are viewing the back face so polygon is obscured. . Tree construction and traversal (object-space ordering algorithm – good for relatively few static. Occluded surfaces: hidden surface removal (visibility). ▫ Back faces: back face culling. ▫ Faces OpenGL): Z-buffer (or depth buffer) algorithm. ▫ Requires lots .

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If all the tests fail, then we split either P or Q using the plane of the other.

### Computer Graphics

There bacface two other matrices obtained by replacing the coefficients in each column by the constants c 1 and c 2 respectively.

Recursively divide each side until each node contains only 1 polygon. The depth and opacity values are used to determine the final color of a pixel. We do not draw polygons facing the other direction Test z component of surface normals. Ray casting is easy to implement for polygonal models because the only calculation required is the intersection of a line with a plane. Even if we use another algorithm for visible surface determination, the back-face cull is a good pre-processing step.

Pixel positions across each scan-line are processed from left to right.

## Back Face Removal

Removwl any polygon lying on both sides. However, we have node C at back of node Bso node C will go to the right side of node B. The intensity field then stores the RGB components of the surface color at that point and the percent of pixel coverage.

Start at root polygon. The intensity field then stores a pointer to a linked list of surface data. Choose polygon arbitrarily 3 4 1 2 5 5a 5b back front Choose polygon arbitrarily Divide scene into front relative to normal and back half-spaces.

When projecting surfaces, check for intersections.

As shown in figure cwe will first process the node B. For a 3 x remocal matrix of coefficients the determinant may be expressed as either: It is applied very efficiently on surfaces of polygon. For a 3 x 3 matrix of coefficients the determinant may be expressed as either:.

A simple object space algorithm is Back-Face removal or back face cull where no faces on the back of the object are displayed. Back face culling View-frustrum culling Detail culling Occlusion culling.

You only need to perform depth calculations when multiple surfaces have their flags turned on at a certain scan-line position. Binary space partitioning is used to calculate visibility. The area-subdivision method takes advantage by locating those view areas that represent part of a single surface.

Cells or rooms Edges: Cells or rooms F Model scene as a graph: This method has a depth information for only single scan-line. The scan conversion of the polygon surfaces is performed in image space. Add subsequent edges based on whether they are inside or outside. Reject intersections that lie outside the polygon. Their determinants are det A x and det A y respectively. Feedback Privacy Policy Feedback. Partition all the other polygons in the scene to the back left subtree or the front right backfade.

## Visible Surface Detection

Same as with normal vector. The z-coordinates are usually normalized to the range [0, 1]. If negative — cull, since normal points away from viewer. To facilitate the search for surfaces crossing a given scan-line, an active list of edges is formed. Also, for color shading we must compute the normal for all of the polygons anyway. Similar methods can be used in packages that employ a left-handed viewing system. Download ppt “Hidden Surface Removal”. The key data structure in the A-buffer is the accumulation buffer.

Can be used to generate correct visibility for arbitrary views. Standard equation of a plane in 3 space: It is just the opposite condition for a right handed viewing system. The sides will be invisible for an orthographic projection, but not for a perspective projection. Point 2 – 1. Need to decide the order to draw — far objects first 7 7.

Spatial Sorting Chung Ji Hye. For the scene below, the visible surfaces are different for orthographic or perspective projection. In general, if V is a vector in the viewing direction from the eye or “camera” position, then this polygon is a back face if.

Initialise frame buffer to background colour.

When object description is converted to projection coordinates x,y,z each pixel position on the view plane is specified by x and y coordinate and z value gives the depth information. When we view a picture containing non-transparent objects and surfaces, then we cannot see those objects from view which are behind from objects closer to eye.

### Explain Back – Face Removal Algorithm.

Lecture 9 Choose polygon arbitrarily 3 4 1 2 5 5a 5b back front Choose polygon arbitrarily Divide scene into front relative to normal and back half-spaces.

Recursively calculate x-coordinate values removak left edge of polygon 3. Recursively descend the tree. Choose polygon arbitrarily 5 5a 5b 2 Choose polygon arbitrarily Divide scene into front relative to normal and back half-spaces.