Scan Line Polygon Fill Algorithm in Computer Graphics MCQ Questions and Answers
1. The Scan-Line Polygon Fill Algorithm is primarily used to
A) Draw outlines of polygons
B) Fill the interior of polygons efficiently by processing one scan line at a time
C) Draw circles
D) Render text
Answer: B
2. The main data structures used in the Scan-Line Fill Algorithm are
A) Frame buffer and pixel list
B) Edge Table (ET) and Active Edge Table (AET)
C) Flood table and boundary queue
D) Polygon stack and vertex map
Answer: B
3. The Scan-Line Fill Algorithm works by processing pixels along
A) Horizontal lines (scan lines)
B) Vertical lines
C) Diagonal lines
D) Circular arcs
Answer: A
4. The intersection points of polygon edges with a scan line are used to
A) Compute centroids
B) Determine start and end points of fill spans
C) Store vertex normals
D) Clip lines
Answer: B
5. The Edge Table (ET) stores
A) Polygon vertices only
B) Edges sorted by their minimum y-coordinate
C) Pixel colors
D) Bounding boxes
Answer: B
6. The Active Edge Table (AET) contains edges
A) Not used
B) From all polygons
C) That intersect the current scan line
D) That are vertical only
Answer: C
7. The ET is sorted by
A) Edge slope
B) Minimum y-coordinate (Ymin)
C) Edge length
D) Polygon ID
Answer: B
8. The AET is sorted by
A) Polygon ID
B) Edge length
C) x-intercept values (current x of intersections)
D) Color intensity
Answer: C
9. Each edge in the Edge Table stores
A) Color of the polygon
B) Polygon name
C) Ymax, Xmin, and inverse slope (1/m)
D) Only slope
Answer: C
10. The polygon fill process begins at the
A) Minimum y-coordinate (Ymin) of the polygon
B) Maximum y-coordinate
C) Polygon centroid
D) Screen origin
Answer: A
11. In Scan-Line Filling, edges are added to the Active Edge Table when
A) The current scan line equals the edge’s Ymin
B) The scan line equals Ymax
C) The polygon is closed
D) The edge is horizontal
Answer: A
12. Edges are removed from the AET when
A) x-intercept is zero
B) The current scan line reaches the edge’s Ymax
C) The polygon is completed
D) The slope becomes infinite
Answer: B
13. Horizontal edges in a polygon are
A) Always filled directly
B) Ignored for intersection computations
C) Stored twice
D) Treated as vertical
Answer: B
14. The basic idea of Scan-Line Filling is based on the
A) Point-in-polygon rule
B) Parity (odd-even) rule
C) Flood propagation
D) Region growing
Answer: B
15. According to the odd-even rule, pixels are filled when
A) They lie outside the polygon
B) The number of edge crossings before them is odd
C) The number of edge crossings is even
D) The y-value is maximum
Answer: B
16. The Scan-Line Fill Algorithm can handle
A) Circles only
B) Any convex or concave polygon (non-self-intersecting)
C) Only convex polygons
D) Polylines only
Answer: B
17. The algorithm assumes polygons are
A) 3D
B) Open
C) Closed and planar
D) Unbounded
Answer: C
18. For efficient implementation, edges in ET are stored in
A) One list
B) Buckets corresponding to each scan line
C) Pixel grid
D) Color array
Answer: B
19. The term “scan line” refers to
A) A horizontal line across the display that defines one row of pixels
B) A vertical line
C) A diagonal traversal
D) A recursive path
Answer: A
20. The slope inverse (1/m) is used to
A) Compute polygon area
B) Increment x-intercepts between scan lines
C) Find Ymax
D) Identify vertex type
Answer: B
21. When moving from one scan line to the next, x-intercepts in AET are updated by
A) Adding slope
B) Adding the inverse slope (1/m) to current x
C) Subtracting y
D) Doubling x
Answer: B
22. To fill between intersections, we
A) Fill alternate pixels
B) Pair up intersection points and fill between them
C) Skip first half
D) Fill only vertices
Answer: B
23. The Active Edge Table (AET) is updated every time
A) The polygon changes
B) The scan line moves to the next y value
C) A new vertex appears
D) A pixel is filled
Answer: B
24. The Scan-Line Algorithm’s major efficiency comes from
A) Random filling
B) Incremental updates of edge intersections instead of recomputation
C) Using recursion
D) Matrix operations
Answer: B
25. The scan-line approach fills polygons in
A) Random order
B) Top to bottom order (increasing y)
C) Bottom to top
D) Diagonal order
Answer: B
26. The odd-even parity test ensures
A) The outline is drawn twice
B) Only the inside region of polygon is filled
C) Boundary pixels are skipped
D) Flooding is avoided
Answer: B
27. The inverse slope (1/m) of a vertical edge is
A) Zero
B) Infinite
C) One
D) Negative
Answer: A
28. The inverse slope (1/m) of a horizontal edge is
A) Zero
B) Infinite (undefined)
C) Positive
D) Negative
Answer: B
29. A vertex connecting two edges with the same Ymax is
A) Counted once to avoid double filling
B) Counted twice
C) Ignored
D) Split into two vertices
Answer: A
30. Which of the following algorithms is a boundary-based filling algorithm?
A) Scan-line
B) Boundary-fill algorithm
C) Polygon clipping
D) DDA line algorithm
Answer: B
31. The Scan-Line Fill Algorithm requires edges to be sorted primarily by
A) Ymin
B) Xmax
C) Ymax
D) Edge ID
Answer: A
32. In AET, edges are sorted by
A) Color
B) Current x-intersection value
C) Slope sign
D) Length
Answer: B
33. Filling stops when
A) Current scan line exceeds maximum Ymax of polygon
B) Current x < 0
C) Parity rule fails
D) Polygon edges overlap
Answer: A
34. When a scan line crosses exactly through a polygon vertex, care must be taken to
A) Skip it
B) Avoid counting it twice
C) Count it always
D) Round it up
Answer: B
35. The scan-line polygon fill algorithm works well for polygons that are
A) Open
B) Simple (non-self-intersecting)
C) 3D curved
D) Overlapping
Answer: B
36. The Scan-Line Fill Algorithm is more efficient than flood fill because
A) It processes one line at a time rather than recursion per pixel
B) It uses recursion
C) It ignores edges
D) It fills per polygon vertex
Answer: A
37. The scan-line algorithm complexity is proportional to
A) Number of polygons squared
B) Number of scan lines intersecting polygon edges
C) Pixel count of the frame buffer
D) Total color depth
Answer: B
38. The main purpose of maintaining the AET is
A) To store the polygon color
B) To track currently active edges for filling between intersections
C) To sort by vertex
D) To draw outlines
Answer: B
39. After computing intersections in AET, we
A) Remove all edges
B) Pair them up and fill pixels between each pair
C) Sort by slope again
D) Restart algorithm
Answer: B
40. The scan-line algorithm is implemented in which coordinate space?
A) Normalized device coordinates
B) Screen coordinates (pixel grid)
C) World coordinates
D) Homogeneous coordinates
Answer: B
41. The polygon edges are assumed to be
A) Curved
B) Straight line segments
C) Parabolic
D) Circular
Answer: B
42. The ET and AET are typically represented as
A) Linked lists or arrays of edge records
B) Binary trees
C) Stacks
D) Queues only
Answer: A
43. The scan-line polygon fill algorithm can fail for
A) Convex polygons
B) Self-intersecting polygons
C) Triangles
D) Quadrilaterals
Answer: B
44. When two adjacent edges share a vertex with the same Ymax, the vertex is
A) Always filled
B) Counted once (to avoid double-counting)
C) Ignored completely
D) Filled twice
Answer: B
45. The scan-line algorithm can handle polygons with
A) 2 vertices
B) Any number of vertices (≥3)
C) 4 only
D) Infinite edges
Answer: B
46. The main disadvantage of scan-line fill is that
A) It cannot easily handle self-intersecting or complex polygons
B) It uses recursion
C) It fills too slowly
D) It only fills circles
Answer: A
47. The scan-line polygon fill algorithm is used in which graphics pipeline stage?
A) Geometry transformation
B) Viewing transformation
C) Rasterization (scan conversion)
D) Clipping
Answer: C
48. The edge records in ET and AET are updated for
A) Every vertex
B) Every scan line
C) Only at polygon start
D) Only at polygon end
Answer: B
49. The scan-line algorithm requires sorting at two stages — which are they?
A) Vertex list and color
B) Ymin sorting in ET and x-intersection sorting in AET
C) Edge list only
D) Polygon IDs
Answer: B
50. The term “Active Edge Table” means
A) All polygon edges
B) Non-horizontal edges
C) Edges currently intersected by the current scan line
D) Vertical edges only
Answer: C
51. The Edge Table is cleared and rebuilt
A) After each pixel fill
B) After each vertex
C) Once before polygon filling begins
D) After all polygons are filled
Answer: C
52. The main loop of the scan-line algorithm iterates over
A) All edges
B) All scan lines between Ymin and Ymax
C) All vertices
D) All colors
Answer: B
53. To reduce computational cost, the algorithm updates intersections using
A) Trigonometric functions
B) Incremental addition of the inverse slope (1/m)
C) Random selection
D) Recalculation of equations
Answer: B
54. When filling pixels between intersection pairs, the algorithm must
A) Round intersection x-values to nearest integers before filling
B) Skip first pair
C) Round to Ymax only
D) Fill diagonally
Answer: A
55. The primary reason horizontal edges are ignored in ET is that
A) They do not contribute to parity changes along scan lines
B) They have infinite slope
C) They are difficult to compute
D) They cause recursion
Answer: A
56. For vertical edges, the inverse slope value (1/m) is
A) Infinite
B) Zero
C) Undefined
D) Negative
Answer: B
57. The ET is typically implemented as
A) A stack
B) A hash map
C) A list of buckets indexed by scan line (Ymin)
D) A tree
Answer: C
58. The AET changes dynamically as
A) The slope changes
B) The polygon color changes
C) Scan lines progress from top to bottom
D) Ymax remains fixed
Answer: C
59. Which of the following steps occurs first in Scan-Line Polygon Fill Algorithm?
A) Filling interior pixels
B) Building Edge Table (ET)
C) Removing inactive edges
D) Updating AET
Answer: B
60. When a scan line moves up, the new x-intersections are found by
A) Redrawing edges
B) Adding 1/m to previous x values for each active edge
C) Recomputing slopes
D) Multiplying by m
Answer: B
61. The fill between intersection pairs is done in which direction?
A) Left to right
B) Top to bottom
C) Diagonal
D) Circular
Answer: A
62. The scan-line algorithm ensures pixels are filled continuously between
A) Alternate scan lines
B) Each consecutive pair of intersections in sorted order
C) Even-numbered scan lines only
D) Random intersections
Answer: B
63. The scan-line algorithm is more memory efficient than flood fill because
A) It stores only colors
B) It fills line-by-line rather than storing all pixel states recursively
C) It uses no loops
D) It fills at once
Answer: B
64. If a polygon is not simple (self-intersecting), the scan-line algorithm may
A) Fill correctly
B) Produce incorrect fill regions due to ambiguous parity
C) Ignore edges
D) Fill only outer boundary
Answer: B
65. For concave polygons, the scan-line algorithm
A) Fails
B) Works correctly with parity control
C) Requires clipping
D) Uses recursion
Answer: B
66. The polygon interior is filled according to the
A) Even-even rule
B) Odd-even parity rule
C) Flood-fill rule
D) Convex rule
Answer: B
67. The term “active edge” refers to
A) Edge used for clipping
B) Edge intersecting current scan line
C) Edge with zero slope
D) Edge above Ymax
Answer: B
68. The process of filling polygons using scan-line algorithm is a part of
A) Rasterization (Scan Conversion)
B) Modeling
C) Shading
D) Projection
Answer: A
69. Each polygon edge is represented by its
A) Endpoints only
B) Ymax, current X, and slope inverse (1/m)
C) Area
D) Centroid
Answer: B
70. The Edge Table is built from
A) The list of polygon vertices
B) Pixel buffer
C) Frame buffer
D) Color map
Answer: A
71. When multiple edges intersect the same scan line, the intersections must be
A) Ignored
B) Filled randomly
C) Sorted in increasing x order before filling
D) Grouped by slope
Answer: C
72. If the number of intersections in AET is even, the algorithm fills
A) None
B) Between every pair of intersections
C) Every alternate pixel
D) Only first half
Answer: B
73. The scan-line polygon fill algorithm guarantees
A) 100% accuracy for any shape
B) Correct filling for simple polygons
C) Correct filling for 3D models
D) Partial fill only
Answer: B
74. A horizontal scan line will always intersect an even number of polygon edges because
A) Edges repeat
B) Each polygon edge enters and leaves the polygon boundary
C) The polygon is open
D) X values reset
Answer: B
75. The scan-line algorithm can fill
A) Only triangles
B) Any polygon (triangle, quad, pentagon, etc.)
C) Only rectangles
D) Only convex polygons
Answer: B
76. The slope inverse (1/m) helps compute
A) Y coordinates
B) Incremental x-intersections
C) Colors
D) Normal vectors
Answer: B
77. The ET removes which type of edges during initialization?
A) All edges
B) Horizontal edges (dy = 0)
C) Vertical edges
D) Diagonal edges
Answer: B
78. The polygon boundary edges define transitions between
A) Bright and dark pixels
B) Inside and outside regions of the polygon
C) Upper and lower halves
D) Scan lines
Answer: B
79. The parity (odd-even) logic ensures that
A) Every pixel is filled twice
B) Pixels alternate between inside and outside states
C) Only vertical lines are drawn
D) Edges are skipped
Answer: B
80. In practice, how often is the AET updated per frame?
A) Once
B) Once per scan line
C) Once per vertex
D) Never
Answer: B
81. The polygon’s Ymax value defines
A) The lowest pixel to fill
B) The last scan line to process
C) The number of intersections
D) The slope direction
Answer: B
82. The efficiency of scan-line algorithm mainly depends on
A) Sorting and maintaining active edge lists efficiently
B) Random filling
C) Frame buffer size
D) Hardware color depth
Answer: A
83. For convex polygons, the scan-line algorithm
A) Requires clipping
B) Produces exactly one pair of intersections per scan line
C) Needs flood-fill correction
D) Uses multiple AETs
Answer: B
84. For concave polygons, the scan-line algorithm
A) Fails completely
B) Produces multiple pairs of intersections per scan line
C) Fills nothing
D) Needs recursion
Answer: B
85. When the scan line moves from Y to Y+1, the AET
A) Stays same
B) Updates each edge’s x by adding inverse slope (1/m)
C) Removes all edges
D) Adds all edges again
Answer: B
86. The scan-line fill algorithm complexity is roughly
A) O(n²)
B) O(n + k) where n = edges, k = scan lines
C) O(log n)
D) O(k²)
Answer: B
87. The algorithm ensures pixel-level accuracy by
A) Rounding x-intersections before drawing spans
B) Using floating values
C) Using anti-aliasing
D) Ignoring fractional parts
Answer: A
88. The scan-line algorithm does not use
A) Polygon vertices
B) Recursive region growing
C) Edge intersections
D) Edge sorting
Answer: B
89. The ET and AET together ensure that
A) Only convex polygons are filled
B) Only inside regions are colored, edges excluded
C) Flood fill occurs
D) Clipping is done
Answer: B
90. The term “edge coherence” in scan-line algorithm refers to
A) Vertex repetition
B) Reuse of edge intersection data between adjacent scan lines
C) Edge duplication
D) Random access
Answer: B
91. When two polygons share a boundary, scan-line filling can cause
A) Double fill on shared edges
B) Overdraw or gaps if parity not handled correctly
C) Perfect separation
D) Random colors
Answer: B
92. The scan-line polygon fill algorithm is most similar in principle to
A) Flood-fill algorithm
B) Line drawing algorithms (incremental computation)
C) Recursive functions
D) 3D shading
Answer: B
93. The intersection x values are recomputed each time
A) From scratch
B) Incrementally using last x and 1/m
C) Using trigonometry
D) From vertex table
Answer: B
94. The scan-line fill method assumes the polygon vertices are given in
A) Clockwise or counter-clockwise order
B) Either clockwise or counter-clockwise consistently
C) Random order
D) Sorted by color
Answer: B
95. The scan-line polygon fill algorithm differs from the flood-fill algorithm because
A) Scan-line uses edge coherence; flood-fill uses pixel adjacency
B) Both are recursive
C) Flood-fill is faster
D) Scan-line needs recursion
Answer: A
96. AET entries are removed when
A) Polygon closes
B) Current scan line equals Ymax of the edge
C) Ymin reached
D) x=0
Answer: B
97. In hardware graphics pipelines, the scan-line concept is fundamental to
A) Scene modeling
B) Rasterization and span generation
C) Z-buffer initialization
D) Illumination models
Answer: B
98. When multiple polygons overlap, the scan-line algorithm can be extended to support
A) Vertex merging
B) Z-buffer or depth sorting per scan line
C) Flood-fill recursion
D) Polygon subdivision
Answer: B
99. The scan-line fill algorithm is unsuitable for
A) 2D polygons
B) Self-intersecting or complex polygons without preprocessing
C) Convex polygons
D) Simple concave polygons
Answer: B
100. The final step after processing all scan lines is to
A) Rebuild AET
B) Clear ET and AET to free memory
C) Reverse fill order
D) Recompute slopes
Answer: B