Data Set Generation for Rectangular Placement Problems


 Marcus Jacobs
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1 Data Set Generation for Rectangular Placeent Probles Christine L. Valenzuela (Muford) Pearl Y. Wang School of Coputer Science & Inforatics Departent of Coputer Science MS 4A5 Cardiff University George Mason University 5 The Parade Fairfax, VA Cardiff CF24 3AA U.S.A United Kingdo Published in 21 Abstract This paper describes a recursive process for generating data sets of rigid rectangles that can be placed into rectangular regions with zero waste. The generation procedure can be odified to guarantee that the aspect and area ratios of the rectangles in the generated data sets satisfy userspecified paraeters. This recursive process can thus be eployed to create a variety of data sets that can be used to evaluate the efficiency and scalability of rectangular cutting and packing algoriths. Keywords: cutting, packing, rectangular placeent 1 Introduction Many rectangular cutting and packing algoriths have appeared in the literature in the last three decades. These solution procedures have often been evaluated by using a variety of test data sets. For cutting probles, several popular benchark data sets [1, 4] have been utilized for solving constrained and unconstrained probles. The nuber of rectangles appearing in these data sets typically ranges fro the tens to hundreds of pieces, and the optial solution for each data set ay or ay not be known. Siilarly, any rectangular bin packing heuristics have relied on data sets for deonstrating their effectiveness. For exaple, bin packing data sets have been used in [2, 5, 6, 7]; these contain at ost hundreds of rectangles and several share the property that an optial solution for the data set is known. Other bin packing data sets (e.g. [3]) contain rectangles This work was partially supported by the NASA Goddard Space Flight Center (NAG ) whose heights and widths have been randoly generated and whose optial solution has waste that is unknown but can be bounded below by suing the areas of the rectangles to be packed. The rectangles appearing in the published data sets for both cutting and packing display a range of properties. Soe data sets contain rectangles that appear to be nearly square. Others contain rectangles that are ostly tall and thin or short and fat, while other data sets contain both types of rectangles. Additionally, any contain rectangles that are either very large or very sall in area, while others contain rectangles that all have siilar area. This variety of rectangle sizes and areas enables researchers to deterine if their proposed algoriths are biased towards any particular types of data. However, the sall sizes of these data sets do not enable a deterination to be ade of whether cutting and packing algoriths will scale to large proble sizes, and often, the quality of the solution can only be approxiated since the optial solution is not known. Due to this sparsity of large benchark data sets for the proble of cutting or packing rectangles into rectangular regions, we have developed a recursive routine for generating data sets of rigid rectangles which can be packed into a zerowaste rectangular region. More iportantly, this procedure perits the user to specify a range of variation in the diensions and areas of the generated rectangles. Section 2 describes the basic approach used by the data set generation algorith which, siply stated, recursively cuts a user specified input rectangle into saller subrectangles. This ethod can be odified so that the heighttowidth ratios of the resulting rectangles is controlled as proven in section 3. Further, section 4 illustrates how the iposition of restrictions governing the choice of which subrectangles can be recursively sliced will yield data sets where 1
2 H Figure 1: A zero waste packing W the axiutoiniu area of resulting rectangles can be liited. Next, we show how these procedures can be cobined to produce data sets containing rectangles whose heighttowidth and area ratios are controlled as described in section 5. Section 6 characterizes soe saple data sets that were generated by these procedures. A description of how these algoriths and data sets can be used is presented in section 7. Finally, areas of ongoing research are described in section 8. 2 Generating unconstrained rectangles A basic procedure can be forulated which generates data sets containing n rectangles with no restrictions being placed on the relative height h i and width w i of each rectangle. Recursive slicing of a large (stock) rectangle is perfored by applying vertical or horizontal cuts with equal probability. At each step, slicing positions are chosen with unifor probability. By reversing the slicing process, the rectangles can be reassebled into a zerowaste packing. In this anner, rectangles such as those shown in Figure 1 can easily be generated. At the start of the data set generation process, the user is asked to input the diensions of the stock rectangle and also the precise nuber of rectangular pieces desired. The input paraeters consist of n, the nuber of desired rectangles, and H and W, the height and width of the stock rectangle being cut. Algorith 1 describes the basic technique for generating a data set. It is clear that this Θ(n) process generates a set of rectangles with realvalued diensions that can be reassebled into a stock rectangle of size H W with zero waste. Algorith 1 Generating Unconstrained Rectangles Input: n, H, and W while n rectangles have not yet been generated do choose a rectangle R randoly choose a vertical or horizontal slicing direction randoly choose a rando position to cut R in the chosen direction perfor the cut, generating two subrectangles replace R in the list with the two new subrectangles end while 3 Generating rectangles satisfying the aspect ratio constraint The aspect ratio of a rectangle with height h i and width w i is defined to be the ratio hi w i. Algorith 1 can be odified to produce a set of rectangles whose aspect ratios fall within a userspecified range of [1/ρ, ρ] where ρ 2. To ensure this, additional constraints ust be satisfied during the generation process. First, the input stock rectangle ust satisfy an initial condition based on the value of the ρ paraeter. Next, positions at which successive rando cutting of the initial stock piece and the interediate subrectangles ust be restricted. To prove that the final set of generated rectangles have the desired aspect ratio, the following theores are noted. For convenience, a rectangle R of height H and width W is said to have aspect ratio ρ if 1/ρ H/W ρ. 3.1 Matheatical conditions for aspect ratio cutting Lea 1 Let R be a rectangle having height H and width W that is sliced vertically into two subrectangles R 1 and R 2. If W > 2ρH for a given ρ, then R 1 and R 2 cannot both have aspect ratio ρ. Proof. Suppose R is sliced at position x to for two subrectangles R 1 and R 2 as shown in Figure 2. Let W > 2ρH and assue that R 1 has aspect ratio ρ. It follows that x < W/2 and W x > W/2 because = H/x 1/ρ = x ρh < ρw/(2ρ) = W/2 = x < W/2 = W x > W/2 2
3 H fro which R 1 R 2 x Wx Figure 2: Vertical slicing of an H W rectangle = H/(W x) < H/(W/2) = H/(W x) < 2H/W < 2H/(2ρH) = 1/ρ = H/(W x) < 1/ρ which iplies that R 2 does not have aspect ratio ρ. A siilar arguent can be used to establish that R 1 will not have aspect ratio ρ if W > 2ρH and R 2 has aspect ratio ρ. Lea 2 Let R be a rectangle having height H and width W that is sliced vertically into two subrectangles R 1 and R 2. If W < 2H/ρ for a given ρ, then R 1 and R 2 cannot both have aspect ratio ρ. Proof. Let W < 2H/ρ and assue that R 1 has aspect ratio ρ. Then it can be shown that x > W/2 and W x < W/2: so that = H/x ρ = H ρx = ρw/2 < H ρx = W/2 < x = W x < W/2 = H/(W x) 2H/W > 2H/(2H/ρ) = ρ = H/(W x) > ρ which iplies that R 2 does not have aspect ratio ρ. Siilarly, if W < 2H/ρ and R 2 has aspect ratio ρ, then it can be shown that R 1 does not have aspect ratio ρ. Lea 3 Let R be a rectangle having height H and width W that is sliced horizontally into two subrectangles R 1 and R 2. If H > 2ρW or if H < 2W/ρ for a given ρ value, then R 1 and R 2 cannot both have aspect ratio ρ. Proof. This lea can be established by first observing that 1/ρ H/W ρ iplies that 1/ρ W/H ρ and then applying the sae arguents used in the proofs of Leas 1 and 2 with H and W interchanged. The proof techniques used for the above leas provide clues for obtaining soe conditions which guarantee that a rectangle can be vertically (or horizontally) sliced into two subrectangles, each having an aspect ratio of ρ. For exaple, note that Leas 1 and 2 have indicated that if there is to be a chance that a rectangle can be cut vertically into two acceptable subrectangles, then it should probably have height H and width W satisfying 2H ρ W 2ρH. We now show that this ust be the case. Theore 1 A rectangle with height H and width W can be sliced vertically into two subrectangles with aspect ratio ρ if W satisfies 2H ρ W 2ρH. Proof. If the height H and width W of the rectangle to be cut satisfy any of the conditions in Leas 1, 2 or 3, then the two resulting subrectangles cannot both have aspect ratio ρ. Thus, suppose that the width W of a rectangle satisfies 2H ρ W 2ρH. (Note that this is equivalent to H ρ W/2 ρh.) Now observe that any vertical cut at position x definitely dictates that H/ρ x ρh: if not, then x < H/ρ iplies that H/x > ρ and x > ρh iplies that H/x < 1/ρ. These conditions cause the left subrectangle, R 1, which is fored by the cut at x, to lack the desired aspect ratio property. However, it is not clear that cutting the rectangle at position x where H/ρ x ρh will guarantee that R 1 has aspect ratio ρ. To verify this, first assue that H/ρ x W/2. If this is true, then H/x 2H/W 1/ρ and H/x H/(H/ρ) = ρ, and so R 1 will have aspect ratio ρ. For a cut position x, W/2 < x ρh, H/x < H/(W/2) ρ so H/x < ρ. Also H/x H/ρH = 1/ρ. Thus the resulting R 1 will again have aspect ratio ρ. Cutting the rectangle at position x where H/ρ x ρh does not, however, necessarily guarantee that the subrectangle created to the right of the x cut will also have aspect ratio ρ. In order for this to be true, the x cut will have to be liited to the range W ρh x W H/ρ as seen in Figure 3. Arguents siilar to those used above can be applied to prove this restriction, since cutting at these positions is syetric with respect to the idpoint W/2 of the rectangle width. It follows that in order to cut the rectangle so that both subrectangles have aspect ratio ρ, the vertical cut ust be 3
4 W  ρ H R 2 cuts W  H/ ρ Theore 3 If the height H and width W of a rectangle satisfies the relation 2H/ρ W Hρ/2 where ρ 2, then it can be cut horizontally and vertically to yield two subrectangles with aspect ratio ρ. H Proof. Using the inequalities 2H/ρ W Hρ/2 and 1/2 < 2, it follows that H/(2ρ) 2H/ρ W Hρ/2 2ρH. H/ρ R 1 cuts ρ H W < 2ρ H Figure 3: Legal vertical slicing positions of an H W rectangle restricted to positions ax(h/ρ, W Hρ) x in(hρ, W H/ρ). Note that if W = 2H/ρ or W = 2ρH, it can easily be shown that only one vertical cut position of x = W/2 can be used which will produce two subrectangles with aspect ratios of ρ. Corollary 1 If an H W rectangle with 2H ρ is sliced vertically at any position x where ax(h/ρ, W Hρ) x in(hρ, W H/ρ) W 2ρH then the resulting two subrectangles have aspect ratio ρ. Equivalent results can also be derived for horizontally cutting a rectangle of height H and width W so that two resulting subrectangles with aspect ratio ρ are obtained. These results can be obtained by interchanging H and W in the above proof, hence Theore 2 and Corollary 2. Theore 2 A rectangle with height H and width W can be sliced horizontally into two subrectangles with aspect ratio ρ if H satisfies 2W ρ H 2ρW. Corollary 2 If an H W rectangle with 2W ρ H 2ρW is sliced horizontally at any position y where ax(w/ρ, H W ρ) y in(w ρ, H W/ρ) then the resulting two subrectangles have aspect ratio ρ. We can cobine these two theores to obtain a condition that will guarantee that both horizontal and vertical slicing will yield two subrectangles with aspect ratio ρ. which eets the conditions of both Theores 1 and 2. Theore 3 can be used to ensure that the initial stock rectangle to be recursively cut by our revised algorith will generate two resulting rectangles with aspect ratio ρ. What reains to be proved is that the recursive cutting of these resulting rectangles will continue to generate subrectangles with aspect ratio ρ. We now show that this is the case if ρ is chosen so that ρ 2. Theore 4 Suppose a rectangle R has aspect ratio ρ where ρ 2. If R cannot be sliced vertically (horizontally) to produce subrectangles with aspect ratio ρ, then it can be sliced horizontally (vertically) to yield subrectangles with aspect ratio ρ. Proof. Suppose R has aspect ratio ρ 2 and R cannot be sliced vertically to yield two subrectangles with aspect ratio ρ. By Theore 1, its diensions satisfy either W < 2H/ρ or W > 2Hρ. The only possibility is W < 2H/ρ since the second inequality would contradict the assuption that R has aspect ρ: W > 2Hρ iplies that H/W < 1/(2ρ) < 1/ρ. Now assuing that W < 2H/ρ and ρ 2, we have ρ 2 4 so that W 4W/ρ 2. Cobining this with the first inequality, then 2H/ρ > 4W/ρ 2 or H > 2W/ρ. We saw earlier in Theore 2 that if 2W/ρ H 2ρW then R can be cut horizontally to give two subrectangles with aspect ratio ρ. So it reains to show that H 2ρW. If H > 2ρW, then H/W > 2ρ > ρ contradicting the initial assuption that R has aspect ratio ρ, thus R satisfies the conditions of Theore 2 and can be cut horizontally to produce subrectangles with aspect ratio ρ. Analogously, it can be shown if that if R has aspect ratio ρ and it cannot be sliced horizontally, then it can be sliced vertically. If R cannot be cut horizontally, then H < 2W/ρ or H > 2ρW. The case where H > 2ρW conflicts with the assuption that H/W ρ. Now assuing that H < 2W/ρ and ρ 2, we have ρ 2 4 so that H 4H/ρ 2. Cobining this with the first inequality, then 2W/ρ > 4H/ρ 2 or W > 2H/ρ. We saw 4
5 earlier in Theore 1 that if 2H/ρ W 2ρH then R can be cut horizontally to give two subrectangles with aspect ratio ρ. So it reains to show that W 2ρH. If W > 2ρH, then H/W < 1/(2ρ) < 1/ρ contradicting the initial assuption that R has aspect ratio ρ, thus R satisfies the conditions of Theore 1 and can be cut vertically to produce subrectangles with aspect ratio ρ. Thus, any rectangle R with aspect ratio ρ 2 can be sliced vertically or horizontally, (or both ways) if the conditions of Theore 4 are et. 3.2 Algorith for generating aspect ratio data sets It is now possible to design an algorith that slices a stock rectangle into a set of subrectangles having the sae aspect ratio ρ 2. First, start with a rectangle R having aspect ratio ρ whose height and width satisfy the conditions of Theore 3. R can be sliced either vertically or horizontally to yield two subrectangles with the sae aspect ratio ρ: after selecting a direction randoly, choose a rando slicing position dictated by the appropriate Corollary (1 or 2). The resulting two subrectangles will have aspect ratio ρ. Next, randoly select a subrectangle and deterine a slicing direction: (horizontal, vertical, or either if possible). Having chosen the direction to slice, select an appropriate rando position and cut the subrectangle. Replace the rectangle in the list with these two subrectangles. This process is then reapplied to the list of subrectangles: since each subrectangle has aspect ratio ρ, its subrectangles will also have aspect ratio ρ (Theore 4) and these slicing steps can be repeated. The process terinates when the list contains the desired nuber of subrectangles. The second Θ(n) data generation procedure can be written as expressed in Algorith 2. 4 Generating rectangles satisfying the area ratio constraint The data sets generated by Algorith 2 consist of rectangles h i w i with aspect ratio ρ (i.e. 1/ρ h i /w i ρ). The areas of these rectangles, however, often vary as widely as those produced by the basic algorith alone. To control the range of areas, a second paraeter, γ is now introduced. A odification to Algorith 1 can be ade to generate data sets whose rectangles satisfy a userspecified area ratio γ 2. That is, the ratio of the areas of any two rectangles in the Algorith 2 Controlling the Aspect Ratio Input the paraeters n, ρ 2, H, and then W where 2H/ρ W ρh/2 while n rectangles not yet generated do choose a rectangle R at rando {Theore 4 guarantees that it can be cut in at least one direction} randoly choose a vertical or horizontal slicing direction, if possible; otherwise select the vertical or horizontal direction as appropriate {Theore 2 or 3} randoly choose a cutting position within the legal range of slicing positions {Corollary 1 or 2} perfor the cut on R, generating two subrectangles replace R in the list with the two subrectangles end while data set ust fall in the interval [1/γ, γ]. To ensure that the generated rectangles satisfy this constraint, the following properties are noted. Theore 5 Let γ 2 and {R i } be a set of n rectangles with area ratio γ which are ordered by nonincreasing areas: area (R ) area (R 1 ) area (R 2 )... area (R n 1 ) Any R j where area (R j ) 2 area (R )/γ can be sliced vertically into two subrectangles so that the resulting set of n + 1 subrectangles will have area ratio γ. Proof. Assue that rectangle R j is selected for cutting, and let the vertical slicing of R j take place at position x. Since cuts at x and w j x yield syetric subrectangles, we liit the choice of x to x w j /2. To ensure that the two subrectangles resulting fro this slicing both have areas of at least area (R )/γ when γ 2, we restrict x further so that x area (R ). It is possible to restrict x this way because area (R j ) = h j w j 2 area (R )/γ, i.e. w j /2 area (R ). (1) The initial set of n sorted rectangles were assued to have area ratio γ; in particular 1/γ area (R p )/ area (R q ) γ for all p, q j and this expression still holds after R j has been sliced. Let A and B denote the resulting subrectangles fro the slicing of R j as shown in Figure 4. The reaining area 5
6 h j and A B area (B) area (R )/γ cobine to yield area (A)/ area (B) γ, and so 1/γ area (A)/ area (B) γ. x w j Figure 4: Vertical slicing of rectangle R j = h j w j rectangle ratios that ust be exained are: (i) area (R i )/ area (A) and area (R i )/ area (B) for all i j and (ii) area (A)/ area (B) case(i) Since area (A) area (R j ) and area (R j )/ area (R n 1 ) γ, then area (A)/ area (R n 1 ) γ and area (R n 1 )/ area (A) 1/γ. But area (R i ) area (R n 1 ), so area (R i )/ area (A) 1/γ for all i j. Furtherore, the vertical cut position x was chosen so that area (A) area (R )/γ, iplying that area (A)/ area (R ) 1/γ so that area (R )/ area (A) γ. Since area (R i ) area (R ), we have area (R i )/ area (A) γ for all i j. Thus, 1/γ area (R i )/ area (A) γ for all i j and siilarly, 1/γ area (R i )/ area (B) γ for all i j. case (ii) The vertical cut position guarantees that area (A) area (R )/γ so area (A)/ area (B) area (R )/(γ area (B)). Further, area (B) area (R j ) area (R ) yields area (R )/ area (B) 1, so area (A)/ area (B) 1/γ. Siilarly, This copletes the proof that the set of n + 1 resulting subrectangles satisfies the area ratio constraint. Corollary 3 Let denote the area of the rectangle having the axiu area in a list of n rectangles with area ratio γ. Select any rectangle R j where area (R j ) 2/γ and slice it vertically at position x where x w j /2. The resulting set of n + 1 rectangles has area ratio γ. It can be shown that a siilar condition for horizontal cutting exists. For brevity, we state only the corresponding corollary. Corollary 4 Let denote the area of the rectangle having the axiu area in a list of n rectangles with area ratio γ. Select any rectangle R j where area (R j ) 2/γ and slice it horizontally at position y where γw j y h j /2. The resulting set of n + 1 rectangles has area ratio γ. By incorporating these observations into the basic algorith, an O(n 2 ) data generation procedure that creates a set of rectangles satisfying the area ratio constraint can be written as expressed in Algorith 3. Algorith 3 Controlling the Area Ratio Input the paraeters n, γ 2, H, and W while n rectangles not yet generated do let be the area of the largest rectangle in the current set choose a rectangle R fro all subrectangles whose areas are greater than 2/γ randoly choose a vertical or horizontal slicing direction randoly choose a cutting position within the legal range of slicing positions {Corollary 3 or 4} perfor the cut on R, generating two subrectangles replace R in the list with the two subrectangles end while area (A) area (R j ) area (R ) 6
7 5 Cobining aspect and area ratio constraints The algoriths developed thus far generate data sets where the sizes and areas of the rectangles can be constrained by either aspect ratio or area ratio. In any instances, it is desirable to eploy data sets where both the aspect ratio and the axiutoiniu area ratio are bounded. To accoplish this, the algoriths derived in sections 3 and 4 can be cobined. However, erging the two ethods requires that the conditions in Corollaries 1, 2, 3 and 4 be et. To prove that these conditions do not conflict, the following theore is established. where = area (R ). If the vertical slicing position x can be selected to satisfy both inequalities 3 and 4, then the resulting set of rectangles will have aspect ratio ρ and area ratio γ. A proof by contradiction establishes that x can be so chosen. Suppose there is no x that satisfies both inequalities (3) and (4). Then either (i) w j /2 < ax(h j /ρ, w j h j ρ) or (ii) > in(h j ρ, w j h j /ρ). Theore 6 Let ρ, γ 2 and {R i } be a set of n rectangles with aspect ratio ρ and area ratio γ which are ordered by nonincreasing areas: area (R ) area (R 1 ) area (R 2 )... area (R n 1 ) Any R j where area (R j ) 2 area (R )/γ can be sliced into two subrectangles so that the resulting set of n + 1 subrectangles will have aspect ratio ρ and area ratio γ. Proof. Assue that rectangle R j = h j w j is selected for cutting because R j eets the condition area (R j ) 2 area (R )/γ. Note this iplies that inequality (1) in Section 4 holds. Fro Theore 4, we know that R j can also be sliced either vertically or horizontally. (If Theore 3 is satisfied, R j can be sliced in either direction.) Specifically, if the conditions of Theore 1 hold, then R j can be sliced vertically; if the conditions of Theore 2 hold, then R j can be sliced horizontally. Suppose that the conditions of Theore 1 hold: 2h j ρ w j 2ρh j, (2) and so Corollary 1 defines the positions x for slicing vertically so that the resulting two subrectangles will have aspect ratio ρ: ax(h j /ρ, w j h j ρ) x in(h j ρ, w j h j /ρ) (3) Siilarly, in order to ensure that the n + 1 subrectangles resulting fro the cut will have area ratio γ, Corollary 3 dictates the slicing positions as x w j /2 (4) case(i) If w j /2 < ax(h j /ρ, w j h j ρ), then either (a) w j /2 < h j /ρ or (b) w j /2 < w j h j ρ. Inequality (a) iplies that w j < 2h j /ρ and (b) iplies that 2ρh j < w j which both contradict inequality (2). case(ii) If that w j > h j ρ or (b) > hjρ w j 1 ρ, this leads to the con and since γ tradiction that 1 > 1. > in(h j ρ, w j h j /ρ), then either (a) > w j h j /ρ. Inequality (a) iplies h jw j and h j w j Since w j h j /ρ w j /2 using inequality (2), inequality (b) siplifies to > w j /2 which contradicts inequality (1) in section 4. Thus, x can be chose to satisfy both conditions of Corollaries 1 and 3. Using siilar techniques, a proof for the case where R j is to be sliced horizontally can be derived by applying Theore 2 and Corollaries 2 and 4. Corollary 5 Let denote the area of the rectangle having the axiu area in a list of n rectangles with aspect ratio ρ and area ratio γ. Select any rectangle R j where area (R j ) 2/γ and slice it vertically at position x where ax(h j /ρ, w j h j ρ, ) x in(h j ρ, w j h j /ρ, w j /2). The resulting set of n + 1 rectangles has aspect ratio ρ and area ratio γ. Corollary 6 Let denote the area of the rectangle having the axiu area in a list of n rectangles with aspect 7
8 Data Set Height Width Nae n ax in avg ax in avg path path path path e path path e path e e path 1t e e path 2t e e path 5t e e nice nice nice nice nice nice nice nice 1t nice 2t nice 5t Table 1: Height and width statistics for exaple data sets ratio ρ and area ratio γ. Select any rectangle R j where area (R j ) 2/γ and slice it horizontally at position y where ax(w j /ρ, h j w j ρ, γw j ) y in(w j ρ, h j w j /ρ, h j /2). The resulting set of n + 1 rectangles has aspect ratio ρ and area ratio γ. A fourth O(n 2 ) data generation procedure can now be written as expressed in Algorith 4. 6 Saple Data Sets To illustrate the differences in the data sets that can be generated by the approaches described in this paper, we exaine soe saple data sets that were produced by Algoriths 1 and 4. For each of these sets, an initial rectangle of size 1 2 was recursively sliced as discussed in sections 2 and 5. The characteristics of the resulting data sets are suarized in this section. Algorith 4 Controlling the Aspect and Area Ratio Input the paraeters n, {γ, ρ 2}, H, and then W, where 2H/ρ W ρh/2 while n rectangles not yet generated do let be the area of the largest rectangle in the current set choose a rectangle R fro all subrectangles whose areas are greater than 2/γ if possible, randoly choose a vertical or horizontal slicing direction; otherwise select the vertical or horizontal direction as appropriate {Theore 2 or 3} randoly choose a cutting position within the legal range of slicing positions {Corollary 5 or 6} perfor the cut on R, generating two subrectangles replace R in the list with the two subrectangles end while The first group of data sets shown in Table 1 were generated by the basic routine given in Algorith 1. Recall that no restrictions are placed on aspect ratio or area ratio in this case rectangles are randoly selected for cutting, and slices are equally likely to be ade in rando directions. This procedure results in generating the rectangles whose height and width diensions are shown in Figure 5 for n = 5, n = 5, and n = 5. We refer to these sets as pathological data sets because there is a large variance in the heights and widths of the generated rectangles. As the data sets get larger in size, 8
9 Data Set Aspect ratio = Height/Width Area Nae n ax in ax in γ = ax/in path path path path e e e+8 path e e+7 path e e e+9 path e e e+12 path 1t e e e+1 path 2t e e e e+15 path 5t e e e e+17 nice nice nice nice nice nice nice nice 1t nice 2t nice 5t Table 2: Height/Width and area ratios for saple data sets either the height or width of the rectangles also see to get very sall. The second group of data sets shown in Table 1 were generated using Algorith 4 where the slicing positions are controlled so that resulting rectangles will have aspect ratio ρ and the data set will have area ratio γ. The values ρ = 4 and γ = 7 were selected for these data sets. Thus each generated rectangle has a height/width ratio lying between [.25,4]. Siilarly, the ratio of the largest area to sallest rectangle area in any data set does not exceed 7. Figure 6 plots the rectangles for the data sets of size n = 5, n = 5, and n = 5. The sets can be thought of as nice data sets because the rectangles characteristics fall within specified ranges: there are no long flat or tall thin rectangles and the areas of the rectangles are of the sae agnitude. The characteristics of these two types of data sets are further illustrated by regarding their observed values of ρ and γ. Note that in Table 2 there tends to be at least two agnitudes of difference in the rectangles height/width ratios within every pathological data set where n > 3. For the nice data sets generated by Algorith 4, each rectangle s aspect ratios is at ost 4 and no less than.25 for all data sets. Figure 7 plots the sorted height/width ratios for rectangles in the pathological and nice data sets where n = 5. The graphs shows how the height/width ratios are distributed within each data set and reflects the difference in agnitude shown in Table 2. The distribution of height/width ratios for the pathological rectangles range fro any sall ratios to several larger ratios. The aspect ratios for the nice data sets fall only between.25 and 4. The differences in the areas of the rectangles belonging to the pathological and nice data sets are also shown in Table 2. For the pathological data sets, the ratio of the largest rectangle area to sallest rectangle area appears to increase by orders of agnitude as the nuber of rectangles in the sets grows larger. For the nice data sets, this ratio is at ost 7, the value specified as the input paraeter γ. In addition to finding the axiu and iniu area values for each data set, the average rectangle area could be calculated, but this value will always equal the total area of the initial rectangle divided by the nuber of rectangles generated and so does not provide uch any additional inforation about the data set. However, the areas of the rectangles in each data set can easily be plotted to show the range of their distribution. In Figure 8, the distribution of area values includes any sall and soe very large rectangles for the pathological data set. As expected, the area variance in the nice data set is far saller due to its being bounded by a axiu area ratio equal to 7. 9
10 2 path_5 8 nice_ Width 1 Width Height Height 2 path_5 8 nice_ Width 1 Width Height Height 2 path_5t 8 nice_5t Width 1 Width Height Height Figure 5: Height and width of rectangles in pathological data sets 7 Using the Algoriths to Generate Test Data Sets The procedures outlined in this report perit the generation of sets of n rigid rectangles which can be packed or cut fro rectangular regions with zero waste. The sizes and areas of these rectangles ay have large variance as produced by Algorith 1 or can be restricted to satisfy user specified aspect and axiutoiniu area ratios using Algoriths 2, 3 or 4. Data sets of any size can be obtained in this anner. Figure 6: Height and width of rectangles in soe nice data sets These data sets can be used to evaluate the perforance of algoriths which are intended to solve the proble of packing a given set of rectangles into a single larger twodiensional rectangular region with iniu total waste, or equivalently, inial total height in the case of twodiensional strip packing. Several data sets can also be replicated or cobined to yield test suites for solving cutting stock probles. Individual rectangles can be rotated, or different stock sizes could be used when generating the data. In these cases, a range of aspect or area ratios ight also be specified. The optial solution would still have zero 1
11 4 "path_5" "nice_5" 35 3 Height/Width Ratio Figure 7: Height/Width coparisons for data sets of size n = 5 Figure 9: A pathological data set of 5 rectangles with integer diensions 8 7 "path_5" "nice_5" 6 5 Area Rectangles Sorted by Area Figure 8: Area coparisons for data sets of size n = 5 waste as it would consist of all the original rectangles that were sliced to generate the data sets. The zero waste property, while not reflective of realworld cutting probles, still provides researchers with a eans for coparing the quality of their solutions against a known optial. One characteristic of the exaple data sets described in the previous section is clearly that the diensions of the rectangles are given as real values. The precision of these values is dictated by the default arithetic precision of the prograing language and coputer syste on which the algoriths are ipleented and executed. By liiting the precision of the output stateents to the nuber of desired decial places (i.e. by rounding the values), we have found that we can usually overcoe the difficulties inherent with finite precision arithetic. It is also possible to utilize our algoriths to generate data Figure 1: A nice data set of 5 rectangles with integer diensions sets containing integer valued rectangles, i.e. rectangles whose heights and widths are integers. One way to accoplish this is by rounding the cut positions to the nearest integer. Two exaples of data sets that were generated by rounding the cut positions are shown in Figures 9 and 1 which were generated using Algoriths 1 and 4, respectively. However, it is not guaranteed that a rounded cut position will satisfy the conditions needed to ensure that the aspect and area ratios of the final rectangles will fall within the specified ranges. For exaple, if the input rectangle to be sliced has size 1 5, the axiu nuber of integer diension rectangles that could be generated by arbitrarily slicing this rectangle would be n = 5 rectangles of size 1 1. Requesting that the procedure generate 1 rectangles with integer dien 11
12 sions would result in at least 5 final rectangles having a height or width of zero. Thus, the choice of the size of the input rectangle ust be carefully considered along with the corresponding n value. In Figures 9 and 1, a 1 1 rectangle was sliced to generate the data sets of n = 5 rectangles. The ρ and γ values that were input to Algorith 4 to obtain the data in Figure 1 were 4 and 7, respectively. The algoriths successfully generated rectangles having nonzero integer diensions. However, because of the rounding process, the data set generated by Algorith 4 has an aspect ratio of 4.18 and an area ratio of 7.32 which are greater than the specified values. Nevertheless, the generated data sets still have bounded aspect and area ratios, and we believe that soe additional analysis of our ethods will show that the aspect and area ratios of the generated data sets can be predicted when the rectangles are rounded to integer diensions. 8 Ongoing Research Current versions of our algoriths give the user control over the range of variation within data sets for aspect ratio and area, but no control over the distribution of shapes and sizes. It is possible that sall alterations to our data generation algoriths would result in different types of data sets which a researcher ay prefer. There are any possibilities here: for exaple, the largest rectangle at any stage could be the one chosen to be sliced, or slicing positions could be restricted to favor ore or less extree aspect ratios. The distribution of rectangle sizes and areas generated by our algoriths depends on the probability distributions which govern the rando choices for the rectangles to be recursively sliced as well as the direction and the position of the slice, to the extent dictated by the appropriate theores and corollaries. A probabilistic analysis of the size distribution of the final rectangles, or an analysis of the ipact of changing the slicing probabilities would be interesting to pursue. Extensions of our general ethod can also be ade so that data sets can be generated (with bounded aspect and/or area ratios) whose optial packing (or cutting) solutions consist of nonslicing (i.e. nonguillotine) patterns. This research area is currently under study. In addition we are also considering odifications to our software that will ake it possible to generate data sets with known optia for testing algoriths for VLSI floorplanning. Acknowledgeents We would like to thank Yun You for odifying the software and the PLT drawing tool which were used to create the illustrations used in this paper. The authors would also like to thank the reviewers for their insightful coents and suggestions for iproving the paper. References [1] J.E. Beasley. ORLibrary: distributing test probles by electronic ail. Journal of the Operational Research Society, 41(11): , 199. url: astjjb/jeb/info.htl [2] BengtErik Bengtsson. Packing Rectangular Pieces A Heuristic Approach. The Coputer Journal, 25(3): , [3] J.O. Berkey and P.Y. Wang. TwoDiensional Finite BinPacking. Journal of the Operational Research Society, 38(5): , [4] ESICUP The Special Interest Group on Cutting and Packing. esicup/tikiindex.php. [5] S.S. Israni and J.L. Sanders. Perforance testing of rectangular partsnesting heuristics. International Journal of Production Research, 23: , [6] Stefan Jakobs. On Genetic Algoriths for the Packing of Polygons. European Journal of Operational Research, 88: , [7] Dequan Liu and Hongfei Teng. An Iproved BLalgorith for Genetic Algorith of the Orthogonal Packing of Rectangles. European Journal of Operational Research, 112:412 42, Finally, a public doain software package consisting of the algoriths described in this paper as well as visualization tools for drawing the data sets are under developent and will be distributed in the near future. 12
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