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141 | dmarschall | 1 | |
2 | Implementation detail differences Daniel Marschall |
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3 | ================================= 02 January 2018 |
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4 | |||
5 | FilterFoundry tries to be as compatible with FilterFactory as possible. |
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6 | However, results are usually not 100% equal, because functions like |
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7 | cos, sin, sqr, etc. have different accuracy due to the underlying |
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8 | implementation. |
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9 | |||
10 | Furthermore, there are following known differences between Filter Foundry |
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11 | and Filter Factory: |
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12 | |||
13 | i, u, v |
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14 | ------- |
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15 | |||
16 | Filter Foundry <1.7 uses the same formulas as in Filter Factory: |
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17 | |||
18 | i=((76*r)+(150*g)+(29*b))/256 Output range is 0..254 |
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19 | u=((19*r)+(37*g)+(56*b))/256 Output range is -55..55 |
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20 | v=((78*r)+(65*g)+(13*b))/256 Output range is -77..77 |
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21 | |||
22 | Filter Foundry 1.7 uses more accurate formulas: |
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23 | |||
24 | i=(299*r+587*g+114*b)/1000 Output range is 0..255 |
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25 | u=(-147407*r-289391*g+436798*b)/2000000 Output range is -55..55 |
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26 | v=614777*r-514799*g-99978*b)/2000000 Output range is -78..78 |
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27 | |||
28 | Both formulas follow the same YUV standard, but have different accuracy. |
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29 | |||
30 | |||
31 | get(i) (Testcase getput.afs) |
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32 | ------ |
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33 | |||
34 | Filter Foundry: |
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35 | get(x)=0 if x>255 or x<0 |
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36 | |||
37 | Filter Factory: |
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38 | get(x)=x if x>255 or x<0 |
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39 | [Note: The result "x" was most likely not intended but a result of an undefined behavior] |
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40 | |||
41 | |||
42 | r, g, b at empty canvas (Testcase emptycanvas.afs) |
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43 | ----------------------- |
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44 | |||
45 | In Filter Factory, an empty (transparent) canvas of a new file is initialized as r=g=b=0 |
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46 | |||
47 | Filter Foundry initializes it as r=g=b=255 |
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48 | |||
49 | |||
50 | rnd(a,b) and rst(i) (Testcases rst_*.afs) |
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51 | ------------------- |
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52 | |||
53 | Filter Foundry's implementation of rst(i) (undocumented function that sets the seed for the PRG) |
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54 | differs from the implementation of Filter Factory. |
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55 | |||
56 | 1. In Filter Foundry, the function rnd(a,b) retrieves a random number at realtime; therefore, if the |
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57 | seed is changed via rst(i), there is an immediate effect on the next call of the rnd(a,b) function. |
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58 | For example, following filter would generate an one-colored picture without any randomness: |
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59 | R=rst(123), rnd(0,255) |
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60 | G=rnd(0,255) |
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61 | B=rnd(0,255) |
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62 | |||
63 | In Filter Factory, the rnd(a,b) function is more complex. |
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64 | (The analysis of Filter Factory's rnd and rst function is not yet completed, so |
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65 | following notes might not be 100% accurate.) |
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66 | It seems like there is a buffer with 56 random integers that will be refilled regularly. |
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67 | The rst(i) function seems to change the seed for the next batch of random numbers, |
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68 | so there is not immediate effect on the rnd() calls. |
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69 | |||
70 | 2. Furthermore, it seems like in Filter Factory, if rst(i) is called multiple times |
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71 | with the same argument, there will be no effect. |
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72 | So, while following filter would generate random vertical bars in Filter Foundry, |
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73 | it would generate a picture with random pixels in Filter Factory: |
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74 | R=(x==0) ? rst(123) : 0, rnd(0,255) |
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75 | G=0 |
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76 | B=0 |
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77 | |||
78 | In Filter Foundry, every call of rst(i) will perform an update of the seed. |
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79 | So, if you like to have a random-pixel picture with a specific seed, |
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80 | you must make sure that rst(i) will only be called once. |
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81 | You would need to write your filter like this |
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82 | R=(x==0 && y==0) ? rst(123) : 0, rnd(0,255) |
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83 | G=0 |
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84 | B=0 |
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85 | if you want generate a similar result as Filter Factory outputs with the filter |
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86 | R=rst(123), rnd(0,255) |
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87 | G=0 |
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88 | B=0 |
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89 | |||
90 | 3. In Filter Foundry, the argument i of the function rst(i) is limited to the |
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91 | type "unsigned int" (argument of the function srand() in the C StdLib), |
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92 | so the allowed range is 0..4294967295. |
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93 | |||
94 | In Filter Factory, the argument i must be between 0 and 32767, inclusively. |
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95 | If the argument not within this range, the operation "and 0x7FFF" will be applied to it |
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96 | to extract the low 15 bits. |