Fix rounding error in fnumber calculation

webmeister · webmeister · commit e0b2022acf58 · 2020-12-09T16:55:55.000+01:00
The mathematical calculation of fnumbers does not always match the expected
values: For example for f/3.5 the precise mathematical value is 3.564...,
which gets rounded to 3.6. Fix this special case by returning a value
closer to the expected value.
diff --git a/src/tags_int.cpp b/src/tags_int.cpp
@@ -2204,7 +2204,11 @@ namespace Exiv2 {
 
     float fnumber(float apertureValue)
     {
-        return std::exp(std::log(2.0F) * apertureValue / 2.F);
+        float result = std::exp(std::log(2.0F) * apertureValue / 2.F);
+        if (std::abs(result - 3.5) < 0.1) {
+            result = 3.5;
+        }
+        return result;
     }
 
     URational exposureTime(float shutterSpeedValue)
diff --git a/test/data/exiv2-test.out b/test/data/exiv2-test.out
@@ -1077,7 +1077,7 @@ File  6/15: 20030925_201850.jpg
 20030925_201850.jpg   Exif.CanonCs.0x0015                          Short       1  32767
 20030925_201850.jpg   Exif.CanonCs.LensType                        Short       1  n/a
 20030925_201850.jpg   Exif.CanonCs.Lens                            Short       3  18.0 - 55.0 mm
-20030925_201850.jpg   Exif.CanonCs.MaxAperture                     Short       1  F3.6
+20030925_201850.jpg   Exif.CanonCs.MaxAperture                     Short       1  F3.5
 20030925_201850.jpg   Exif.CanonCs.MinAperture                     Short       1  F22
 20030925_201850.jpg   Exif.CanonCs.FlashActivity                   Short       1  Did not fire
 20030925_201850.jpg   Exif.CanonCs.FlashDetails                    Short       1  
@@ -2690,7 +2690,7 @@ Compare image data and extracted data ------------------------------------
 < 20030925_201850.jpg   Exif.CanonCs.0x0015                          Short       1  32767
 < 20030925_201850.jpg   Exif.CanonCs.LensType                        Short       1  n/a
 < 20030925_201850.jpg   Exif.CanonCs.Lens                            Short       3  18.0 - 55.0 mm
-< 20030925_201850.jpg   Exif.CanonCs.MaxAperture                     Short       1  F3.6
+< 20030925_201850.jpg   Exif.CanonCs.MaxAperture                     Short       1  F3.5
 < 20030925_201850.jpg   Exif.CanonCs.MinAperture                     Short       1  F22
 < 20030925_201850.jpg   Exif.CanonCs.FlashActivity                   Short       1  Did not fire
 < 20030925_201850.jpg   Exif.CanonCs.FlashDetails                    Short       1  
@@ -4231,7 +4231,7 @@ Compare image data and extracted data ------------------------------------
 > 20030925_201850.exv   Exif.CanonCs.0x0015                          Short       1  32767
 > 20030925_201850.exv   Exif.CanonCs.LensType                        Short       1  n/a
 > 20030925_201850.exv   Exif.CanonCs.Lens                            Short       3  18.0 - 55.0 mm
-> 20030925_201850.exv   Exif.CanonCs.MaxAperture                     Short       1  F3.6
+> 20030925_201850.exv   Exif.CanonCs.MaxAperture                     Short       1  F3.5
 > 20030925_201850.exv   Exif.CanonCs.MinAperture                     Short       1  F22
 > 20030925_201850.exv   Exif.CanonCs.FlashActivity                   Short       1  Did not fire
 > 20030925_201850.exv   Exif.CanonCs.FlashDetails                    Short       1  
@@ -5992,7 +5992,7 @@ Compare original and inserted image data ---------------------------------
 < 20030925_201850.jpg   Exif.CanonCs.0x0015                          Short       1  32767
 < 20030925_201850.jpg   Exif.CanonCs.LensType                        Short       1  n/a
 < 20030925_201850.jpg   Exif.CanonCs.Lens                            Short       3  18.0 - 55.0 mm
-< 20030925_201850.jpg   Exif.CanonCs.MaxAperture                     Short       1  F3.6
+< 20030925_201850.jpg   Exif.CanonCs.MaxAperture                     Short       1  F3.5
 < 20030925_201850.jpg   Exif.CanonCs.MinAperture                     Short       1  F22
 < 20030925_201850.jpg   Exif.CanonCs.FlashActivity                   Short       1  Did not fire
 < 20030925_201850.jpg   Exif.CanonCs.FlashDetails                    Short       1  
@@ -7533,7 +7533,7 @@ Compare original and inserted image data ---------------------------------
 > 20030925_201850.exv   Exif.CanonCs.0x0015                          Short       1  32767
 > 20030925_201850.exv   Exif.CanonCs.LensType                        Short       1  n/a
 > 20030925_201850.exv   Exif.CanonCs.Lens                            Short       3  18.0 - 55.0 mm
-> 20030925_201850.exv   Exif.CanonCs.MaxAperture                     Short       1  F3.6
+> 20030925_201850.exv   Exif.CanonCs.MaxAperture                     Short       1  F3.5
 > 20030925_201850.exv   Exif.CanonCs.MinAperture                     Short       1  F22
 > 20030925_201850.exv   Exif.CanonCs.FlashActivity                   Short       1  Did not fire
 > 20030925_201850.exv   Exif.CanonCs.FlashDetails                    Short       1  

Original file line number	Diff line number	Diff line change
`@@ -2204,7 +2204,11 @@ namespace Exiv2 {`
`2204`	`2204`
`2205`	`2205`	`float fnumber(float apertureValue)`
`2206`	`2206`	`{`
`2207`		`- return std::exp(std::log(2.0F) * apertureValue / 2.F);`
	`2207`	`+ float result = std::exp(std::log(2.0F) * apertureValue / 2.F);`
	`2208`	`+ if (std::abs(result - 3.5) < 0.1) {`
	`2209`	`+ result = 3.5;`
	`2210`	`+ }`
	`2211`	`+ return result;`
`2208`	`2212`	`}`
`2209`	`2213`
`2210`	`2214`	`URational exposureTime(float shutterSpeedValue)`