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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
image display. Therefore, one cannot conclude that one 
image is better than another if the display condition is 
not the same. Unfortunately, no display conditions were 
clearly described in many visual comparisons, including 
those in the IEEE GRSS 2006 Data Fusion Contest. This 
ambiguity in display conditions significantly reduced the 
reliability of the visual comparison results. 
• For quantitative evaluation, different evaluation 
results can often be obtained when different quantitative 
measures or indicators are selected for the evaluation. 
Therefore, whether, or not, a given quantitative index 
can measure image fusion quality or measure quality 
difference between two images is still an open question. 
Among numerous quantitative evaluation indicators, the 
Mean Bias (MB), Variance Difference (VD), Standard 
Deviation Difference (SDD), Correlation Coefficient 
(CC), Spectral Angle Mapper (SAM), Relative 
Dimensionless Global Error (ERGAS), and Q4 Quality 
Index (Q4) have been often used in image fusion 
publications. They were also used in the IEEE GRSS 
2006 Data Fusion Contest for quantitative evaluation. 
Therefore, this paper focuses on the evaluation and discussion 
of how display conditions affect visual comparison and whether, 
or not, the seven often used quantitative indicators (MB, VD, 
SDD, CC, SAM, ERGAS, and Q4) can provide convincing 
results to tell the quality difference or similarity of two images. 
This evaluation is conducted based on the assumption that 
(1) if two images of the same area can present 
identical information, including colour, spatial detail and 
image depth, under the same visualization condition, and 
(2) if the two images can also provide the same 
classification result using the same classifier under the 
same processing condition, 
the two images can be defined and accepted as having the same 
image quality. 
This assumption is true for remote sensing imagery and remote 
sensing applications, because the two foremost important 
applications of remote sensing imagery are (1) visualization and 
(2) classification. If two images can provide the same results for 
visualization and classification under the same condition, they 
will not make any difference for remote sensing applications, 
and they can be equally accepted by remote sensing users. 
For the evaluation and discussion, some testing images having 
the same image quality are generated; the seven quality 
indicators are applied to the testing images to check their ability 
to measure the quality similarity among the images; and the 
fusion quality evaluations by Alparone, et al. (2004) and 
Alparone, et al. (2007) are reviewed and analyzed to see 
whether, or not, the quality indicators of the evaluations 
provided convincing results. 
2. TESTING IMAGES 
An original Ikonos MS image of Fredericton, NB, Canada, 
collected on October 1, 2001, is used for the evaluation. The 
image contains 4 spectral bands and is stored in 16 bits. For 
visual comparison purpose and to test the performance of the 
seven quantitative indicators, the original Ikonos image (Ik-Orig) 
is altered through mean shifting, histogram stretching, and 
histogram stretching plus mean shifting, resulting in a mean 
shifted Ikonos image (Ik-Shift), a histogram stretched image 
(Ik-Str), and a histogram stretched and mean shifted image (Ik- 
Str-Shift). The detailed alteration of the Ikonos image is 
described in Table 1. 
2.1 Visual comparison 
To prove that the four images (Ik-Orig, Ik-Shift, Ik-Str, and Ik- 
Str-Shift) have the same image quality for visualization, they 
are displayed under the same display conditions and compared 
with each other. The histogram stretchings used are zero 
stretching (i.e. no stretching), linear stretching, root stretching, 
adaptive stretching, and equalization stretching (Figure 1). It 
can be seen that all of the four images appear very dark without 
any histogram stretching. And, all of the four images appear 
exactly the same when they are stretched using the same 
histogram stretch, regardless what stretch is applied (compare 
images in the same column of Figure 1). This comparison 
demonstrates that the four images have the same quality for 
visualization and visual interpretation. 
On the other hand, it can also be seen from Figure 1 that the 
same image can be displayed and interpreted differently as if the 
source image had different qualities, if the image is not 
displayed under the same condition. For example, the same 
original Ikonos image (Ik-Orig) in Figure 1 appears 
significantly differently under different display conditions. 
Some appear darker than others, and some look noisier than 
others. If the image source information and the image stretching 
information were not given in Figure 1, one must say that the 
images in different columns of Figure 1 have different qualities. 
Ik-Orig 
Ik-Shift 
Ik-Str 
Ik-Str-Shift 
Band 1 
B 
B+100 
Bxl.5 
Bxl.5+100 
Band 2 
G 
G+100 
Gxl.5 
Gxl.5+100 
Band 3 
R 
R+100 
Rxl.5 
Rx 1.5+100 
Band 4 
NIR 
NIR+100 
NIRxl.5 
NIRx 1.5+100 
Table 1. Alteration of the spectral bands of the original Ikonos MS (Ik-Orig) to obtain other testing images 
(Ik-Shift, Ik-Str, and Ik-Str-Shift)