Find Similar Pictures

This search method enables you to find similar or almost identical pictures. For each image will be a checksum calculated for further comparison with all other images.

Comparison Method
Hints
Image Formats
Percentage Match
Picture Area
Compare Size
Checksum
Compare only pictures with the same properties
Detect picture modifications (slower)
Recognition Rate Test
Examples

Comparison Method

The comparison methods aHash, bHash, dHash, mHash and pHash enables you to find similar or almost identical pictures by using a percent match lower than 100%. If you want find exactly the same pictures you have to use a percent match of 100% or the comparison methods MD5/SHA. An overview of the recognition rate of the comparison methods aHash, bHash, dHash, mHash and pHash can be found here.

aHash
The comparison method aHash (Average Hash) resizes the image to 8x8 or 16x16 pixel. The image is then converted to grayscale and the average color value of all image pixels is calculated. Then all image pixels are compared with the average color value and the checksum is calculated.
bHash
The comparison method bHash (Blockhash) resizes the image to 128x128, 256x256 or 512x512 pixel. The image will be divided into a block matrix and the median value of all blocks will be calculated to create the checksum. The options "Fast" and "Precise" enables you to influence the accuracy of the checksum calculation.
dHash
The comparison method dHash (Difference Hash) resizes the image to 8x8 or 16x16 pixel. After that, the image is converted to grayscale and the checksum is created by comparing the difference in brightness values between all neighboring pixels.
mHash
The comparison method mHash (Median Hash) resizes the image to 8x8 or 16x16 pixel. The image is then converted to grayscale and the mean color value of all image pixels is determined. Then all image pixels are compared with the mean color value and the checksum is calculated.
pHash
The comparison method pHash (Perceptual Hash) resizes the image to 32x32 pixel. The image is then converted to grayscale and transformed by a discrete cosine transform (DCT). Next, the mean color value of all pixels in the image area (8x8) in the top left of the image is determined. Then the checksum is calculated by comparing the color value of all pixels from the top left image area with the mean color value.
MD5, SHA
These comparison methods can only be used to find exactly the same images. The following comparison methods are available:
- MD5 (128-Bit)
- SHA-1 (160-Bit)
- SHA-2 (256-Bit)
- SHA-2 (384-Bit)
- SHA-2 (512-Bit)

For more information on the aHash, bHash, dHash, mHash and pHash comparison methods, see articles Testing different image hash functions and Detection of Duplicate Images Using Image Hash Functions.

Calculation time of the checksums

Using the comparison methods aHash, bHash, dHash, mHash and pHash, we have calculated the time required to create a checksum and listed it in the following table:

Comparison Method	Picture Area	Checksum	Expenditure of Time
aHash	8x8	64-bit	0,0450 ms
aHash	16x16	256-bit	0,1425 ms
bHash (fast)	256x256	256-bit	9,6030 ms
bHash (precise)	256x256	256-bit	28,4792 ms
dHash	8x8	64-bit	0,0458 ms
dHash	16x16	256-bit	0,0988 ms
mHash	8x8	64-bit	0,1435 ms
mHash	16x16	256-bit	1,1012 ms
pHash	32x32	64-bit	8,6922 ms

Hints

The following files will be automatically excluded from the search:

Files with a size of 0 bytes
Pictures with a width or height smaller than the specified compare size
Corrupted, invalid or incomplete pictures (*)
Files with a blocked read access (*)

(*) An error message will be shown at the log pane.

Image Formats

Here you can specify which image formats should be checked during a search. Image files with the following file extensions are supported: 3FR, ARW, BMP, CR2, CRW, CUT, DCR, DIB, DNG, EMF, ERF, GIF, HDP, ICO, IFF, J2C, J2K, JP2, JPE, JPG, JPEG, JPX, JFIF, KDC, MDC, MEF, MOS, MRW, NEF, ORF, PEF, PBM, PCX, PGM, PNG, PPM, PSD, RAF, RAS, RAW, RW2, SRW, TGA, TIF, TIFF, RAS, RLE, WBMP, WEBP, WMF and X3F.

Percentage Match

Here you can specify the minimum percentage matching of two pictures. The calculated percentage matching between two pictures will be shown at the column Match. The percentage always refers to the reference picture of a group which will be shown in a different text color.

Picture Area

This option enables you to specify the picture area to be used to create the checksum. The following options are available:

entire picture
area at the upper left corner
area at the upper right corner
area at the lower left corner
area at the lower right corner

When using the options 2, 3, 4 or 5, the size of the picture area can be specified in pixel. For example, a value of 16 pixel will be using a picture area with a width and height of 16 pixel at the selected corner.

Compare Size

Here you can specify the maximum width and height of the pictures to be compared. A lower compare size finds more similar pictures and speeds up the comparison time. A higher compare size finds more identical pictures and less similar pictures and of course needs more time to compare them.

Checksum

The size of the checksum in bits is displayed here. The checksum can only be changed when using the bHash comparison method.

Compare only pictures with the same properties

This option will be performed before the option Detect picture modifications.

File Name
This option enables you to compare only pictures with the same file name.
File Extension
This option enables you to compare only pictures with the same file extension.
Width and Height
This option enables you to compare only pictures with the same width and height.
Orientation
This option enables you to compare only pictures with the same orientation (portrait or landscape).
Aspect Ratio
This option enables you to compare only pictures with the same aspect ratio. The calculation of the aspect ratio is done by the formula "width divided by height". The result of the calculation is truncated to one decimal place. A picture with 1920x1080 pixel has an aspect ratio of "1.7".

Detect picture modifications (slower)

This option enables you to detect different picture modifications when comparing two pictures. For this purpose, each picture modification will be performed with the picture to be compared and in each case an additional checksum will be created. The following picture modifications can be detected:

Rotated 90° to the right
Rotated 180° to the right
Rotated 90° to the left
Flipped horizontally
Rotated 90° to the right and flipped horizontally
Flipped vertically
Rotated 90° to the left and flipped horizontally

Recognition Rate Test

We have carried out tests with various comparison methods to determine the recognition rate for different image changes. For this purpose, 29 copies of a JPEG image (1600x1200px, 606KB) were created and their color, size and format changed. The tests always compared the original JPEG image and a modified copy of the image.

The following comparison methods were used in the test:

Comparison Method	Compare Size	Checksum
aHash¹	8x8	64-bit
aHash²	16x16	256-bit
bHash¹ (fast)	256x256	256-bit
bHash² (precise)	256x256	256-bit
dHash	16x16	256-bit
mHash	16x16	256-bit
pHash	32x32	64-bit

The percentage values in the table below show how well a comparison method recognizes the change in the image copy:

Image Modification	aHash¹	aHash²	bHash¹	bHash²	dHash	mHash	pHash
Image reduction to 75%	100%	100%	100%	100%	100%	100%	97%
Image reduction to 50%	100%	100%	100%	100%	99%	100%	100%
Image reduction to 25%	100%	100%	100%	100%	99%	100%	97%
Image enlargement to 150%	100%	100%	100%	100%	100%	100%	100%
Image enlargement to 200%	100%	100%	100%	100%	100%	100%	100%
Conversion to grayscale	97%	98%	97%	93%	97%	96%	97%
Brightness increased by 30%	95%	99%	100%	99%	98%	97%	91%
Brightness decreased by 30%	95%	95%	89%	93%	92%	94%	91%
Contrast increased by 30%	100%	99%	95%	96%	96%	96%	94%
Contrast decreased by 30%	100%	99%	100%	100%	98%	100%	94%
JPEG quality reduced by 25%	100%	100%	100%	100%	100%	100%	100%
JPEG quality reduced by 50%	100%	100%	100%	100%	99%	100%	100%
JPEG quality reduced by 75%	100%	100%	100%	100%	99%	100%	94%
Image rotated 90° to the left	47%	52%	68%	54%	46%	50%	47%
Image rotated 90° to the right	47%	52%	54%	55%	48%	50%	44%
Image flipped vertically	81%	81%	77%	73%	71%	80%	50%
Image flipped horizontally	69%	59%	62%	64%	54%	61%	47%
Added white frame (30px)	84%	82%	68%	61%	86%	82%	88%
Added black frame (30px)	94%	94%	92%	89%	92%	92%	88%
Image height reduced to 80%	100%	100%	100%	100%	100%	100%	100%
Image width reduced to 80%	100%	100%	100%	100%	99%	100%	100%
Conversion to PNG 24bit	100%	100%	100%	100%	100%	100%	100%
Conversion to BMP 24bit	100%	100%	100%	100%	100%	100%	100%
Conversion to GIF 256 colors	100%	100%	100%	99%	99%	100%	97%
Cropped 100 pixel from the left edge	92%	91%	88%	71%	82%	92%	78%
Cropped 100 pixel from the right edge	97%	92%	86%	86%	85%	90%	91%
Cropped 100 pixel from the top edge	94%	88%	86%	83%	86%	89%	84%
Cropped 100 pixel from the bottom edge	98%	91%	87%	86%	86%	91%	84%
Cropped 100 pixel from all edges	89%	80%	77%	75%	70%	80%	62%

The following table gives you an overview of how many duplicates were found with the various comparison methods in the test. The evaluation was carried out with a Percentage Match of at least 70%, 80% and 90%.

Comparison Method	70%	80%	90%
aHash¹ (64-bit)	26	26	23
aHash² (256-bit)	26	26	22
bHash¹ (fast)	25	23	18
bHash² (precise)	25	22	18
dHash (256-bit)	26	24	19
mHash (256-bit)	26	26	22
pHash (64-bit)	26	24	19