How to read a tif image with transparent pixels in opencv - c++

Question

I have a tif image that has transparent pixels (I can see them in paint.net as transparent pixels).
I am trying to read them into a Mat in openCV and I am using this code:
Mat image=imread(imagePathname,CV_LOAD_IMAGE_UNCHANGED);
auto x=image.channels();
based on I my understanding, since the input image has transparency, channels() should return 4, but it return 3.
How can I read a tif image with transparent pixels and checking if a pixel is transparent in opencv?
edit1
Result of running imagemagick:
Image: layer0003.tif
Format: TIFF (Tagged Image File Format)
Mime type: image/tiff
Class: DirectClass
Geometry: 10000x5000+0+0
Resolution: 150x150
Print size: 66.6667x33.3333
Units: PixelsPerInch
Type: TrueColorAlpha
Base type: TrueColor
Endianess: MSB
Colorspace: sRGB
Depth: 8-bit
Channel depth:
red: 8-bit
green: 8-bit
blue: 8-bit
alpha: 1-bit
Channel statistics:
Red:
min: 0 (0)
max: 255 (1)
mean: 23.6472 (0.0927342)
standard deviation: 37.6851 (0.147785)
kurtosis: 8.93054
skewness: 2.28009
Green:
min: 0 (0)
max: 255 (1)
mean: 22.8353 (0.0895504)
standard deviation: 37.6516 (0.147653)
kurtosis: 10.4255
skewness: 2.52881
Blue:
min: 0 (0)
max: 255 (1)
mean: 22.798 (0.0894041)
standard deviation: 37.6575 (0.147677)
kurtosis: 10.9059
skewness: 2.58999
Alpha:
min: 0 (0)
max: 255 (1)
mean: 89.055 (0.349235)
standard deviation: 121.566 (0.476728)
kurtosis: -1.59995
skewness: -0.632496
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 58.8064 (0.230613)
standard deviation: 68.9821 (0.270518)
kurtosis: 8.35337
skewness: 3.53852
Alpha: none #00000000
Rendering intent: Perceptual
Gamma: 0.454545
Chromaticity:
red primary: (0.64,0.33)
green primary: (0.3,0.6)
blue primary: (0.15,0.06)
white point: (0.3127,0.329)
Background color: white
Border color: srgba(223,223,223,1)
Matte color: grey74
Transparent color: none
Interlace: None
Intensity: Undefined
Compose: Over
Page geometry: 10000x5000+0+0
Dispose: Undefined
Iterations: 0
Compression: LZW
Orientation: TopLeft
Properties:
date:create: 2014-03-01T13:11:12+00:00
date:modify: 2014-02-28T17:48:41+00:00
signature: dfa3e35c35345ef3440ff15d15ad37222f9cf0376bed7b7710dd95f4e537e210
tiff:alpha: unassociated
tiff:endian: lsb
tiff:photometric: RGB
tiff:rows-per-strip: 1
tiff:timestamp: 2014:02:28 17:48:38
xmp:CreatorTool: Microsoft Windows Live Photo Gallery 15.4.3555.308
Profiles:
Profile-xmp: 12702 bytes
Artifacts:
filename: layer0003.tif
verbose: true
Tainted: False
Filesize: 20.42MB
Number pixels: 50M
Pixels per second: 60.24MB
User time: 0.827u
Elapsed time: 0:01.829
Version: ImageMagick 6.8.8-7 Q16 x64 2014-02-13 http://www.imagemagick.org

Answer 1

I just had some time to revisit this :-)
Basically, I think the issue is that ImageMagick will adopt the most economical, in terms of file size, when writing its output files. So, if your image is basically greyscale but the alpha (opacity) channel is only 0 or 1, ImageMagick will encode that with 16-bit greyscale for the data and single-bit alpha channel. I guess that is beyond OpenCV as it seems to expect the alpha to be encoded with the same depth as the data.
So, the question becomes... "How can you force the combination you need of data/alpha (both 8-bit, not 8-bit plus 1-bit) greyscale/colour (8-bit greyscale or 8-bit colour)?"
There may be other ways, but for now, I can envisage adding a row across the bottom of the image that forces ImageMagick's hand and which you can hopefully readily remove once it has done the trick for you in OpenCV.
So, let's create a 16-bit greyscale image with a single-bit alpha where it makes the square transparent hole in the middle:
convert -size 300x300 gradient:black-white -alpha set -region 100x100+100+100 -alpha transparent image.tif
Let's check what we have:
identify -verbose image.tif | head -14
Image: image.tif
Format: TIFF (Tagged Image File Format)
Mime type: image/tiff
Class: DirectClass
Geometry: 300x300+0+0
Units: PixelsPerInch
Type: GrayscaleAlpha
Base type: Grayscale
Endianess: LSB
Colorspace: Gray
Depth: 16-bit
Channel depth:
gray: 16-bit <--- 16-bit greyscale data
alpha: 1-bit <--- 1-bit alpha
Now let's force it to 8-bit alpha and 8-bit data:
# Force to at least 8-bit greyscale (but maybe RGB depending on rest of image) plus 8-bit alpha...
# ... by adding a line across the bottom with alpha varying from 0 to 1 (i=x coordinate,w=width)
convert image.tif \
\( +clone -resize x1! -channel A -fx "i/w" \) \
-append -depth 8 result.tif
Let's check it worked:
identify -verbose result.tif | head -16
Image: result.tif
Format: TIFF (Tagged Image File Format)
Mime type: image/tiff
Class: DirectClass
Geometry: 300x301+0+0 <--- one extra row
Units: PixelsPerInch
Type: GrayscaleAlpha
Base type: Grayscale
Endianess: LSB
Colorspace: Gray
Depth: 8-bit
Channel depth:
gray: 8-bit <--- 8-bit data
alpha: 8-bit <--- 8-bit alpha
Now let's force 8-bit RGB plus 8-bit alpha:
# Force to 8-bit RGB plus 8-bit alpha...
# ... by adding a coloured line across the bottom with alpha varying from 0 to 1 (i=x coordinate,w=width)
convert image.tif \
\( +clone -resize x1! -channel RGBA -fx "rand()" \) \
-append -depth 8 result.tif
And let's check again:
Image: result.tif
Format: TIFF (Tagged Image File Format)
Mime type: image/tiff
Class: DirectClass
Geometry: 300x301+0+0 <--- 1 extra row
Units: PixelsPerInch
Type: TrueColorAlpha
Base type: TrueColor <--- Truecolour
Endianess: LSB
Colorspace: sRGB <--- RGB
Depth: 8-bit
Channel depth:
red: 8-bit <--- 8-bit red
green: 8-bit <--- 8-bit green
blue: 8-bit <--- 8-bit blue
alpha: 8-bit <--- 8-bit alpha