Re: imread (repost)

[Thomas L. Scofield]

In Octave:

octave:4> im = imread("magnolia.png");

octave:5> whos im

Variables in the current scope:

  Attr Name        Size                     Bytes  Class

  ==== ====        ====                     =====  ===== 

       im        480x640x4                1228800  uint8

Total is 1228800 elements using 1228800 bytes

octave:6> max(max(im(:,:,4)))

ans = 247

octave:7> min(min(im(:,:,4)))

octave:8> [im,map,alpha] = imread("magnolia.png");

octave:9> whos im map alpha

Variables in the current scope:

  Attr Name        Size                     Bytes  Class

  ==== ====        ====                     =====  ===== 

       im        480x640x4                1228800  uint8

       map         0x0                          0  double

       alpha       0x0                          0  double

Total is 1228800 elements using 1228800 bytes

In Matlab:

>> im = imread('magnolia.png');

>> whos im

  Name        Size              Bytes  Class    Attributes

  im        480x640            307200  uint8              

>> [im, map, alpha] = imread('magnolia.png');

>> whos im map alpha

  Name         Size              Bytes  Class     Attributes

  alpha        0x0                   0  double              

  im         480x640            307200  uint8               

  map        250x3                6000  double    

For this same image, here is the output of the GraphicsMagick shell command  "gm identify -verbose magnolia.png":

Image: /Users/scofield/images/generic/magnolia.png

  Format: PNG (Portable Network Graphics)

  Geometry: 640x480

  Class: DirectClass

  Type: true color with transparency

  Depth: 8 bits-per-pixel component

  Channel Depths:

    Red:      8 bits

    Green:    8 bits

    Blue:     8 bits

    Opacity:  8 bits

On Aug 5, 2008, at 10:34 PM, John W. Eaton wrote:

On  5-Aug-2008, Thomas L. Scofield wrote:

| 

| On Aug 5, 2008, at 4:53 PM, Søren Hauberg wrote:

| 

| > tir, 05 08 2008 kl. 22:50 +0200, skrev Søren Hauberg:

| >> tir, 05 08 2008 kl. 10:35 -0400, skrev Thomas L. Scofield:

| >>> That's not the kind of modification I was suggesting for imshow.  I

| >>> was suggesting this much simpler change (pseudocode):

| >>>

| >>>

| >>>   if (length (size (image)) == 3 && size (image, 3) == 4)

| >>>     show image(:, :, 1:3)

| >>>   else

| >>>     show image

| >>>   endif

| >>

| >> I can only see the point of this behaviour if 'imread' actually  

| >> returns

| >> NxMx3 arrays.

| >

| > Aaarghh, I of course ment to write MxNx4 here...

| >

| 

| 

| That brings us back around to my original post.  I have examples of  

| png files for which

| 

|    im = imread ("file.png");

|    size (im, 3)

| 

| produces the output 4.

Can you find out what Matlab's imread returns for this file?  I'm also

curious about what happens if you call imread like this:

  [im, map, alpha] = imread ('file.png')

Thanks,

jwe

Thomas L. Scofield

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Associate Professor

Department of Mathematics and Statistics

Calvin College

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