Re: Re: Reading alpha data from CMYK TIFF

Aha!  I know that GetIndexes() is really an overloaded function.  For
images with a CLUT, it returns the pixel values as lookup table
indices (hence the name), and for CMYK images with alpha channels, it
returns the alpha data.  I had thought that the reference to
GetImagePixels/SetImagePixels was only for the first use, but
apparently I need to do that in the second case also.

Thanks *very* much for catching this.  There also seems to be a bug
somewhere in my code for generating the PostScript, but I can easily
fix that.

Thanks again!

-pd


On Mon, Feb 10, 2003 at 09:58:56PM -0000, el_supremo01 <[email protected]> wrote:
> Hi Peter,
> 
> --- In [email protected], pd@w... wrote:
> > I'm trying to read the alpha data for a CMYK TIFF image using
> > ImageMagick 5.5.4 on Windows 2000.  I'm using the call to GetIndexes
> ()
> > which, as I understand it, is the only way to get the alpha data for
> > CMYK.
> 
> I haven't used the GetIndexes call but the documentation says:
> "Method GetIndexes() returns the indexes associated with the last 
> call to SetImagePixels ( ) or GetImagePixels ( )"
> 
> which implies you must have called Set or Get ImagePixels before you 
> call GetIndexes. 
> 
> I tried adding a GetImagePixels() call and the program terminated 
> without crashing, but my ghostscript said there was an error in the 
> resulting .ps file.
> 
> Here's how I changed the code (with an additional declaration at the 
> top of the file of PixelPacket *pp;)
> 
>                     // Make 1-bit/pixel string from data
>                     bitData = 0;
>                     for (row = 0; row < image->rows; row++)
>                     {
> 						pp = GetImagePixels
> (image,0,row,image->columns,1);
> 		                if((alpha = GetIndexes(image)) == 
> NULL)printf("OOOPS\n\n\n");
>                         for (col = 0; col < image->columns; col++)
>                         {
>                             bitData *= 2;
>                             //bitData |= (alpha[col, row] == 0) ? 0 : 
> 1;
> //                            if (alpha[(row*(image->columns))+col])
>                            if (alpha[col])
>                                 bitData |= 1;
> 
> 
> I put the getimagepixels and getindexes calls inside the outer for 
> loop so that it fetches one row of the image at a time which requires 
> changing the way "alpha" is accessed.
> As I say, I don't really know what I'm doing but perhaps this'll get 
> you a bit further.
> 
> Best Wishes
> Pete
> 
> 
> 
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