QImage and 8 bit unsigner char **

[fyanardi]

Hi all,

I need to display a tiff image on a QWidget, the image is 8 bit-depth, and I need to represent it using 2 dimensional unsigned char so that it can be processed later. I've successfully read the tiff image into the char, but I can't draw it on the QWidget. What I've tried is:

unsigned char pic[512][512] = TiffIO::tiffReadPic("test.tif", pic, &width, &height);
QImage img(width, height,QImage::Format_Indexed8);
 
for(int i = 0; i < 255; i++)
    {
        myColor = qRgb(i, i, i);
        img.setColor(i, myColor);
    }
 
for(int i = 0; i<width; i++)
    for(int j = 0; j<height; ++j)
         img.setPixel(i, j, pic[i][j]);

unsigned char pic[512][512] = TiffIO::tiffReadPic("test.tif", pic, &width, &height);
QImage img(width, height,QImage::Format_Indexed8);

for(int i = 0; i < 255; i++)
    {
        myColor = qRgb(i, i, i);
        img.setColor(i, myColor);
    }

for(int i = 0; i<width; i++)
    for(int j = 0; j<height; ++j)
         img.setPixel(i, j, pic[i][j]);

To copy to clipboard, switch view to plain text mode 

But it doesn't give anything, the QWidget remains blank and no error message at all

Any suggestion?

Thanks...

[jacek]

How do you draw that image on QWidget? How does this image look like if you save it to a file using QImage::save()?

[fyanardi]

Thanks for your very fast reply,

I'm sorry, I got error message with that code from QImage that the pixel I try to set is always out of range (QImage::setPixel: index=15 out of range). Then I tried to print img.numColors(), and I found that it always returns 0, even though I've set the format to 8 bit indexed (QImage::Format_Indexed8). However if I set the numColors manually via img.setNumColors(256), that piece of code works, the QWidget and also img.save() produce correct images.

I still don't understand why 8 bit indexed can have 0 numColors() ? From the doc, I read QImage will not return 0 from image that uses color table. Or maybe I miss something?

Thanks btw

[wysota]

If you choose the format of QImage, you only set a way of storing pixel data in the object. You have to set the number of colours manually using setNumColors() then probably to set the proper size of the CLT.

[fyanardi]

If you choose the format of QImage, you only set a way of storing pixel data in the object. You have to set the number of colours manually using setNumColors() then probably to set the proper size of the CLT.

Oh, I didn't know about that before, thanks for your explanation