cannot produce Targa(tga) image with transparent background

[academy25]

Hi all, I have a QImage (QT's image class) with transparent background. I use the following code to convert it to a Magick image.

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Magick::Image* TgaFileType::toImage(QImage& qimage)
{

	Magick::Image *newImage ;
	double scale = 1 / 255.0;
	Magick::PixelPacket *pixels;
	Magick::ColorRGB mgc;

	newImage = new Magick::Image(Magick::Geometry(qimage.width(), qimage.height()), Magick::ColorRGB(0.5, 0.2, 0.3));

	newImage->modifyImage();

	for (int y = 0; y < qimage.height(); y++) {
		pixels = newImage->getPixels(0, y, newImage->columns(), 1);
		for (int x = 0; x < qimage.width(); x++) {
			
			QRgb pix = qimage.pixel(x, y);

			mgc.red( scale*qRed(pix) );
			mgc.green( scale*qGreen(pix)  );
			mgc.blue( scale*qBlue(pix) );
			mgc.alpha( scale*qAlpha(pix) );

			*pixels++ = mgc;
		}
		newImage->syncPixels();
	}

	return newImage;
}

But when I write the resulting Magick image to file, the background all seems black. However, when I save the original QImage as png via

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qimage.save(dstFilename)

, the transparency is preserved.

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Magick::Image* image = toImage(*imgOriginal);
image->write(dstFilename);

The more weird thing is, even if I set the alpha directly to zero, via

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mgc.alpha( 0 );

and save the Magick image, it does the same thing resulting with a fully black background image.
How can I set alpha values to produce a transparent Targa image?

[magick]

Call newImage->matte(true) to tell ImageMagick you want to respect the matte channel, otherwise the alpha values are ignored. Also make sure that there is not a problem with the TGA image format. Save to PNG with newImage->write("image.png"). Otherwise your code should work (we tried a mock example of your code and returned the correct results with ImageMagick 6.6.8-4).

[academy25]

hello, I have modified the above toImage function as follows,

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Magick::Image* TgaFileType::toImage(QImage& qimage)
{

	Magick::Image *newImage ;
	double scale = 1 / 255.0;
	Magick::PixelPacket *pixels;
	Magick::ColorRGB mgc;

	newImage = new Magick::Image(Magick::Geometry(qimage.width(), qimage.height()), Magick::ColorRGB(0.5, 0.2, 0.3));

	newImage->modifyImage();

	for (int y = 0; y < qimage.height(); y++) {
		pixels = newImage->getPixels(0, y, newImage->columns(), 1);
		for (int x = 0; x < qimage.width(); x++) {
			
			QRgb pix = qimage.pixel(x, y);

			mgc.red( scale*qRed(pix) );
			mgc.green( scale*qGreen(pix)  );
			mgc.blue( scale*qBlue(pix) );
			mgc.alpha( scale*qAlpha(pix) );

			*pixels++ = mgc;
		}
		newImage->syncPixels();
	}

	newImage->matte(true);

	newImage->magick("PNG");
	newImage->write(&blob);
	newImage->read(blob);

	newImage->write("C:\\tgapng.png");

	return newImage;
}

but again the tga output and png output has a black background with no transparency.
The following is the input png image with a transparent background(you can save and open it via photoshop or any software that you can check its transparent part) ,
http://img820.imageshack.us/i/transparanlena.png/
And following is the png result written by the above code
http://img828.imageshack.us/i/tgapng.png/

[magick]

A couple of things to try. Move the matte() method before the loop. Its possible your QImage pixels are not correct. Instead set

• mgc.red( scale*qRed(pix) );

to

• mgc.red( scale*127.0 );

and do that for green, blue, and alpha. See if you get a semitransparent gray image.

[academy25]

It works fine for png, I got a semi-transparent image. But it fails for tga, there is no transparency, I just got a gray image.
Did you succeed for the tga case? Would you please try to obtain a semi-transparent tga image with the same code?

[fmw42]

This works for me in CLI

convert logo: -transparent white logot.png
convert logot.png logot.tga

The colors are slightly different but the transparency is there.

[academy25]

The problem is solved, didn't know that the opacity values are inverted in ImageMagick: 0 for fully opaque and 1.0 for fully transparent !!
So I subtracted the alpha value that belongs to QImage from 255, before scaling the result by 1/255.

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mgc.alpha( scale*(255-qAlpha(pix)) );

Setting the matte to 'true' before the loop, and considering the inverted alpha values solved the problem.