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Bug: ImageBuffer::alpha not initialised on X11 [message #15575]

Tue, 29 April 2008 14:34

mrjt
Messages: 705
Registered: March 2007
Location: London

Contributor

The alpha channel is not correctly initialised on X11. This causes corruption (second image is what it should look like):
index.php?t=getfile&id=1177&private=0

Everything works correctly for Win32. The code below exibits the problem:

class ImageDrawTest : public TopWindow {
public:
	typedef ImageDrawTest CLASSNAME;
	Image img;
	
	ImageDrawTest() {
		Sizeable();
		GenerateImage();
	}
	
	void GenerateImage() {
		Size sz(100, 100);
		ImageDraw w(sz);
//		w.Alpha().DrawRect(sz, Black());
		w.Alpha().DrawRect(30, 30, 40, 40, White());
		w.DrawRect(sz, Red);
		img = w;
	}
	
	virtual void Paint(Draw &w) {
		w.DrawRect(GetSize(), SColorFace());
		w.DrawImage(0, 0, img);	
	}
	
	virtual void LeftUp(Point p, dword keyflags) {
		GenerateImage();	
		Refresh();
	}
};

If you uncomment the line above, the channel is initialised and everything works correctly.

Edit: Looking at the manual for XCreatePixmap, it says the pixmap contents are undefined, which I guess is fair enough. My main issue here is the inconsistency between Win32/Linux, both should be either defined or undefined.

Attachment: ImageDrawTest.png
(Size: 8.79KB, Downloaded 783 times)

[Updated on: Tue, 29 April 2008 14:43]

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Re: Bug: ImageBuffer::alpha not initialised on X11 [message #15589 is a reply to message #15575]

Tue, 29 April 2008 19:33

mirek
Messages: 14258
Registered: November 2005

Ultimate Member

mrjt wrote on Tue, 29 April 2008 08:34

The alpha channel is not correctly initialised on X11. This causes corruption (second image is what it should look like):
index.php?t=getfile&id=1177&private=0

Everything works correctly for Win32. The code below exibits the problem:

class ImageDrawTest : public TopWindow {
public:
	typedef ImageDrawTest CLASSNAME;
	Image img;
	
	ImageDrawTest() {
		Sizeable();
		GenerateImage();
	}
	
	void GenerateImage() {
		Size sz(100, 100);
		ImageDraw w(sz);
//		w.Alpha().DrawRect(sz, Black());
		w.Alpha().DrawRect(30, 30, 40, 40, White());
		w.DrawRect(sz, Red);
		img = w;
	}
	
	virtual void Paint(Draw &w) {
		w.DrawRect(GetSize(), SColorFace());
		w.DrawImage(0, 0, img);	
	}
	
	virtual void LeftUp(Point p, dword keyflags) {
		GenerateImage();	
		Refresh();
	}
};

Well, this is rather missing docs... It was intended to have both "body" and "alpha" uninitialized, the same situation as with e.g. Paint. After all, you never know whether for specific use it is better to have alpha =0 or 255 (or even something else).

OTOH, if you believe it should be initialized, we can do that too.

Mirek

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Re: Bug: ImageBuffer::alpha not initialised on X11 [message #15600 is a reply to message #15589]

Wed, 30 April 2008 11:23

mrjt
Messages: 705
Registered: March 2007
Location: London

Contributor

To be honest it doesn't make much difference to me now since I know about the problem Smile

. But IMO ImageDraw should behave in exactly the same way on both platforms, it's annoying to write code that works on Win32 but not X11.

IMO the ideal solution would be to have it undefined in Win32 too. But if you think that is a bad idea for backwards-compatability reasons then yes, ImageDraw::alpha should be initialized to 0. Perhaps this could be done on the first call to Alpha() so there is no additional overhead for none-alpha situations?

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Re: Bug: ImageBuffer::alpha not initialised on X11 [message #15601 is a reply to message #15600]

Wed, 30 April 2008 11:55

mirek
Messages: 14258
Registered: November 2005

Ultimate Member

mrjt wrote on Wed, 30 April 2008 05:23

IMO the ideal solution would be to have it undefined in Win32 too.

Should I draw random data into alpha? Smile

Win32 clears it for me.

Quote:

ImageDraw::alpha should be initialized to 0. Perhaps this could be done on the first call to Alpha() so there is no additional overhead for none-alpha situations?

Should not it be rather 255?

Mirek

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Re: Bug: ImageBuffer::alpha not initialised on X11 [message #15602 is a reply to message #15601]

Wed, 30 April 2008 12:56

mrjt
Messages: 705
Registered: March 2007
Location: London

Contributor

luzr wrote on Wed, 30 April 2008 10:55

Should I draw random data into alpha? Smile

Win32 clears it for me.

You could write in Jesus's face for a Turrin shroud effect, that'd be cool Razz

luzr wrote on Wed, 30 April 2008 10:55

Should not it be rather 255?

Windows clears it to 0 does it not? So unless you want to change that behaviour too then use 0. I agree it's more intuitive for it to be 255 but we write software, I think we can handle it Smile

[Updated on: Wed, 30 April 2008 13:04]

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Re: Bug: ImageBuffer::alpha not initialised on X11 [message #15603 is a reply to message #15602]

Wed, 30 April 2008 13:21

mirek
Messages: 14258
Registered: November 2005

Ultimate Member

OK then.

Image.cpp:

Draw& ImageDraw::Alpha()
{
	if(!has_alpha) {
		alpha.DrawRect(size, GrayColor(0));
		has_alpha = true;
	}
	return alpha;
}

Mirek

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Re: Bug: ImageBuffer::alpha not initialised on X11 [message #15606 is a reply to message #15601]

Wed, 30 April 2008 16:00

mr_ped
Messages: 826
Registered: November 2005
Location: Czech Republic - Praha

Experienced Contributor

luzr wrote on Wed, 30 April 2008 11:55

Should I draw random data into alpha? Smile

Win32 clears it for me.

In case it is supposed to be uninitialized, and the OS does initialize it for you, I suggest to use random bytes in DEBUG mode trough #ifdef.
That will make such errors to appear in debug mode, yet it will not slow down release.

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