Qt-interest Archive, February 2007
QT4.3 coordinate oddness
Message 1 in thread
Hello-
Just when I thought I had the coordinate stuff nailed,
I find I dont quite know it yet.
Im drawing lines and text onto the QGraphicsView/QGraphicsScene.
All of these values come to me from some other program.
All these values happen to be negative coordinates ie:
line: -500, -450 to -560, -450
etc
I expect something like:
------- |
| |
| instead I get: |
| -------
What's odd is that the drawing appears on the screen
to be 'flipped' or mirrored across the X axis and Im
at a loss to figure out why. Obviously this could easily
be my problem, but I wanted to ask here in case there's
some coordinate gotcha I dont already know about. Each
item is an individual QGraphicsItem.
Thank you,
Jeff
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Message 2 in thread
On 28.02.07 00:28:19, Jeff Lacki wrote:
> Im drawing lines and text onto the QGraphicsView/QGraphicsScene.
> All of these values come to me from some other program.
> All these values happen to be negative coordinates ie:
> line: -500, -450 to -560, -450
> etc
Well, thats easily changed by multiplying whatever you get with -1.
> I expect something like:
>
> ------- |
> | |
> | instead I get: |
> | -------
>
> What's odd is that the drawing appears on the screen
> to be 'flipped' or mirrored across the X axis and Im
> at a loss to figure out why. Obviously this could easily
> be my problem, but I wanted to ask here in case there's
> some coordinate gotcha I dont already know about. Each
> item is an individual QGraphicsItem.
The question is hardly possible to answer because you don't show any
code how this drawing is to be done. Having had a quick look at the docs
I understood that the coordinate system starts a the upper right corner,
the x axis grows to the left, the y axis to the bottom. If you create a
line with the coordinates x1=10,y1=5,x2=10,y2=15 and then another line
with coordinates x1=0,y1=4,x2=20,y2=4 you should get exactly the result
you were looking for.
Andreas
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Message 3 in thread
Andreas Pakulat wrote:
> On 28.02.07 00:28:19, Jeff Lacki wrote:
>> Im drawing lines and text onto the QGraphicsView/QGraphicsScene.
>> All of these values come to me from some other program.
>> All these values happen to be negative coordinates ie:
>> line: -500, -450 to -560, -450
>> etc
>
> Well, thats easily changed by multiplying whatever you get with -1.
>
>> I expect something like:
>>
>> ------- |
>> | |
>> | instead I get: |
>> | -------
>>
>> What's odd is that the drawing appears on the screen
>> to be 'flipped' or mirrored across the X axis and Im
>> at a loss to figure out why. Obviously this could easily
>> be my problem, but I wanted to ask here in case there's
>> some coordinate gotcha I dont already know about. Each
>> item is an individual QGraphicsItem.
>
> The question is hardly possible to answer because you don't show any
> code how this drawing is to be done. Having had a quick look at the docs
> I understood that the coordinate system starts a the upper right corner,
> the x axis grows to the left, the y axis to the bottom. If you create a
> line with the coordinates x1=10,y1=5,x2=10,y2=15 and then another line
> with coordinates x1=0,y1=4,x2=20,y2=4 you should get exactly the result
> you were looking for.
Actually, I think that the coordinate system starts in the upper left
corner of the screen. The X axis grows to the right and the Y axis grows
downwards. The probable reason that the images are flipped is that
they were created with a system that has coordinates that start in the
lower left corner of the screen (such as OpenGL?).
>
> Andreas
>
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Message 4 in thread
On Feb 28, 2007, at 6:16 AM, John McClurkin wrote:
> Andreas Pakulat wrote:
>> On 28.02.07 00:28:19, Jeff Lacki wrote:
>>> Im drawing lines and text onto the QGraphicsView/QGraphicsScene.
>>> All of these values come to me from some other program.
>>> All these values happen to be negative coordinates ie:
>>> line: -500, -450 to -560, -450
>>> etc
>> Well, thats easily changed by multiplying whatever you get with -1.
>>> I expect something like:
>>>
>>> ------- |
>>> | |
>>> | instead I get: |
>>> | -------
>>>
>>> What's odd is that the drawing appears on the screen
>>> to be 'flipped' or mirrored across the X axis and Im
>>> at a loss to figure out why. Obviously this could easily
>>> be my problem, but I wanted to ask here in case there's
>>> some coordinate gotcha I dont already know about. Each
>>> item is an individual QGraphicsItem.
>
>> The question is hardly possible to answer because you don't show any
>> code how this drawing is to be done. Having had a quick look at
>> the docs
>> I understood that the coordinate system starts a the upper right
>> corner,
>> the x axis grows to the left, the y axis to the bottom. If you
>> create a
>> line with the coordinates x1=10,y1=5,x2=10,y2=15 and then another
>> line
>> with coordinates x1=0,y1=4,x2=20,y2=4 you should get exactly the
>> result
>> you were looking for.
>
> Actually, I think that the coordinate system starts in the upper
> left corner of the screen. The X axis grows to the right and the Y
> axis grows downwards. The probable reason that the images are
> flipped is that they were created with a system that has
> coordinates that start in the lower left corner of the screen (such
> as OpenGL?).
>> Andreas
I had similar issues. Even though the coordinate system is
documented in the view/scene
as having the origin in the upper left, we all grew up learning to
think of the origin in the
lower left. I solved the problem by using a call to scale to flip
the view/scene around
X axis:
originalDataScene_ = new QGraphicsScene(this);
ui.originalDataView->setScene(originalDataScene_);
ui.originalDataView->scale(1.0,-1.0);
-Dennis
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