Move objects with parent

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Hello!

I'm implementing a vertical scrollbar for a small game project and i'm having some issues with calculating local positions of the scrollbar Thumb and Content when the Viewport rectangle moves.

At the moment, when i move the viewport, the track, thumb and content flyes all over the place.

- What i'm trying to achieve:

. Calculate the Thumb and Content "local positions".

. When the Viewport is moved, the Thumb and Content should keep their local positions in relation to the new viewport position.

I'm using C#, Windows Forms and the System.Drawing Rectangle.
I'm not using the Windows Forms controls, they are all custom.

Here is a snippet of the code:

C#

var track = new Rectangle(0, 0, 50, 100);
var thumb = new Rectangle(0, 0, 50, 100);
var content = new Rectangle(0, 0, 100, 100);

// Called when the viewport rectangle position is changed:
public void OnViewportPositionChanged(Point position)
{
	viewport.X = position.X;
	viewport.Y = position.Y;
	
	thumb.X = viewport.X;
	thumb.Y = thumb_local_position;
	
	content.X = viewport.X;
	content.Y = content_local_position;
}

public void CalculateLocalPositions()
{
	content_local_position = ???;
	thumb_local_position   = ???;
}

Thanks in advance!

What I have tried:

I'm trying to solve these two problems since yesterday but could not find the right calculations on the web.

Posted 24-Aug-21 6:15am

strangerick

Updated 25-Aug-21 7:12am

v23

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Comments

strangerick 24-Aug-21 14:23pm

Someone could provide a direct answer with the math to solve this problem?

BillWoodruff 24-Aug-21 20:29pm

repost of; https://www.codeproject.com/Questions/5310526/How-to-implement-a-simple-scrollbar

strangerick 25-Aug-21 6:44am

Hi, i posted this question but these problems was already solved, could you help with this new question?

Richard MacCutchan 25-Aug-21 4:04am

I assume you mean when the parent control is resized, rather than moved. In which case you just need to calculate the ratio of the original scrollbars to the window. You can then calculate the new scrollbar sizes from those values when the parent has been resized.

strangerick 25-Aug-21 6:43am

Hi, what i need is the math to move the thumb and content so they keep their last Y positions and move to the new position relative to it.
Also, i'm not using the Winform controls, they are all custom.

Richard MacCutchan 25-Aug-21 7:58am

You will have to work that out for yourself. We have no idea how your implementation works. A much better solution would be to use a control that includes automatic scrolling.

strangerick 25-Aug-21 11:21am

Hi, i'm not using Winforms controls nor inheriting from it.

I need the math for the Thumb and Content to calculate thei local positions and keep it when the Viewport is moved.

Richard MacCutchan 25-Aug-21 11:28am

How can anyone answer such a question? We have no idea how your code works.

strangerick 25-Aug-21 11:38am

As you can see from the code i posted the thumb and content and viewport are rectangles and the content is moved up and down by the thumb.
These are all working correctly but when i move the viewport, the thumb and content should keep their local positions but i don't know how to calculate it...

Richard MacCutchan 25-Aug-21 11:46am

The best I can offer is what I already suggested above: "you just need to calculate the ratio of the original scrollbars to the window. You can then calculate the new scrollbar sizes from those values when the parent has been resized." That is to say, the scrollbar values should be calculated relative to the parent window size and position. The thumb control should reflect the percentage of information that is displayed in the viewport, relative to the total information that can be viewed. Microsoft provide a good explanation of it at Viewports and content | Microsoft Docs[^].

strangerick 25-Aug-21 12:00pm

I don't think you get the question, the scrollbar functionalities are working perfectly but when i move the parent of the scrollbar (wich i call viewport) i want the Thumb and Content to keep their Y local positions but i don't know how to calculate their local positions.

Richard MacCutchan 25-Aug-21 12:03pm

I don't know how many times I need to repeat myself on this: all your positions need to be relative to the viewport. When the viewport changes you use the relative position to calculate the absolute. If your viewport dimension is 100 and the scrollbar position is 50, what do you think it should be if the viewport dimension becomes 200?

strangerick 25-Aug-21 12:06pm

Could you post a normal answer with the math to do this so i can see if it works and close the topic?

Richard MacCutchan 25-Aug-21 12:39pm

See below.

2 solutions

Solution 1

The problem is probably caused because control coordinates are recalculated internally and have different values than you expect (depending on Windows scale and Font settings).
See: Control.PointToScreen(Point) Method[^]
And: Automatic Scaling - Windows Forms .NET Framework | Microsoft Docs[^]

Quote:
Custom Controls: Another way to create a control is to create one substantially from the beginning by inheriting from Control.

See: Varieties of Custom Controls - Windows Forms .NET Framework | Microsoft Docs[^]

Posted 24-Aug-21 8:36am

RickZeeland

Updated 24-Aug-21 9:02am

v3

Comments

strangerick 24-Aug-21 14:41pm

Hello, thanks for the answer, but i'm not using Windows Forms controls, they are all custom controls made for a game project.

Could you provide a answer with the math to update the positions of the thumb and content rectangles so they can move to the new Y position relative to their last Y position?

RickZeeland 24-Aug-21 14:50pm

It's been long ago that I struggled with a similar problem, and being an old fart I can't remember how I solved it sadly ... maybe after a good night's rest :)
Oh, and custom controls are controls too !

strangerick 24-Aug-21 14:56pm

Ok, that's fine. Maybe later you can provide a answer :)
Also, i'm using only the PictureBox from Winforms, the rest are all custom controls that does not inherit from Control.

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Richard MacCutchan · Accepted Answer · 2021-08-25T06:39:00

Solution 2

C++

// When the parent is resized, the scrollbar should be resized
// to fit the new size of the viewport (excluding borders etc).
// It is then just a matter of calculating the ratio of visible
// data to the size the relevant dimension of the viewport.
//
ratio = data.dimension / viewport.dimension; // the percentage of data being displayed
thumb.dimension = scrollbar.dimension / ratio;
//
// So if the viewed data is one twentieth of the total available
// the thumb control should be one twentieth of the scrollbar.
// i.e it can move to view each twentieth section of the data.
// this applies in the horizontal and vertical planes.

Posted 25-Aug-21 6:39am

Richard MacCutchan

Comments

strangerick 25-Aug-21 13:10pm

Hello, thanks for the answer but the thumb size has nothing to do with the question in hand, I also tried applying it to the positions but it did not worked.

As I wrote in the original post:

. Calculate the Thumb and Content "LOCAL POSITIONS".
. Then, when the Viewport is MOVED: the Thumb and Content should always KEEP THEIR LOCAL POSITIONS IN RELATION TO THE NEW VIEWPORT POSITION.

Could you please edit your answer?

Richard MacCutchan 26-Aug-21 4:07am

If the viewport is of size 100 and the data is size 1000 then you can only view one tenth of the data at any time. So your thumb position can be in one of ten places depending on which 'page' is being viewed*. If the viewport size increases to 150 then the data that can be viewed increases to one sixth of the total. So you recalculate the positions and sizes with those values. You need to do this calculation every time you refresh the view, as it may be changing all the time.

In reality you may be moving the view in smaller increments.