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Render an image and overlaid controls in exact pixels?

I have a background image that consists of an outline, with 10 rectangles placed specifically on the outline. I want to display that image on the screen, and overlay transparent buttons that match up to the rectangles on the image. When the button is pressed, the button's outline will turn to orange to indicate the rectangle has been selected.
I'm finding that simply setting the width and height of the buttons to match the pixel width and height of the rectangles in the image always renders a button much larger than the corresponding rectangle.
The image itself is 669 x 871px, and the rectangles are all 87 x 143px. Setting the Image's WidthRequest and HeightRequest in code or in XAML seems to render it at a different resolution than it actually is, thereby leading to the issue I described - if a button's size is set to 87 x 143 (again using HeightRequest and WidthRequest) it always renders larger than the rectangle.
Furthermore, if I contain the buttons in a StackLayout (as I often need to do) and specify a margin to push the layout downwards to align with the rectangles on the image, again the correct number of pixels never matches up with the distance the layout is moved.
I'm aware of how to overlay elements using a Grid with a single row and column and simply defining multiple layouts in the same space and triggering visibility in code. What I don't know how to do is display the elements in exactly the right pixel sizes that I need for the controls to always align with the image.
The diagram I'm working with:
Desired behaviour:
Actual behaviour:

I've worked out how to do this by using Grids within a Grid. By setting the Grid's row and column height and width to "*" (use all available space), defining a ContentView containing the diagram, then several smaller grids to lay over the top with their row height and column width matching the dimensions of the diagram image (i.e. if the image is 600 pixels wide, the nested Grid's ColumnDefinition's Width="600"),
I was then able to define the buttons inside a StackLayout, inside the nested Grids. I've tried this out across a few different resolutions and it seems to work just fine for my purposes.
I had to set Padding to 0, and used the Margin to vertically align the StackLayout as needed (i.e. for the first row of buttons at the top of the diagram, their StackLayout's Margin was "19,89,19,0" - 19 pixels from either side, 89 pixels from the top).

Fill with Measure/ArrangeOverride inside a scrollviewer

I have a custom drawn control containing some plots.
I've put it inside a scrollviewer since later I want to be able to zoom the control's contents.
I want this control to have a specific minimum height and if the available visible space is bigger than that it should fill the space.
However using MeasureOverride I just get infinity for the available height (because of the scrollviewer).
How can I get the actual available visible space during MeasureOverride to stretch my control to this height?

You should not need to do any layout overriding for that, just set the alignments to Stretch and a MinHeight.

WPF: How to find space available for Canvas?

I'm using WPF shapes to create Hexagons (for a game map) on a Canvas. After some playing around with ScrollViewer, I've decided to implement the scrolling and zoom of the map myself rather than using WPF functionality, just using WPF to get the events for mouse wheel, arrow keys etc. I'm placing the (Hex Map) Canvas as the last child inside a Dock Panel so it will get all the available remaining space. The Dock Panel will be set to be the content of the Main Window. But I want to find out how big the Canvas can be before I put any Children on the Canvas so that I can centre the screen over the Hex I want and only add the Shapes (Hexs) that can actually be seen. When zoomed out, a long way I will remove Polygons altogether and use another method of rendering and when zoomed in a long way I will add more details.
Is there any neat way of getting the available space? The only way that I can think of that will hopefully work is to get the current dimensions of the windows and subtract the dimensions of the outer elements of the Dock Panel, but that feels rather messy.

You may use the ActualWidth and ActualHeight properties of Canvas to determine size available to it. Be sure that HorizontalAlignment and VerticalAlignment are set to Stretch.

Determine the width of the vertical scroll bar in a ScrollViewer

I'm using a ScrollViewer to display an Image. The Image has a ScaleTransform set as one of it's LayoutTransforms. I've got it setup to fit the width of the image into the ActualSize of the ScrollViewer. My problem is that if the image height requires the vertical scrollbar to be present (I have it set to Auto) then my image is scaled just a little bit to much. I know how to determine if the scrollbar would be present and how to get the correct scale, but I cannot figure out how to determine what the actual width of the scrollbar is. I guess I could just guess at it, but I'd like something that would work if I later add styles to my application that would result in the scrollbars being a different size. Additionally I'm also doing Fit to Height and would need to get the Height of the horizontal scrollbar when it would be visible (I'm assuming that the answer to getting the width of the vertical scrollbar would make getting the height of the horizontal scrollbar obvious).

You can use SystemParameters.ScrollWidth.

Using ViewableHeight and ViewableWidth instead of ActualHeight and ActualWidth in my scaling calculations along with setting the scroll bars Visibility to Visible instead of Auto works. However I'll accept another answer that allows the scroll bars to be set to Auto instead.
Edit:
OK, I've now got the scroll bars set to Visible. Then I do my calculation with the ViewableHeight and ViewableWidth. Then I set the scroll bars back to Auto. This seems to work even if it's not all that elegant.

How to resize a Canvas in WPF?

I'm writing a WPF app that has a Canvas in it. This canvas will be custom rendered at runtime. It's sort of like a game in that it needs to be measured in pixels. I need to be able to set my Canvas to 478x478 pixels (client rectangle size). I don't want any scaling or other resolution-independent steps to take place on my Canvas.
I'm not sure if this is possible in WPF, since its nature is to be resolution independent. My questions:
How do I resize my Canvas at runtime (function to call?)
When I resize my Canvas, is the renderable area (the client rectangle) going to be that size? If not, how can I resize it to make sure the client rectangle is a specific width/height?
Is it possible to set the width/height of the Canvas in Pixels? How does the resolution-independent aspect of WPF interfere with what I'm trying to do?
When I resize my Canvas, will other controls resize appropriately as they have been designed to do in the WPF designer?
Thanks in advance.

Any elements positioned in a canvas will not resize or reposition based upon the size of the canvas. So I don't think there's any advantage to setting the size of the canvas. Maybe you should just set a fixed size of the window instead.
Otherwise, just set the Canvas.Width, Height, and ClipToBounds=True and you have a fixed sized canvas that positions its child elements with X/Y coordinates.
Also you should be sure to set SnapsToDevicePixels=True on the canvas so that child elements will have crisp pixel-aligned bounds.

resizing is cake:
MyCanvas.Width = 350;
MyCanvas.Height = 450;
this sets the size, you CAN render to coordinates outside of this, but it will be clipped. You can wrap your canvas inside a scroller to allow the user to see what is outside the height/width of the canvas.
as for the rest of your questions, i think you can see this SO question for your answers

Update 2020: on WPF .NET CORE: If the Canvas is a child of a Viewbox, the content is automatically scaled to the Viewbox height and width.
<Viewbox Stretch="Fill"
Width="50"
Height="50">
<Canvas>
<Path.... />
</Canvas>
</Viewbox>