direct2d: antialiasing and drawing a line with a 1 pixel stroke

http://xboxforums.create.msdn.com/forums/t/61448.aspx

I'm currently porting a number of custom MFC CStatic-derived controls from GDI+ to Direct2D.

I've noticed when drawing horizontal and vertical lines with a 1 pixel wide stroke that the lines are in fact drawn 2 pixels wide and slightly transparent. The same applies to rectangles and rounded rectangles (diagonal lines seem to be drawn ok). The following code snippet is an example of what I'm trying to do.

D2D1_ROUNDED_RECT TempRect = D2D1::RoundedRect(Rectangle, 4.0F, 4.0F);
CComPtr<ID2D1SolidColorBrush> pTempBrush = NULL;

pRenderTarget->CreateSolidColorBrush(D2D1::ColorF(D2D1::ColorF::White), &pTempBrush);

pRenderTarget->DrawRoundedRectangle(TempRect, pTempBrush);
pRenderTarget->DrawLine(D2D1::Point2F(Rectangle.left, Rectangle.top), D2D1::Point2F(Rectangle.right, Rectangle.top), pTempBrush, 1.0F);

If a specify a 2 pixel stroke then the lines and rectangles are drawn correctly. That is, exactly 2 pixels wide and 100% opaque.

I've tried playing around with the antialiasing mode on the render target, but if I set the render target as follows:

pRenderTarget->SetAntialiasMode(D2D1_ANTIALIAS_MODE_ALIASED);

then the lines look terrible as they are all jagged.

Is there any way that I can force lines to have a 1 pixel stroke while in antialiased mode?

Regards
Martin

I noticed this too. From what I know you can't force it, but you can get around it by adding/subtracting .5 to your positions.
Eg. here is a rectangle:

RT->DrawRectangle(&D2D1::RectF(3.5f, 3.5f, 75.5f, 65.5f), Brush, 1, NULL); 

 

Re: Direct2D: Antialiasing and drawing a line with a 1 pixel stroke

 

Reply Quote

 

 Waterfox:

I noticed this too. From what I know you can't force it, but you can get around it by adding/subtracting .5 to your positions.
Eg. here is a rectangle:

RT->DrawRectangle(&D2D1::RectF(3.5f, 3.5f, 75.5f, 65.5f), Brush, 1, NULL); 

This fixes the problem, but I'm not really sure why I need to do this? It looks like it has something to do with the rectangle not aligning precisely to physical pixel boundaries. 

I understand that in Direct2D all drawing is done using device independent pixels (DIP) but I'm drawing to a 96 DPI display therefore there should be a 1:1 mapping between DIPs and physical pixels. In WPF there is a property called UseLayoutRounding which ensures that rounding behaviour aligns to whole pixel boundaries. Is there an equivalent in Direct2D? I haven't been able to find anything.

In DirectX pixel "positions" are at the center of the pixel.  For instance, position 0.0f, 0.0f is the exact center of the upper left pixel.

So, if you try to draw a 1 pixel wide line using a rect from (10,3) to (100,4) the rect doesn't align with the pixels.  Instead for each X position it covers half of X,3 and X,4.  This is why you are seeing the 2 pixel wide line with half transparency.

To solve this subract half a pixel from your coordinates in both directions.