clear screen

[mic]

Hi,

i draw some primitives like lines and text.
Then I want to show only a part of the screen like scissor, but not a rectangle. I want a circle.
Are there any solution?



thanks

[jamesbowman]

Yes, you can use Stencil to do this.

The book only mentions it briefly, on p.106.  You draw the circle into the "stencil buffer", then draw everything else using a stencil test (called "StencilFunc") so that only pixels inside the circle are touched.  Here is a fragment of code to do it:

// First draw some things 'A' here...

// Draw a circle into the stencil
GD.SaveContext();
GD.ColorMask(0,0,0,0);    // Don't touch color
GD.StencilOp(INCR, INCR); // incrementing stencil
GD.PointSize(135 * 16);
GD.Begin(POINTS);
GD.Vertex2ii(240, 136);
GD.RestoreContext();

// Set the stencil test, to only inside the circle
GD.StencilFunc(EQUAL, 1, 255);

// Things drawn 'B' here are stenciled to inside the circle



FT800's stencil functions are very similar to OpenGL's: en.wikibooks.org/wiki/OpenGL_Programming/Stencil_buffer

[mic]

Fantastic!

Thanks a lot.