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line and rect coordinates

edited February 2012 in General Posts: 2

I'm finding glitches when trying to draw one line to the corner of another.

How do I interpret the parameters for line and rect?

Assuming noSmooth() and strokeWidth(0) and recMode(CORNERS):

How many pixels are colored with: line (x, y, x + 10, y)?

How many pixels wide is: rect(x, y, x + 10, y + 10)?

Can I assume that the answer for both is either: 10 or 11?

X-Windows would draw 11 pixels but GDI would draw 10 pixels.

Could someone explain the drawing model for Codea (e.g. Apple documentation, underlying drawing API)?


  • SimeonSimeon Admin Mod
    edited February 2012 Posts: 5,417

    Codea uses OpenGL ES 2 for drawing. Lines, rects and all other primitives are implemented as individual fragment shaders in the backend.

    One exception is noSmooth() lines: these are implemented using the GL_LINE primitive. Which duplicates the line pixels along the line axis based on the strokeWidth() — you can't have a 0 width line using GL_LINE.

    For your example:

    rect(x, y, x + 10, y + 10)

    Coordinates for rectMode(CORNERS) are computed as follows:

    x = x, y = y, w = (x+10), h = (y+10)
    //t is a temporary
    if x > w 
        t = x
        x = w
        w = t
    if y > h 
        t = y
        y = h
        h = t                
    w = w - x
    h = h - y

    These coordinates are then sent to OpenGL as vertex coordinates for a GL_TRIANGLE_STRIP primitive. The vertex data would look something like:

    GLfloat rectVerts[] = 
        x,   y,
        x+w, y,
        x,   y+h,
        x+w, y+h,

    You can assume it will be 10 pixels wide if:

    • you use the noSmooth() shader path.
    • you are using the identity transform
    • You should expect slightly less than 10 pixels wide if you use the smooth() shader path — this is due to the anti-aliasing in the shader.
  • edited February 2012 Posts: 622

    Edit: if you use scale(8) and draw near 0,0 it will be able to see the above

  • edited February 2012 Posts: 2

    So why does there appear to be a glitch when drawing a simple rectangle with lines. Try running this and watching the top-left corner as you change the "Glitch" parameter from 0 to 1.

    function setup()
        iparameter ("glitch", -2, 2)
        glitch = 0
    function draw()            
        background(255, 255, 255)
        stroke (0, 0, 0, 255)
        local x1, x2 = 100, 306
        local y1, y2 = 220, 413
        line (x1, y1, x2, y1) -- bottom line
        line (x1, y2, x2, y2) -- top line
        line (x1, y1, x1, y2 + glitch) -- left line
        line (x2, y1, x2, y2) -- right side
  • SimeonSimeon Admin Mod
    edited February 2012 Posts: 5,417

    Your code seems to render with the top right corner not lining up, for me (200% zoom):


    This looks like it might be specific to the GL_LINE implementation. I'll investigate further after 1.3.1 is out.

    Consider lodging it on the issue tracker here:

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