[Gambas-user] gb3: OpenGL, rotation and translation of matrix affecting multiple quads

[Kevin Fishburne]

Here's looking at you Tomek, although anyone who knows give me a shout out.

I'm using a nested For...Next loop to render a 2D grid of quads. I need 
the grid of quads to be rotated about its "center", but would like to 
make a single matrix transformation instead of doing one for each quad. 
Is this possible, and if so, how? Even if I have to do multiple matrix 
transformations, that is fine, as long as I don't have to do one for 
every single quad.

Right now my code (removing all experiments, just bare code) looks like 
this:

Public Sub Tile_Grid()

   ' Draw tile grid texture array in render window.

   ' General declarations.
   Dim TileGridX As Short  ' Tile in tile grid texture array being rendered.
   Dim TileGridY As Short  ' Tile in tile grid texture array being rendered.
   Dim TileGridX1 As Short ' Starting tile in tile grid texture array to 
be rendered.
   Dim TileGridY1 As Short ' Starting tile in tile grid texture array to 
be rendered.
   Dim TileGridX2 As Short ' Ending tile in tile grid texture array to 
be rendered.
   Dim TileGridY2 As Short ' Ending tile in tile grid texture array to 
be rendered.
   Dim PixelX As Short     ' Pixel coordinates of tile being rendered.
   Dim PixelY As Short     ' Pixel coordinates of tile being rendered.
   Dim OffsetX As Short    ' Number of pixels to offset matrix to 
accommodate camera position and tile grid resolution.
   Dim OffsetY As Short    ' Number of pixels to offset matrix to 
accommodate camera position and tile grid resolution.
   Dim StepX As Short      ' Direction to step through tile grid texture 
array loop.
   Dim StepY As Short      ' Direction to step through tile grid texture 
array loop.

   ' Assign initial values to variables.
   TileGridX1 = TileGrid.CenterX
   TileGridY1 = TileGrid.CenterY
   TileGridX2 = TileGrid.CenterX + TileGrid.Size - 1
   TileGridY2 = TileGrid.CenterY + TileGrid.Size - 1
   If TileGridX1 < TileGridX2 Then
     StepX = 1
   Else
     StepX = -1
   Endif
   If TileGridY1 < TileGridY2 Then
     StepY = 1
   Else
     StepY = -1
   Endif

   ' Rotate and translate matrix.
   Gl.LoadIdentity()
   Gl.Translatef(- OffsetX, - OffsetY, 0)
   Gl.Rotatef(- Client.Orientation, 0, 0, 1)
   Gl.Translatef(OffsetX, OffsetY, 0)

   ' Render specified tiles in cell grid to tile grid.
   For TileGridY = TileGridY1 To TileGridY2 Step StepY
     For TileGridX = TileGridX1 To TileGridX2 Step StepX
       ' Select texture using its ID.
       Gl.BindTexture(Gl.GL_TEXTURE_2D, tTileGrid[Convert.Wrap_Short(0, 
TileGrid.Size - 1, TileGridX), Convert.Wrap_Short(0, TileGrid.Size - 1, 
TileGridY)][0])
       ' Create the quad the texture is drawn on.
       Gl.Begin(Gl.GL_QUADS)
         ' Bottom-left vertex.
         Gl.TexCoord2i(0, 0)
         Gl.Vertex3i(PixelX + OffsetX, PixelY + OffsetY, 0)
         ' Bottom-right vertex.
         Gl.TexCoord2i(1, 0)
         Gl.Vertex3i(PixelX + OffsetX + 128, PixelY + OffsetY, 0)
         ' Top-right vertex.
         Gl.TexCoord2i(1, 1)
         Gl.Vertex3i(PixelX + OffsetX + 128, PixelY + OffsetY + 128, 0)
         ' Top-left vertex.
         Gl.TexCoord2i(0, 1)
         Gl.Vertex3i(PixelX + OffsetX, PixelY + OffsetY + 128, 0)
       Gl.End()
       ' Adjust pixel position.
       PixelX = PixelX + 128
       If PixelX = TileGrid.Size * 128 Then PixelX = 0
     Next
     ' Adjust pixel positions.
     PixelX = 0
     PixelY = PixelY + 128
     If PixelY = TileGrid.Size * 128 Then PixelY = 0
   Next

   ' Reset matrices so subsequent SDL writes won't get botched.
   Gl.LoadIdentity()

End

Notice I didn't assign values to OffsetX or OffsetY. Didn't want to 
include any failed experiments. I also attached the code for better 
legibility.

I can offset the position of the quads easily using Gl.Translatef, but 
the group ALWAYS seems to rotate with the upper-left corner as the 
origin. I need to be able to control the origin. I can do this easily 
with a single quad, but seem to have trouble doing it "globally" with 
the For...Next loops of quads.

-- 
Kevin Fishburne
Eight Virtues
www:http://sales.eightvirtues.com
e-mail:sales at ...1887...
phone: (770) 853-6271

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