Legacy GM Explain pr_trianglestrip?

[Guest User 1]

I'm using primitives to draw the sides of an isometric cube. The documentation described a set of commands that made a rectangle. Thinking I understood this, I went and made the 3 polygons, but this didn't work. There is a corner missing from each polygon, and triangles appear to be being drawn on top of each-other.

Here is my code:

draw_primitive_begin( pr_trianglestrip ) ;
draw_vertex_colour( 16 ,  0 , $009900 , 1 ) ;
draw_vertex_colour( 32 ,  8 , $009900 , 1 ) ;
draw_vertex_colour( 16 , 16 , $009900 , 1 ) ;
draw_vertex_colour(  0 ,  8 , $009900 , 1 ) ;
draw_primitive_end() ;

draw_primitive_begin( pr_trianglestrip ) ;
draw_vertex_colour( 32 ,  8 , $663300 , 1 ) ;
draw_vertex_colour( 32 , 24 , $663300 , 1 ) ;
draw_vertex_colour( 16 , 32 , $663300 , 1 ) ;
draw_vertex_colour( 16 , 16 , $663300 , 1 ) ;
draw_primitive_end() ;

draw_primitive_begin( pr_trianglestrip ) ;
draw_vertex_colour(  0 ,  8 , $663300 , 0.8 ) ;
draw_vertex_colour( 16 , 16 , $663300 , 0.8 ) ;
draw_vertex_colour( 16 , 32 , $663300 , 0.8 ) ;
draw_vertex_colour(  0 , 24 , $663300 , 0.8 ) ;
draw_primitive_end() ;

While a fixed version would be nice, I'd much rather have an explanation as to how Game Maker draws the triangles based on the points given.

Thanks for any help.

 

[CaffeineJunky]

pr_trianglestrip will reuse the last two vertices as part of the next triangle.

For example, if I want to draw a square, I can define this using only four vertices, instead of six. However, this can be tricky if you are using backface culling, because you still need to have a counter-clockwise orientation for each one.

 

[Guest User 1]

pr_trianglestrip will reuse the last two vertices as part of the next triangle.

For example, if I want to draw a square, I can define this using only four vertices, instead of six. However, this can be tricky if you are using backface culling, because you still need to have a counter-clockwise orientation for each one.

This is helpful. Then if I'm using textures, are the last two values in the vertex function from 0 to 1? or are they pixel coordinates? For instance if I want the bottom right of a 32x32 texture, do I use 1, 1? Or 32, 32?

 

[CaffeineJunky]

The last two values I believe (and you can check the documentation) are numbers between 0 and 1. If you have a texture sheet that is 64x64, and you want to draw the top-right 32x32 square on the geometry, then you would use (0.5, 0), (0.5, 0.5), (1, 0), and (1, 0.5) for the last two parameters.

 

[Guest User 1]

Below I'm using a texture with the 0-1 value for the dimensions of a 32x32 sprite that is a radial gradient going from white in the middle to black around the edges. When I run the code, I get a black rhombus. Right shape, no texture. I tried adding in draw_set_colour(c_white) at the beginning as it does in the documentation, but then I get a white rhombus. What's happening?

draw_primitive_begin_texture( pr_trianglestrip , sprite0 ) ;
draw_vertex_texture( 32 ,  0 , 1 ,  0 ) ;
draw_vertex_texture( 64 , 16 , 1 , 1 ) ;
draw_vertex_texture(  0 , 16 ,  0 ,  0 ) ;
draw_vertex_texture( 32 , 32 ,  0 , 1 ) ;
draw_primitive_end() ;

 

[CaffeineJunky]

You might need to use sprite_get_texture or background_get_texture. See the page in the documentation for draw_primitive_begin_texture.