CODEWITHSUNDEEP
copenglsdl-2
Draeton
Draeton

Reputation: 135

Why doesn't this code draw a triangle?

I'm very new to OpenGL and I just wrote up a section of code using SDL 2 that to my knowledge should have drawn a triangle, but this code doesn't seem to work and so I am not done learning. I've got all the initialization code SDL 2 documentation says I need already written in, and the functions returned by dynamic loading ARE callable. When I execute this code instead of a triangle I get a black (but cleared) window. Why does this code not draw the triangle I want, and why is the window cleared to black by this code? I want to know the technical details behind mainly the first question so I can depend on it later.

(*main_context.glViewport)(0, 0, 100, 100);
(*main_context.glBegin)(GL_TRIANGLES);
(*main_context.glColor4d)(255, 255, 255, 255);
(*main_context.glVertex3d)(1, 1, -50);
(*main_context.glVertex3d)(1, 30, 1);
(*main_context.glVertex3d)(30, 1, 1);
(*main_context.glEnd)();
(*main_context.glFinish)();
(*main_context.glFlush)();
SDL_GL_SwapWindow(window);

Update:

I've revised my code to include different coordinates and I got the triangle to draw, but I cannot get it to draw when farther away. Why is that?

(*main_context.glVertex3d)(2, -1, 1); /* Works. */
(*main_context.glVertex3d)(2, -1, 3); /* Doesn't work. */

Upvotes: 2

Views: 246

Answers (1)

bcrist
bcrist

Reputation: 1530

Unless you are setting up a projection and/or modelview matrix elsewhere in your code, it's using the default (identity matrix) transform, which is an orthographic projection with (-1, -1) at the bottom left and (1, 1) at the top right. glViewport only changes the portion of the default framebuffer being rendered to, it has no bearing on the projection whatsoever.

With an orthographic projection, the Z coordinate does not affect the screen-space position of a point, except that points outside the Z clipping planes will not be rendered. In this case, that's everything outside of -1 <= z <= 1. Given that one of your points is (1, 1, -50), this seems to be your problem.

Upvotes: 1

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