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bumperman

OpenGL How to tell GL_TRIANGLE_STRIP has finished?

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bumperman    122
Hi,

I am a little bit confused about triangle strips and triangles. I am actually programming on OpenGL ES 2.0, but I think it doesn't matter as far as these functions are concerned.

I am trying to draw multiple 2D rectangles. I have a rectangle class for drawing a rectangle. Single rectangle is drawn with glDrawArrays (or glDrawElements, it's convertible). If I want to add second rectangle, it, as expected, continues drawing from the last vertex of the last rectangle.

I probably can achieve this with GL_TRIANGLES instead of GL_TRIANGLE_STRIP. I, however, wonder whether it's possible to tell OpenGL that GL_TRIANGLE_STRIP has ended and I want to make new triangle strips?

Thanks!

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bumperman    122
I may have found a solution, but that didn't work, may be I am doing it wrong.

I call glDisableVertexAttribArray for disabling the previous attribute (vertex position) but that didn't work either. Here is a simplified code:


static const GLfloat rect_vertices[] = {
-0.5f, -0.33f,
0.5f, -0.33f,
-0.5f, 0.33f,
0.5f, 0.33f,
};

size_t rect_count = 0;
// for each rectangle...
shader_program& sp = current_rect.get_shader_program();
sp->use_program();
sp->set_uniform<float>("translate", translation);
sp->set_vertex_positions(vertex_data->pointer()); // points to copy of rect_vertices
sp->enable_attribute(0); //0th position is vertex position
// then I call
glDrawArrays(GL_TRIANGLE_STRIP, rect_count * 4, 4);
sp->disable_vertex_positions();
sp->disable_texture_coordinates(); // which is another attribute
sp->dont_use(); // calls glUseProgram(0);
++rect_count;
// go back to foreach, for the next rectangle




Shader is super very simple. Vertex shader:

attribute vec4 position;
attribute vec2 a_texCoord;

varying vec2 v_texCoord;

uniform float translate;

void main()
{
gl_Position = position;
gl_Position.y += sin(translate) / 2.0;

v_texCoord = a_texCoord;
}



Fragment shader:

precision mediump float;
varying vec2 v_texCoord;
uniform sampler2D s_texture;

void main()
{
//vec4 mycol = vec4(0.5, 0.0, 0.75, 1.0);
gl_FragColor = texture2D(s_texture, v_texCoord);
}


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Promit    13246
Generally speaking you would achieve this by using degenerate triangles; you can google for plenty of information. Sometimes devices also support primitive restart, but it's never been very popular and is hardware dependent. For the vast majority of devices, especially ES based ones, you should probably use strip ordered indexed lists.

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bumperman    122
I have rewritten parts of engine code to use glDrawElements with indexed vertices instead. I again use GL_TRIANGLE_STRIP but each 2D rectangle calls glDrawElements with their own vertex coordinates (but indices are the same).

Thank you for the help. I will probably come back to ask how can I deal with z-ordering of these rectangles.

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