Missing faces when displaying STL data - c++

Question

I wrote a simple parser for the ASCII STL format. When I try to render the triangles with the supplied normals, the resulting object is missing many faces:
This is how it should look like:
What I already tried:
explicitly disabled backface culling (though it shouldn't have been active before)
ensured that the depth buffer is enabled
Here is a minimal sample program which reproduces the error:
#include <SDL2/SDL.h>
#include <SDL2/SDL_main.h>
#include <SDL2/SDL_render.h>
#include <SDL2/SDL_opengl.h>
int main(int argc, char **argv) {
SDL_Init(SDL_INIT_VIDEO);
int screen_w=1280,screen_h=720;
SDL_Window * win = SDL_CreateWindow("test", 20, 20, screen_w, screen_h,
SDL_WINDOW_OPENGL);
SDL_GLContext glcontext = SDL_GL_CreateContext(win);
STLParser stlparser;
std::ifstream file(".\\logo.stl");
stlparser.parseAscii(file);
const auto& ndata = stlparser.getNData();
const auto& vdata = stlparser.getVData();
std::cout << "number of facets: " << ndata.size() << std::endl;
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
glMatrixMode(GL_PROJECTION | GL_MODELVIEW);
glLoadIdentity();
glScalef(1.f, -1.f, 1.f);
glOrtho(0, screen_w, 0, screen_h, -screen_w, screen_w);
glClearDepth(1.0f);
glDepthFunc(GL_LEQUAL);
glEnable(GL_DEPTH_TEST);
glDisable(GL_CULL_FACE);
glEnableClientState(GL_NORMAL_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glNormalPointer(GL_FLOAT, 0, ndata.data());
glVertexPointer(3, GL_FLOAT, 0, vdata.data());
SDL_Event event;
bool quit = false;
while (!quit) {
while (SDL_PollEvent(&event))
switch(event.type) {
case SDL_QUIT: quit = true; break;
}
;
// Drawing
glClearColor(255,255,255,255);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glTranslatef(screen_w/2,0,0);
glRotatef(0.5,0,1,0);
glTranslatef(-screen_w/2,0,0);
glPushMatrix();
glTranslatef(screen_w/2,screen_h/2,0);
glColor3f(0.5,0.5,0);
glDrawArrays(GL_TRIANGLES, 0, vdata.size());
glPopMatrix();
SDL_GL_SwapWindow(win);
SDL_Delay(10);
}
SDL_DestroyWindow(win);
SDL_Quit();
return 0;
}
The STLParser methods getNData() and getVData() have the following signatures:
const std::vector<std::array<float,3>>& getNData() const;
const std::vector<std::array<std::array<float,3>,3>>& getVData() const;
STLParser output should be correct, but I can provide the sources as well if needed.
What am I doing wrong?

Answer 1

You should change
glDrawArrays(GL_TRIANGLES, 0, vdata.size());
to
glDrawArrays(GL_TRIANGLES, 0, 3 * vdata.size());
I.e. count should be vertex count, but not triangle count.