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[solved] Modified opengl-tutorial.org code access violation at 0


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#1 nonsenseinc   Members   -  Reputation: 102

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Posted 18 November 2012 - 02:17 AM

Greetings,

I am trying to learn OpenGL with the tutorials at opengl-tutorial.org and started modifying tutorial two that basically draws a simple triangle. In addiction to changing some of the utility code that seems to work for me I tried to add another triangle to draw a square that I would want to texture later.

Now I ran into my first problem that I cannot identify. I am getting an access violation at position 0 at glDisableVertexAttribArray.

My Code:
#include <GL/glew.h>
#include <GL/glfw.h>
#include <glm/glm.hpp>

#include <string>
#include <fstream>
#include <streambuf>
#include <iostream>

using namespace glm;
using namespace std;

GLuint loadShaders(const string vertexShaderPath, const string fragmentShaderPath) {
	GLuint vertexShaderID = glCreateShader(GL_VERTEX_SHADER);
	GLuint fragmentShaderID = glCreateShader(GL_FRAGMENT_SHADER);

	string vertexShaderCode;
	ifstream vertexShaderStream(vertexShaderPath);

	vertexShaderStream.seekg(0, ios::end);
	vertexShaderCode.reserve(vertexShaderStream.tellg());
	vertexShaderStream.seekg(0, ios::beg);

	vertexShaderCode.assign((istreambuf_iterator<char>(vertexShaderStream)), istreambuf_iterator<char>());

	string fragmentShaderCode;
	ifstream fragmentShaderStream(fragmentShaderPath);

	fragmentShaderStream.seekg(0, ios::end);
	fragmentShaderCode.reserve(fragmentShaderStream.tellg());
	fragmentShaderStream.seekg(0, ios::beg);

	fragmentShaderCode.assign((istreambuf_iterator<char>(fragmentShaderStream)), istreambuf_iterator<char>());

	const char *vertexShaderSource = vertexShaderCode.c_str();
	glShaderSource(vertexShaderID, 1, &vertexShaderSource, NULL);
	glCompileShader(vertexShaderID);

	const char *fragmentShaderSource = fragmentShaderCode.c_str();
	glShaderSource(fragmentShaderID, 1, &fragmentShaderSource, NULL);
	glCompileShader(fragmentShaderID);

	GLuint programID = glCreateProgram();
	glAttachShader(programID, vertexShaderID);
	glAttachShader(programID, fragmentShaderID);
	glLinkProgram(programID);

	glDeleteShader(vertexShaderID);
	glDeleteShader(fragmentShaderID);

	return programID;
}

int main(int argc, char* argv[]) {
	glfwInit();

	glfwOpenWindowHint(GLFW_FSAA_SAMPLES, 4);
	glfwOpenWindowHint(GLFW_OPENGL_VERSION_MAJOR, 3);
	glfwOpenWindowHint(GLFW_OPENGL_VERSION_MINOR, 3);
	glfwOpenWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

	glfwOpenWindow(1024, 768, 0, 0, 0, 0, 32, 0, GLFW_WINDOW);
	glfwSetWindowTitle("RTS");
	glfwEnable(GLFW_STICKY_KEYS);
	
	glewExperimental = GL_TRUE;
	glewInit();

	glClearColor(0.0f, 1.0f, 0.0f, 0.0f);

	GLuint VertexArrayID;
	glGenVertexArrays(1, &VertexArrayID);
	glBindVertexArray(VertexArrayID);
	
	GLuint programID = loadShaders("VertexShader.glsl", "FragmentShader.glsl");

	const GLfloat vertexBufferData[] = {
		-1.0f, -1.0f, 0.0f,
		-1.0f,  1.0f, 0.0f,
		 1.0f, -1.0f, 0.0f,
		 1.0f, -1.0f, 0.0f,
		 1.0f,  1.0f, 0.0f,
		-1.0f,  1.0f, 0.0f
	};

	GLuint vertexBuffer;
	glGenBuffers(1, &vertexBuffer);
	glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
	glBufferData(GL_ARRAY_BUFFER, sizeof(vertexBufferData), vertexBufferData, GL_STATIC_DRAW);

	do {
		glClear(GL_COLOR_BUFFER_BIT);

		glUseProgram(programID);

		glEnableVertexAttribArray(0);
		glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
		glVertexAttribPointer(0, 6, GL_FLOAT, GL_FALSE, 0, (void*)0);
		glDrawArrays(GL_TRIANGLES, 0, 6);

		glDisableVertexAttribArray(0);

		glfwSwapBuffers();
	}
	while(glfwGetKey(GLFW_KEY_ESC) != GLFW_PRESS && glfwGetWindowParam(GLFW_OPENED));

	glDeleteBuffers(1, &vertexBuffer);
	glDeleteProgram(programID);
	glDeleteVertexArrays(1, &VertexArrayID);

	glfwTerminate();

	return 0;
}
(original on http://www.opengl-tu...first-triangle/)

The shader code is the same as the original, it compiles fine and the original code does run perfectly.

Please help me fix my code, thank you!

Cheers,
Jan

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#2 Sponji   Members   -  Reputation: 1249

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Posted 18 November 2012 - 04:04 AM

I think your glVertexAttribPointer call is wrong, the size should be 3 instead of 6.
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);

Edited by Sponji, 18 November 2012 - 04:05 AM.

Derp

#3 nonsenseinc   Members   -  Reputation: 102

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Posted 18 November 2012 - 04:12 AM

Why is that? Running it with "3" (as the tutorial does) does actually work, but renders only one triangle (the first three vertices). I do aim to render six vertices making up two triangles forming a square.

//Edit: having VertexAttribPointer at 3 and DrawArrays at 6 seems to work and makes sense now. I did not see the functions context and that it does not refer to the vertex count of the entire buffer but to the format thereof.

Edited by nonsenseinc, 18 November 2012 - 04:16 AM.





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