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Author Topic: Why it doesn't draw a rectangle ?  (Read 125 times)

t0m

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Why it doesn't draw a rectangle ?
« on: August 15, 2017, 14:00:39 »

Hi everyone, newbie here :)
I have followed some tutorials to simply draw triangles with OpenGL using LWJGL.
Here is my code :
Code: [Select]
/**** Game.java ****/
public class Game {
private Screen screen;
private String title;
private int width, height;
private long windowID;
public Game(String title, int width, int height) {
this.init();
}

private void init() {
GLFWErrorCallback.createPrint(System.err).set();
if ( !glfwInit() )
throw new IllegalStateException("Unable to initialize GLFW");
System.out.println("GLFW initalized !");
}
public void create(Screen screen) {
glfwDefaultWindowHints();
                // Asking for OpenGL 3.X, cause OS X use 2.X by default
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
glfwWindowHint(GLFW_VISIBLE, GLFW_FALSE);
glfwWindowHint(GLFW_RESIZABLE, GLFW_FALSE);
windowID = glfwCreateWindow(this.width, this.height, this.title, NULL, NULL);
if ( windowID == NULL )
throw new RuntimeException("Failed to create the GLFW window");
try ( MemoryStack stack = stackPush() ) {
IntBuffer pWidth = stack.mallocInt(1);
IntBuffer pHeight = stack.mallocInt(1);
glfwGetWindowSize(windowID, pWidth, pHeight);
GLFWVidMode vidmode = glfwGetVideoMode(glfwGetPrimaryMonitor());
glfwSetWindowPos(
windowID,
(vidmode.width() - pWidth.get(0)) / 2,
(vidmode.height() - pHeight.get(0)) / 2
);
}
glfwMakeContextCurrent(windowID);
glfwSwapInterval(1);
glfwShowWindow(windowID);
GL.createCapabilities();

                // Here is called the Screen.init() method
setScreen(screen);

loop();
}

private void loop() {
if (screen != null) {
glClearColor(screen.getClearColor().x, screen.getClearColor().y, screen.getClearColor().z, screen.getClearColor().w);
} else {
glClearColor(1.0f, 0.0f, 0.0f, 0.0f);
}
while ( !glfwWindowShouldClose(windowID) ) {
render();
}
destroy();
}

private void render() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glfwSwapBuffers(windowID);
glfwPollEvents();
if (screen != null) {
this.screen.update();
this.screen.render();
}
}
public void setScreen(Screen screen) {
if (this.screen != null)
this.screen.destroy();
this.screen = screen;
this.screen.init();
}
    }
Code: [Select]
/**** TestScreen.java ****/
public class TestScreen extends Screen {
float[] vertices = {
        // Left bottom triangle
        -0.5f, 0.5f, 0f,
        -0.5f, -0.5f, 0f,
        0.5f, -0.5f, 0f,
        // Right top triangle
        0.5f, -0.5f, 0f,
        0.5f, 0.5f, 0f,
        -0.5f, 0.5f, 0f
};
VAO vao;
public void init() {
//System.out.println("OpenGL v." + GL11.glGetString(GL11.GL_VERSION));
setClearColor(new Vector4f(0.5f, 0.3f, 1f, 1f));
vao = new VAO(vertices);
}
public void render() {
// Bind to the VAO that has all the information about the quad vertices
GL30.glBindVertexArray(vao.getVAO());
GL20.glEnableVertexAttribArray(0);
// Draw the vertices
GL11.glDrawArrays(GL11.GL_TRIANGLES, 0, vao.getVerticesCount());
// Put everything back to default (deselect)
GL20.glDisableVertexAttribArray(0);
GL30.glBindVertexArray(0);
}
public void update() {}
public void destroy() {
vao.destroy();
}
Code: [Select]
/**** VAO.java ****/
public class VAO {
private int vaoID;
private int vboID;
private int verticesCount;
public VAO(float[] vertices) {
// VAO creation
vaoID = GL30.glGenVertexArrays();
// Bind the VAO
GL30.glBindVertexArray(vaoID);
// VBO creation
vboID = GL15.glGenBuffers();
// Bind the VBO
GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER, vboID);
// Push the vertices array in the VBO
GL15.glBufferData(GL15.GL_ARRAY_BUFFER, floatToFloatBuffer(vertices), GL15.GL_STATIC_DRAW);
// Push the VBO in the VAO at index 0
GL20.glVertexAttribPointer(0, 3, GL11.GL_FLOAT, false, 0, 0);
// Deselect all
GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER, 0);
GL30.glBindVertexArray(0);
verticesCount = vertices.length / 3;
}
public FloatBuffer floatToFloatBuffer(float[] vertices) {
FloatBuffer verticesBuffer = BufferUtils.createFloatBuffer(vertices.length);
verticesBuffer.put(vertices);
verticesBuffer.flip();
return verticesBuffer;
}
public void destroy() {
// Delete the VBO
GL15.glBindBuffer(GL15.GL_ARRAY_BUFFER, 0);
GL15.glDeleteBuffers(vboID);
// Delete the VAO
GL30.glBindVertexArray(0);
GL30.glDeleteVertexArrays(vaoID);
}
It does just draw an empty purple screen, but no white rectangle...
Thanks.
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Kai

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Re: Why it doesn't draw a rectangle ?
« Reply #1 on: August 15, 2017, 14:05:25 »

There is at least one issue that can be seen immediately in your code:
Code: [Select]
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glfwSwapBuffers(windowID);
You are clearing the backbuffer (and what was rendered to it previously), and then immediately swap it to front. That does not make any sense. The sequence is:
1. clear
2. render (draw calls)
3. swapbuffers

But your current sequence is:
1. clear
2. swapbuffers
3. render (draw calls)

or equivalently but more obvious:
1. render (draw calls)
2. clear    <-- clearing right after rendering? No.
3. swapbuffers

that means that whatever got rendered in 3 in iteration i-1 will immediately be cleared in iteration i.

If you suspect any more errors in your code, you can also run it with LWJGLX/debug and see what it outputs.
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t0m

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Re: Why it doesn't draw a rectangle ?
« Reply #2 on: August 15, 2017, 14:51:33 »

Thanks for reply, I changed the order of lines as you said, but still no rectangle (but thx for explanations).
I didn't know LWJGL Debug so I will test.
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t0m

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Re: Why it doesn't draw a rectangle ?
« Reply #3 on: August 15, 2017, 17:14:44 »

So I tried LWJGLX Debug and it gives me this error :
Code: [Select]
glDrawArrays produced error: 1282 (GL_INVALID_OPERATION)Any idea ?
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Kai

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Re: Why it doesn't draw a rectangle ?
« Reply #4 on: August 15, 2017, 17:23:39 »

https://stackoverflow.com/questions/12017175/what-can-cause-gldrawarrays-to-generate-a-gl-invalid-operation-error

Work your way through the cases. It seems, though, that the second answer might be your case.
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