[SOLVED] OpenGL Lighting Issue
lazarus / 70-OpenGL

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Handoko · 2020-03-10 13:24

Can anyone tell me what I did wrong why rotating the pyramid will
cause it become darker then gradually bright again?

Download the code and run, press left/right shift to rotate. The demo
requires LazOpenGLContext (and libgl1-mesa-dev if you use Linux).

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i unit
unit1; {$mode objfpc}{$H+} interface uses  Classes, SysUtils, Forms,
Controls, ExtCtrls, LCLType, LclIntf,  OpenGLContext, GL, math; type  
{ TForm1 }   TForm1 = class(TForm)    OpenGLControl1: TOpenGLControl; 
  Timer1: TTimer;    procedure FormCreate(Sender: TObject);   
procedure OpenGLControl1Paint(Sender: TObject);    procedure
Timer1Timer(Sender: TObject);  private    FRotation: GLfloat;   
procedure InitGL;  end; var  Form1: TForm1; const  LightDiffuse:
array[0..3] of Single = (1, 1, 1, 1); implementation procedure
gluPerspective(fovy, aspect, zNear, zFar: GLdouble);var  Width,
Height: Double;begin  Height := tan(fovy * pi / 360) * zNear;  Width
 := Height * aspect;  glFrustum(-Width, Width, -Height, Height, zNear,
zFar);end; {$R *.lfm} { TForm1 } procedure
TForm1.OpenGLControl1Paint(Sender: TObject);const  AlreadyInit:
Boolean = False;begin  if not(AlreadyInit) then    InitGL;  
glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT);  glLoadIdentity; 
glTranslatef(0, 0, -5);  glRotatef(FRotation, 0, 1, 0); 
glBegin(GL_TRIANGLES);    glColor3f (1, 0, 0); glVertex3f( 0,  1,
 0);    glColor3f (0, 1, 0); glVertex3f(-1, -1,  1);    glColor3f (0,
0, 1); glVertex3f( 1, -1,  1);    glColor3f (1, 0, 0); glVertex3f( 0,
 1,  0);    glColor3f (0, 0, 1); glVertex3f( 1, -1,  1);    glColor3f
(0, 1, 0); glVertex3f( 1, -1, -1);    glColor3f (1, 0, 0); glVertex3f(
0,  1,  0);    glColor3f (0, 1, 0); glVertex3f( 1, -1, -1);   
glColor3f (0, 0, 1); glVertex3f(-1, -1, -1);    glColor3f (1, 0, 0);
glVertex3f( 0,  1,  0);    glColor3f (0, 0, 1); glVertex3f(-1, -1,
-1);    glColor3f (0, 1, 0); glVertex3f(-1, -1,  1);  glEnd;  
OpenGLControl1.SwapBuffers;end; procedure TForm1.FormCreate(Sender:
TObject);begin  OpenGLControl1.Anchors := [akTop, akLeft, akBottom,
akRight];  OpenGLControl1.MultiSampling := 8;  FRotation :=
0;end; procedure TForm1.Timer1Timer(Sender: TObject);begin 
OpenGLControl1.Invalidate;   if GetKeyState(VK_RSHIFT) < 0 then   
FRotation := FRotation - 0.8  else    if GetKeyState(VK_LSHIFT) < 0
then      FRotation := FRotation + 0.8;end; procedure
TForm1.InitGL;begin  glShadeModel(GL_SMOOTH);  glClearColor(0, 0, 0,
0);  glClearDepth(1);  glEnable(GL_DEPTH_TEST); 
glDepthFunc(GL_LEQUAL);  glHint(GL_PERSPECTIVE_CORRECTION_HINT,
GL_NICEST);   glViewport(0, 0, OpenGLControl1.Width,
OpenGLControl1.Height);  glMatrixMode(GL_PROJECTION); 
glLoadIdentity;  gluPerspective(45,
OpenGLControl1.Width/OpenGLControl1.Height, 0.1, 100); 
glMatrixMode(GL_MODELVIEW);  glLoadIdentity;  
glEnable(GL_COLOR_MATERIAL);  glEnable(GL_LIGHTING); 
glLightfv(GL_LIGHT1, GL_DIFFUSE, LightDiffuse); 
glEnable(GL_LIGHT1);end; end.

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eljo · 2020-03-10 13:29

I didn't looked at the code yet but from my experience the problem is
usually that you rotate the camera instead of the pyramid and you do
not rotate the light as well.
Just a quick fyi I'll take a closer look later.

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Handoko · 2020-03-10 13:33

That's sound possible. But I don't think it happens on my case.
Actually in my 'complete' code, it has several objects. Only the
object that rotates becomes darker, while the others stay correct as
what they should be (as long as they don't rotate). What I submitted
here was a simplified version, consisted only 1 object.

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eljo · 2020-03-10 13:37

Oh well shoot in dark missed I'll look at the code from home later
tonight.

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Handoko · 2020-03-11 17:47

I just found an article saying that we can get weird result if
enabling light without setting the polygon normal.

Problem solved.

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ChrisR · 2020-03-12 15:15

You might want to look for at the code for plyview
https://github.com/neurolabusc/plyview - it provides Pascal code to
estimate surface normals for any arbitrary triangular mesh.

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Handoko · 2020-03-12 15:44

That's great. I was thinking how to auto calculate the normals. So far
I model using Blender and export to obj file, the normals will be
provided.

Thank you very much.

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wp · 2020-03-12 17:30

Maybe you got the solution already, but your problem motivated me to
brush up my geometry knowledge.

It should be noted that normals usually must point outwards. To find
the correct order of points you must arrange the points such that they
follow the direction of your fingers of your right hand - then the
thumb points outward.

And the normal vectors must be normalized to unit length.

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Handoko · 2020-03-12 18:11

Very educative, thank you for providing that comments in the source
code.

I already knew about the normal must point outside and ccw rules. I am
more interested in the normal calculations. As I am only high school
graduated, I'm very weak in vector equation.

Your improvement of the code works but there was a small not serious
bug in my code. Can you please change the line #139, #140 to:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   if
not(AlreadyInit) then  begin    InitGL;    AlreadyInit := True;  end;
So, we can add the demo to the how-to demo page.

What I suggested to change, almost has no impact on the performance or
whatever. But that was inefficient in the code.

Actually the other way to initialize the OpenGL, as found in Lazarus
tutorials is to include enabling TOpenGLContex.AutoResizeViewport. But
I did it differently, which is more efficient. Because enabling
AutoResizeViewport will always 'reconfigure' the viewport every time
the OnPaint event is called, while mine is only once.  That's why I
need to put the InitGL at the beginning of OnPaint. Of course the
performance gain will be less than 1 millisecond but why not if you
know it.

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wp · 2020-03-12 18:54

OK. Made the requested change. Re-phrased some comments and added on-
screen instructions how to use the program.

Quote from: Handoko on March 12, 2020, 07:11:35 pmAs I am only high
school graduated, I'm very weak in vector equation.
The key word to search for is "vector cross product". This is some
weird way to multiply two vectors in a way that the resulting vector
is perpendicular to the plane defined by the two vectors.

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ChrisR · 2020-03-15 11:37

Handoko, your OpenGL project uses legacy OpenGL. Be aware that GTK3
drops legacy support (requiring OpenGL Core). Likewise, while MacOS
supports legacy OpenGL, it only does so up to OpenGL 2.1. If you want
to use modern OpenGL features in MacOS, you must use OpenGL core.
Therefore, targeting OpenGL core ensures your Lazarus projects are
'write once, compile anywhere' and can target Windows, GTK2, GTK3, QT5
and Cocoa.

You can see some Lazarus OpenGL core demos here
  https://github.com/neurolabusc/OpenGLCoreTutorials
Also, the Metal-Demos can target either Metal or OpenGL-Core on MacOS,
and OpenGL Core on other platforms:
  https://github.com/neurolabusc/Metal-Demos

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Handoko · 2020-03-15 11:54

I knew. I am starting to learn the newer OpenGL but before I really
start it I am now making sure I really fully understand the legacy
mode. And thank you for the links, I am sure to study them but later.