Delphi OpenGL resize - delphi

Question

I've a question about using OpenGL from Delphi.
My intention is to draw a grid of n rows and n columns in a form. i want the cells in the grid to be square like this image:
This is the code I use to create this grid:
unit Main;
interface
uses
Winapi.Windows, Winapi.Messages, Winapi.OpenGL, System.SysUtils,
System.Variants, System.Classes, Vcl.Graphics, Vcl.Controls, Vcl.Forms,
Vcl.Dialogs, Vcl.Menus;
type
TfrmMain = class(TForm)
PopupMenu: TPopupMenu;
mnuDrawGrid: TMenuItem;
procedure FormCreate(Sender: TObject);
procedure mnuDrawGridClick(Sender: TObject);
procedure FormDestroy(Sender: TObject);
procedure FormResize(Sender: TObject);
private
glDC: HDC;
oldW: Integer;
oldH: Integer;
glContext: HGLRC;
errorCode: GLenum;
openGLReady: Boolean;
procedure DisegnaLinea(const aX1, aY1, aX2, aY2: Double; aSpessore,
aOffSet: Integer);
procedure DisegnaTabellone(const aNumRighe, aNumColonne: SmallInt);
public
{ Public declarations }
end;
var
frmMain: TfrmMain;
implementation
{$R *.dfm}
{ Gestione form -------------------------------------------------------------- }
// OpenGL initialization
procedure TfrmMain.FormCreate(Sender: TObject);
var
pfd: TPixelFormatDescriptor;
formatIndex: Integer;
begin
FillChar(pfd, SizeOf(pfd), 0);
with pfd do
begin
nSize := SizeOf(pfd);
nVersion := 1;
dwFlags := PFD_DRAW_TO_WINDOW or PFD_SUPPORT_OPENGL;
iPixelType := PFD_TYPE_RGBA;
cColorBits := 24;
cDepthBits := 32;
iLayerType := PFD_MAIN_PLANE;
end;
glDC := GetDC(Handle);
formatIndex := ChoosePixelFormat(glDC, #pfd);
if formatIndex = 0 then
raise Exception.Create('Choose pixel format failed ' + IntToStr(GetLastError));
if not SetPixelFormat(glDC, formatIndex, #pfd) then
raise Exception.Create('Set pixel forma failed ' + IntToStr(GetLastError));
glContext := wglCreateContext(glDC);
if not glContext = 0 then
raise Exception.Create('Create context failed ' + IntToStr(GetLastError));
if not wglMakeCurrent(glDC, glContext) then
raise Exception.Create('Make current failsed ' + IntToStr(GetLastError));
glMatrixMode(GL_PROJECTION);
glLoadIdentity;
glOrtho(0.0, ClientWidth, 0.0, ClientHeight, 0.0, 1.0);
oldW := ClientWidth;
oldH := ClientHeight;
openGLReady := True;
end;
// OpenGL destruction
procedure TfrmMain.FormDestroy(Sender: TObject);
begin
wglMakeCurrent(Canvas.Handle, 0);
wglDeleteContext(glContext);
end;
// Form resize
procedure TfrmMain.FormResize(Sender: TObject);
begin
if not openGLReady then
exit;
glViewport(0, 0, ClientWidth, ClientHeight);
errorCode := glGetError;
if errorCode <> GL_NO_ERROR then
raise Exception.Create('Form resize: ' + gluErrorString(errorCode));
if (ClientWidth <> oldW) and (ClientHeight <> oldH) then
DisegnaTabellone(10, 10);
oldW := ClientWidth;
oldH := ClientHeight;
end;
{ Gestione menu -------------------------------------------------------------- }
// Menu option grid drawing
procedure TfrmMain.mnuDrawGridClick(Sender: TObject);
begin
DisegnaTabellone(10, 10);
end;
{ OpenGL --------------------------------------------------------------------- }
// Draw a line at aX1, aY1 to aX2, aY2 coordinates
procedure TfrmMain.DisegnaLinea(const aX1, aY1, aX2, aY2: Double; aSpessore,
aOffSet: Integer);
begin
glEnable(GL_LINE_SMOOTH);
glLineWidth(aSpessore);
glBegin(GL_LINES);
glVertex2f(aX1, aY1);
glVertex2f(aX2, aY2);
glEnd;
end;
// Grid design
procedure TfrmMain.DisegnaTabellone(const aNumRighe, aNumColonne: SmallInt);
const
vOffSet = 20;
var
idx: SmallInt;
pX: Double;
pY: Double;
incX: Double;
incY: Double;
hPos: Double;
hWidth: Double;
widthArea: Integer;
heightArea: Integer;
aspectRatio: Double;
begin
glClearColor(1.0, 1.0, 1.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0, 0.0, 0.0);
widthArea := ClientWidth;
heightArea := ClientHeight;
aspectRatio := widthArea / heightArea;
pY := vOffSet / 2;
incY := (heightArea - vOffSet) / aNumRighe;
pX := (widthArea - aNumColonne * incY) / 2;
hPos := pX;
hWidth := hPos + aNumColonne * incY;
incX := incY;
// Draw vertical lines
for idx := 0 to aNumColonne do begin
DisegnaLinea(pX, vOffSet / 2, pX, heightArea - vOffSet / 2, 3, 0);
pX := pX + incX;
end;
// Draw horizontal lines
for idx := 0 to aNumRighe do begin
DisegnaLinea(hPos, pY, hWidth, pY, 3, 0);
pY := pY + incY;
end;
glFlush;
end;
end.
I initialize the pixel format description in the FormCreate event and Ive created a routine called by a popup menu in order to draw the grid. In this routine I make same calculation because I want the grid cells to be square as I said. So I take the height of the form (ClientHeight) and I divide it for the number of rows I need: in this case 10 plus i'ce a little offset in order to have a margin on the top and bottom of the form. Then I calculate the width of the grid for a perfet square. It works well but the problem come when I resize the form.
My intension is to draw a new grid smaller or larger according to the form dimensions, but this is only my intention because the grid does'nt mantein the correct aspet ratio and is drawn in a bad manner as ou can see in the following picure:
I can't understand where is the error in my code. I'm nwe at OpenGL so can someone help me?
Eros

Answer 1

Although you set the new glViewport() on resize, you still use the old orthographic projection matrix which is no more relevant.
So, all you need is an additional glOrtho() before setting a new viewport.
OpenGL immediate mode (glBegin(), glEnd(), glVertex(), etc.) had been obsolete for nearly twenty years; you really shouldn`t be using it in 2017.