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cwinapigdi

The pen join style not applied on all corners of a shape

Line joins

The shape is drawn with the Polyline() function. The relevant code is here:

void DoDrawing(HWND hwnd) {

    LOGBRUSH brush;
    COLORREF col = RGB(0, 0, 0);
    DWORD pen_style = PS_SOLID | PS_JOIN_MITER | PS_GEOMETRIC;

    brush.lbStyle = BS_SOLID;
    brush.lbColor = col;
    brush.lbHatch = 0;       

    PAINTSTRUCT ps;

    HDC hdc = BeginPaint(hwnd, &ps);

    HPEN hPen1 = ExtCreatePen(pen_style, 8, &brush, 0, NULL);
    HPEN holdPen = SelectObject(hdc, hPen1);

    POINT points[5] = { { 10, 30 }, { 100, 30 }, { 100, 100 }, { 10, 100 }, {10, 30}};
    Polyline(hdc, points, 5);
    DeleteObject(hPen1);

    SelectObject(hdc, holdPen);

    EndPaint(hwnd, &ps);  
}

The PS_JOIN_MITER is applied on three corners but not on the top-left corner. On that corner the default PS_JOIN_ROUND is used. How to fix this?

The following is a full working example:

#include <windows.h>

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
void DoDrawing(HWND);

int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    PWSTR lpCmdLine, int nCmdShow) {

    MSG  msg;
    WNDCLASSW wc = {0};

    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.lpszClassName = L"Pens";
    wc.hInstance     = hInstance;
    wc.hbrBackground = (HBRUSH) GetStockObject(WHITE_BRUSH);
    wc.lpfnWndProc   = WndProc;
    wc.hCursor       = LoadCursor(0, IDC_ARROW);

    RegisterClassW(&wc);
    CreateWindowW(wc.lpszClassName, L"Line joins",
          WS_OVERLAPPEDWINDOW | WS_VISIBLE,
          100, 100, 250, 180, NULL, NULL, hInstance, NULL);

    while (GetMessage(&msg, NULL, 0, 0)) {

        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

  return (int) msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg,
    WPARAM wParam, LPARAM lParam) {

    switch(msg) {

        case WM_PAINT:

            DoDrawing(hwnd);
            break;

        case WM_DESTROY:

            PostQuitMessage(0);
            return 0;
    }

    return DefWindowProcW(hwnd, msg, wParam, lParam);
}

void DoDrawing(HWND hwnd) {

    LOGBRUSH brush;
    COLORREF col = RGB(0, 0, 0);
    DWORD pen_style = PS_SOLID | PS_JOIN_MITER | PS_GEOMETRIC;

    brush.lbStyle = BS_SOLID;
    brush.lbColor = col;
    brush.lbHatch = 0;       

    PAINTSTRUCT ps;

    HDC hdc = BeginPaint(hwnd, &ps);

    HPEN hPen1 = ExtCreatePen(pen_style, 8, &brush, 0, NULL);
    HPEN holdPen = SelectObject(hdc, hPen1);

    POINT points[5] = { { 10, 30 }, { 100, 30 }, { 100, 100 }, { 10, 100 }, {10, 30}};
    Polyline(hdc, points, 5);
    DeleteObject(hPen1);

    SelectObject(hdc, holdPen);

    EndPaint(hwnd, &ps);  
}
Solution

  • Polyline does not join the first and last point.

    Use Polygon(hdc, points, 5) instead of Polyline

    Also, select the oldPen in to DC before deleting the existing pen, in this order:

    SelectObject(hdc, holdPen);
DeleteObject(hPen1);

    (although Windows will forgive you if you don't do it in the right order)