C++ SDL2: How to render rects into multiple viewports

问题

I am new into SDL2(version 2.0.10) and I teach it from Lazy Foo tutorial. In Lesson of The ViewPort example code render me only first image in left viewport and others not. Render of rects in different viewports don't work either. What I am doing wrong when I want render rects in different viewports like this:

while( !quit ){

while( SDL_PollEvent( &e ) != 0 ){
if( e.type == SDL_QUIT ){
 quit = true;
}
}

//Clear screen
SDL_SetRenderDrawColor( gRenderer, 0xFF, 0xFF, 0xFF, 0xFF );
SDL_RenderClear( gRenderer );

//Top left corner viewport
SDL_Rect topLeftViewport;
topLeftViewport.x = 0;
topLeftViewport.y = 0;
topLeftViewport.w = SCREEN_WIDTH / 2;
topLeftViewport.h = SCREEN_HEIGHT / 2;
SDL_RenderSetViewport( gRenderer, &topLeftViewport );

SDL_Rect fillRect = { 10, 10, 100, 100 };
SDL_SetRenderDrawColor( gRenderer, 0xFF, 0x00, 0x00, 0xFF );
SDL_RenderFillRect( gRenderer, &fillRect );

SDL_Rect topRightViewport;
topRightViewport.x = SCREEN_WIDTH / 2;
topRightViewport.y = 0;
topRightViewport.w = SCREEN_WIDTH / 2;
topRightViewport.h = SCREEN_HEIGHT / 2;
SDL_RenderSetViewport( gRenderer, &topRightViewport );

SDL_Rect fillRect2 = { 10, 10, 100, 100 };
SDL_SetRenderDrawColor( gRenderer, 0x00, 0xFF, 0x00, 0xFF );
SDL_RenderFillRect( gRenderer, &fillRect2 );

SDL_RenderPresent( gRenderer );
}

回答1:

As I saw, you are drawing a rect at 10, 10 of 100x100 two times, this coordinates doesn't take account of the viewport coordinates. And in a way, if you want to do this by changing the viewport clipping is not relevant... just draw the square where you want.

UPDATE: I tried multiples viewports. Here is a working example. You can choose between the renderer (with viewports) or the classic way.

main.cpp :

#include "Application.hpp"

int
main (int argc, char * argv[])
{
    Application app("SDL 2 Test", 800, 600, true);

    return app.mainLoop();
}

Application.hpp :

#ifndef APPLICATION_HPP
#define APPLICATION_HPP

#include <SDL2/SDL.h>

class Application
{
    public:

        Application (const char * title = "UnknownApplication", int baseWidth = 640, int baseHeight = 480, bool useRenderer = false) noexcept;

        ~Application ();

        int mainLoop () noexcept;

    private:

        int m_width = 0;
        int m_height = 0;
        SDL_Window * m_window = nullptr;
        SDL_Renderer * m_renderer = nullptr;
        bool m_useRenderer = false;
        bool m_isRunning = false;
};

#endif /* APPLICATION_HPP */

Application.cpp :

#include "Application.hpp"

#include <iostream>

#include <SDL2/SDL_image.h>

Application::Application (const char * title, int baseWidth, int baseHeight, bool useRenderer) noexcept
    : m_width(baseWidth), m_height(baseHeight), m_useRenderer(useRenderer)
{
    if ( SDL_Init(SDL_INIT_VIDEO) != 0 )
    {
        std::cerr << "SDL could not initialize! SDL_Error: " << SDL_GetError() << std::endl;

        return;
    }

    m_window = SDL_CreateWindow(title, SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, m_width, m_height, SDL_WINDOW_SHOWN);

    if ( m_window == nullptr )
    {
        std::cerr << "Window could not be created! SDL_Error: " << SDL_GetError() << std::endl;

        return;
    }

    if ( m_useRenderer )
    {
        m_renderer = SDL_CreateRenderer(m_window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC | SDL_RENDERER_TARGETTEXTURE);

        if ( m_renderer == nullptr )
        {
            std::cerr << "Renderer could not be created! SDL_Error: " << SDL_GetError() << std::endl;

            return;
        }
    }

    m_isRunning = true;
}

int
Application::mainLoop () noexcept
{
    SDL_Event event;

    const auto filepath = "SET_IMAGE_YOU_WANT_HERE";

    auto surface = IMG_Load(filepath);

    if ( surface == nullptr )
    {
        std::cerr << "Unable to read image file : " << filepath << std::endl;

        return 1;
    }

    SDL_Rect logoPosition = {8, 8, 32, 32};

    if ( m_useRenderer )
    {
        auto texture = SDL_CreateTextureFromSurface(m_renderer, surface);

        SDL_Rect screenA = {0, 0, m_width / 2, m_height / 2};
        SDL_Rect screenB = {m_width / 2, 0, m_width / 2, m_height / 2};

        while ( m_isRunning )
        {
            while ( SDL_PollEvent(&event) != 0 )
            {
                if ( event.type == SDL_QUIT )
                    m_isRunning = false;
            }

            SDL_SetRenderDrawColor(m_renderer, 0, 0, 0, 255);
            SDL_RenderClear(m_renderer);

            SDL_RenderSetViewport(m_renderer, &screenA);
            SDL_SetRenderDrawColor(m_renderer, 255, 0, 0, 255);
            SDL_RenderFillRect(m_renderer, nullptr);
            SDL_RenderCopy(m_renderer, texture, nullptr, &logoPosition);

            SDL_RenderSetViewport(m_renderer, &screenB);
            SDL_SetRenderDrawColor(m_renderer, 0, 255, 0, 255);
            SDL_RenderFillRect(m_renderer, nullptr);
            SDL_RenderCopy(m_renderer, texture, nullptr, &logoPosition);

            SDL_RenderPresent(m_renderer);
        }

        SDL_DestroyTexture(texture);
    }
    else
    {
        auto windowSurface = SDL_GetWindowSurface(m_window);

        while ( m_isRunning )
        {
            while ( SDL_PollEvent(&event) != 0 )
            {
                if ( event.type == SDL_QUIT )
                    m_isRunning = false;
            }

            SDL_FillRect(windowSurface, nullptr, SDL_MapRGB(windowSurface->format, 0xFF, 0x00, 0xFF));

            //SDL_BlitSurface(surface, nullptr, windowSurface, &logoPosition);
            SDL_BlitScaled(surface, nullptr, windowSurface, &logoPosition);

            SDL_UpdateWindowSurface(m_window);
        }
    }

    SDL_FreeSurface(surface);

    return 0;
}

Application::~Application (void)
{
    if ( m_renderer != nullptr )
        SDL_DestroyRenderer(m_renderer);

    if ( m_window != nullptr )
        SDL_DestroyWindow(m_window);

    SDL_Quit();
}

来源：https://stackoverflow.com/questions/59913756/c-sdl2-how-to-render-rects-into-multiple-viewports