如何在OpenGL中截取屏幕截图

Question

如何在C++中截取OpenGL窗口的截图并将其保存到文件中.

我发现了这个glReadPixels()功能,但我不知道下一步该做什么.例如,我可以在哪里设置文件的路径？

如果不困难,请写代码.

Answer 1

这段代码捕获OpenGL窗口并导出到BMP文件.您必须具有FreeImage库才能运行它.

// Make the BYTE array, factor of 3 because it's RBG.
BYTE* pixels = new BYTE[3 * width * height];

glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, pixels);

// Convert to FreeImage format & save to file
FIBITMAP* image = FreeImage_ConvertFromRawBits(pixels, width, height, 3 * width, 24, 0x0000FF, 0xFF0000, 0x00FF00, false);
FreeImage_Save(FIF_BMP, image, "C:/test.bmp", 0);

// Free resources
FreeImage_Unload(image);
delete [] pixels;

Answer 2

glReadPixels会将这些位复制到您提供的内存缓冲区中.您必须手动格式化数据(到您选择的图像格式)并在 glReadPixels返回后将其写入磁盘.

Answer 3

可运行的例子

每次在窗口上单击鼠标时,tmpX.ppm都会使用当前屏幕截图创建一个文件.

您可以eog在Linux上查看此文件,并使用文本编辑器进行检查.

要在不显示窗口的情况下进行渲染,请参阅:如何使用GLUT/OpenGL渲染文件？

#include <math.h>
#include <stdlib.h>
#include <stdio.h>

#define GL_GLEXT_PROTOTYPES 1
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <GL/glext.h>

static GLubyte *pixels = NULL;
static const GLenum FORMAT = GL_RGBA;
static const GLuint FORMAT_NBYTES = 4;
static const unsigned int HEIGHT = 500;
static const unsigned int WIDTH = 500;
static unsigned int nscreenshots = 0;
static unsigned int time;

/* Model. */
static double angle = 0;
static double angle_speed = 45;

static void init(void)  {
    glReadBuffer(GL_BACK);
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    glViewport(0, 0, WIDTH, HEIGHT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);

    pixels = malloc(FORMAT_NBYTES * WIDTH * HEIGHT);
    time = glutGet(GLUT_ELAPSED_TIME);
}

static void deinit(void)  {
    free(pixels);
}

static void create_ppm(char *prefix, int frame_id, unsigned int width, unsigned int height,
        unsigned int color_max, unsigned int pixel_nbytes, GLubyte *pixels) {
    size_t i, j, k, cur;
    enum Constants { max_filename = 256 };
    char filename[max_filename];
    snprintf(filename, max_filename, "%s%d.ppm", prefix, frame_id);
    FILE *f = fopen(filename, "w");
    fprintf(f, "P3\n%d %d\n%d\n", width, HEIGHT, 255);
    for (i = 0; i < height; i++) {
        for (j = 0; j < width; j++) {
            cur = pixel_nbytes * ((height - i - 1) * width + j);
            fprintf(f, "%3d %3d %3d ", pixels[cur], pixels[cur + 1], pixels[cur + 2]);
        }
        fprintf(f, "\n");
    }
    fclose(f);
}

static void draw_scene() {
    glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    glRotatef(angle, 0.0f, 0.0f, -1.0f);
    glBegin(GL_TRIANGLES);
    glColor3f(1.0f, 0.0f, 0.0f);
    glVertex3f( 0.0f,  0.5f, 0.0f);
    glColor3f(0.0f, 1.0f, 0.0f);
    glVertex3f(-0.5f, -0.5f, 0.0f);
    glColor3f(0.0f, 0.0f, 1.0f);
    glVertex3f( 0.5f, -0.5f, 0.0f);
    glEnd();
}

static void display(void) {
    draw_scene();
    glutSwapBuffers();
    glReadPixels(0, 0, WIDTH, HEIGHT, FORMAT, GL_UNSIGNED_BYTE, pixels);
}

static void idle(void) {
    int new_time = glutGet(GLUT_ELAPSED_TIME);
    angle += angle_speed * (new_time - time) / 1000.0;
    angle = fmod(angle, 360.0);
    time = new_time;
    glutPostRedisplay();
}

void mouse(int button, int state, int x, int y) {
    if (state == GLUT_DOWN) {
        puts("screenshot");
        create_ppm("tmp", nscreenshots, WIDTH, HEIGHT, 255, FORMAT_NBYTES, pixels);
        nscreenshots++;
    }
}

int main(int argc, char **argv) {
    GLint glut_display;
    glutInit(&argc, argv);
    glutInitWindowSize(WIDTH, HEIGHT);
    glutInitWindowPosition(100, 100);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
    glutCreateWindow(argv[0]);
    init();
    glutDisplayFunc(display);
    glutIdleFunc(idle);
    glutMouseFunc(mouse);
    atexit(deinit);
    glutMainLoop();
    return EXIT_SUCCESS;
}

编译:

gcc main.c -lm -lGL -lGLU -lglut

在Ubuntu 15.10,OpenGL 4.5.0 NVIDIA 352.63上测试.

Answer 4

一个简单快速的解决方案。

• 输出 TARGA 文件，但可以轻松转换为 PNG（提供脚本）。
• 不需要额外的库。
• 可以与 C 和 C++ 一起使用（有一些细微的变化）。
• 注意：输出文件应具有tga文件扩展名。

这是代码：

void saveScreenshotToFile(std::string filename, int windowWidth, int windowHeight) {    
    const int numberOfPixels = windowWidth * windowHeight * 3;
    unsigned char pixels[numberOfPixels];

    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    glReadBuffer(GL_FRONT);
    glReadPixels(0, 0, windowWidth, windowHeight, GL_BGR_EXT, GL_UNSIGNED_BYTE, pixels);

    FILE *outputFile = fopen(filename.c_str(), "w");
    short header[] = {0, 2, 0, 0, 0, 0, (short) windowWidth, (short) windowHeight, 24};

    fwrite(&header, sizeof(header), 1, outputFile);
    fwrite(pixels, numberOfPixels, 1, outputFile);
    fclose(outputFile);

    printf("Finish writing to file.\n");
}

并调用该函数：

saveScreenshotToFile("test.tga", 1200, 900);

将 TARGA 文件转换为 PNG 的 bash 脚本：

for oldFileName in *.tga; do
    [ -f "$oldFileName" ] || break         # Break out if no .tga files found.
    newFileName=${oldFileName//.tga/.png}
    convert $oldFileName $newFileName
    rm $oldFileName
    echo "Converted $oldFileName to $newFileName"
done