ラズパイでフレームバッファ(/dev/fb0)を使用して、直接ディスプレイ画像を入出力する ref: https://qiita.com/take-iwiw/items/0a7a2fefec9d93cdf6db

file0.txt

cat /dev/fb0 > aaa.raw

file1.txt

fbset

mode "800x600"
    geometry 800 600 800 600 32
    timings 0 0 0 0 0 0 0
    rgba 8/16,8/

file2.txt

cat /sys/class/graphics/fb0/bits_per_pixel
32

cat /sys/class/graphics/fb0/virtual_size
800,600

getSizeFrameBuffer.cpp

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/errno.h>
#include <sys/fcntl.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <linux/fb.h>
#include <linux/omapfb.h>

void getFrameBufferSize(int* width, int* height, int* colorWidth)
{
    int fd;
    int n;
    char str[64];

    /* Get Bit Per Pixel (usually 32-bit(4Byte) */
    fd = open("/sys/class/graphics/fb0/bits_per_pixel", O_RDONLY);
    n = read(fd, str, sizeof(str));
    str[n] = '\0';
    *colorWidth = atoi(str);
    close(fd);

    /* Get Frame Buffer Size */
    fd = open("/sys/class/graphics/fb0/virtual_size", O_RDONLY);
    n = read(fd, str, sizeof(str));
    str[n] = '\0';
    /* divide "800,600" into "800" and "600" */
    *width  = atoi(strtok(str, ","));
    *height = atoi(strtok(NULL, ","));
    close(fd);
}

void getFrameBufferSize2(int* width, int* height, int* colorWidth)
{
    struct fb_var_screeninfo var;
    int fd;
    fd = open("/dev/fb0", O_RDWR);
    ioctl(fd, FBIOGET_VSCREENINFO, &var);
    *colorWidth = var.bits_per_pixel;
    *width      = var.xres_virtual;
    *height     = var.yres_virtual;
    close(fd);
}

main.cpp

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

void getFrameBufferSize(int* width, int* height, int* colorWidth);
void getFrameBufferSize2(int* width, int* height, int* colorWidth);
void drawColor(int width, int height, int color);
void drawColor2(int width, int height, int color);
void readBuffer(int width, int height, void* dstBuffer);
void readBuffer2(int width, int height, void* dstBuffer);
void saveFileBinary(const char* filename, void* data, int size);

// g++ main.cpp getSizeFrameBuffer.cpp readFrameBuffer.cpp writeFrameBuffer.cpp

int main()
{
    int colorWidth;
    int width;
    int height;

    /* get frame buffer size */
    getFrameBufferSize(&width, &height, &colorWidth);
    printf("%d(W) x %d(H) x %d(Bit)\n", width, height, colorWidth);

    /* screen capture */
    uint32_t *buffer = new uint32_t[width * height];
    readBuffer(width, height, buffer);
    saveFileBinary("capture.raw", buffer, width * height * 4);
    delete[] buffer;

    /* Fill solid color */
    drawColor(width, height, 0xFFFF0000);   // RED
}


void saveFileBinary(const char* filename, void* data, int size)
{
    FILE *fp;
    fp = fopen(filename, "wb");
    fwrite(data, 1, size, fp);
    fclose(fp);
}

readFrameBuffer.cpp

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/errno.h>
#include <sys/fcntl.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <linux/fb.h>
#include <linux/omapfb.h>

void readBuffer(int width, int height, void* dstBuffer)
{
    int fd;
    fd = open("/dev/fb0", O_RDONLY);
    read(fd, dstBuffer, width * height * 4);
    close(fd);
}

void readBuffer2(int width, int height, void* dstBuffer)
{
    int fd;
    fd = open("/dev/fb0", O_RDONLY);
    uint32_t *frameBuffer = (uint32_t *)mmap(NULL, width * height * 4, PROT_READ, MAP_SHARED, fd, 0);
    for(int y = 0; y < height; y++) {
        for(int x = 0; x < width; x++) {
            ((uint32_t*)dstBuffer)[x + y * width] = frameBuffer[x + y * width]; 
        }
    }

    munmap(frameBuffer, width * height * 4);
    close(fd);
}

writeFrameBuffer.cpp

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/errno.h>
#include <sys/fcntl.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <linux/fb.h>
#include <linux/omapfb.h>

// color = AARRGGBB
void drawColor(int width, int height, int color)
{
    uint32_t *canvas = new uint32_t[width * height];
    for(int y = 0; y < height; y++) {
        for(int x = 0; x < width; x++) {
            canvas[x + y * width] = color;  
        }
    }

    int fd;
    fd = open("/dev/fb0", O_WRONLY);
    write(fd, canvas, width * height * 4);
    close(fd);
    delete[] canvas;
}

// color = AARRGGBB
void drawColor2(int width, int height, int color)
{
    int fd;
    fd = open("/dev/fb0", O_RDWR);
    uint32_t *frameBuffer = (uint32_t *)mmap(NULL, width * height * 4, PROT_WRITE, MAP_SHARED, fd, 0);
    for(int y = 0; y < height; y++) {
        for(int x = 0; x < width; x++) {
            frameBuffer[x + y * width] = color; 
        }
    }

    msync(frameBuffer, width * height * 4, 0);
    munmap(frameBuffer, width * height * 4);
    close(fd);
}