How to change the resolution of the image RGB in the buffer?

Well, you can otresayzit any library that works with graphics (opencv, ImageMagic, thousands of them). You can otresayzit its function - bilinear interpolation is written quite simply. Opencv example:

// допустим, картинка хранится в каком-нибудь std::string std::string image_source = "..."; std::vector<unsigned char> bytes(image_source.data(), image_source.data() + image_source.size()); cv::Mat image = cv::imdecode(bytes, CV_LOAD_IMAGE_COLOR); cv::resize(image, image, cv::Size(1280, 720)); // обратно в буфер std::vector<unsigned char> resized; cv::imencode(".png", image, resized); 

I supplement the answer .
At the request in the comments I cite the "manual" implementation of bilinear interpolation. Since the picture needs to be read in some way, for simplicity I have chosen libjpg for this.
Errors (for short) are not processed.
Functions for reading pictures are also provided, so that it is better to understand exactly how the picture is stored in memory (3 channels, one-dimensional array).

Bilinear interpolation is in the im_scale_factor() function.

 #include <jpeglib.h> struct image_t{ image_t() : w(0), h(0), pixels(0) {} unsigned char *pixels; unsigned int w; unsigned int h; }; struct pixel_t { unsigned char b; unsigned char g; unsigned char r; }; bool read_jpg(const char *path, image_t *image) { FILE *file = fopen(path, "rb"); struct jpeg_decompress_struct info; struct jpeg_error_mgr err; info.err = jpeg_std_error( &err ); jpeg_create_decompress( &info ); jpeg_stdio_src( &info, file ); jpeg_read_header( &info, true ); jpeg_start_decompress( &info ); int w = info.output_width; int h = info.output_height; int numChannels = info.num_components; // 3 = RGB, 4 = RGBA unsigned long dataSize = w * h * numChannels; // read RGB(A) scanlines one at a time into data[] unsigned char *data = (unsigned char *)malloc( dataSize ); unsigned char* rowptr; while ( info.output_scanline < h ) { rowptr = data + info.output_scanline * w * numChannels; jpeg_read_scanlines( &info, &rowptr, 1 ); } jpeg_finish_decompress( &info ); fclose( file ); image->pixels = data; image->h = h; image->w = w; return true; } void write_jpg(const char *path, image_t *image) { struct jpeg_compress_struct cinfo; jpeg_create_compress(&cinfo); FILE *fp = fopen(path, "w+"); jpeg_stdio_dest(&cinfo, fp); struct jpeg_error_mgr jerr; cinfo.err = jpeg_std_error(&jerr); cinfo.image_width = image->w; cinfo.image_height = image->h; cinfo.input_components = 3; cinfo.in_color_space = JCS_RGB; jpeg_set_defaults(&cinfo); jpeg_start_compress(&cinfo, FALSE); unsigned char *stride = (unsigned char *)malloc(image->w * 3); unsigned char *data = (unsigned char*)image->pixels; JSAMPROW row_pointer[1]; for (int i = 0; i < image->h; ++i) { memcpy(stride, data + i * image->w * 3, image->w * 3); row_pointer[0] = stride; jpeg_write_scanlines(&cinfo, &stride, 1); } jpeg_finish_compress(&cinfo); jpeg_destroy_compress(&cinfo); fclose(fp); } pixel_t pixel(image_t *im, int h, int w) { pixel_t p; pb = im->pixels[h*im->w*3 + w*3 + 0]; pg = im->pixels[h*im->w*3 + w*3 + 1]; pr = im->pixels[h*im->w*3 + w*3 + 2]; return p; } void im_scale_factor(image_t *src, image_t *dest, float h_factor, float w_factor) { for (int i = 0; i < dest->h; ++i) { float fY = ((float) i) / h_factor; int iY = (int) fY; for (int j = 0; j < dest->w; ++j) { float fX = ((float) j) / w_factor; int iX = (int) fX; pixel_t q11 = pixel(src, iY, iX); pixel_t q12 = pixel(src, iY + 1, iX); pixel_t q21 = pixel(src, iY, iX + 1); pixel_t q22 = pixel(src, iY + 1, iX + 1); unsigned char r1_b = (iX + 1 - fX)/(iX + 1 - iX)*q11.b + (fX - iX)/(iX + 1 - iX)*q21.b; unsigned char r1_g = (iX + 1 - fX)/(iX + 1 - iX)*q11.g + (fX - iX)/(iX + 1 - iX)*q21.g; unsigned char r1_r = (iX + 1 - fX)/(iX + 1 - iX)*q11.r + (fX - iX)/(iX + 1 - iX)*q21.r; unsigned char r2_b = (iX + 1 - fX)/(iX + 1 - iX)*q12.b + (fX - iX)/(iX + 1 - iX)*q22.b; unsigned char r2_g = (iX + 1 - fX)/(iX + 1 - iX)*q12.g + (fX - iX)/(iX + 1 - iX)*q22.g; unsigned char r2_r = (iX + 1 - fX)/(iX + 1 - iX)*q12.r + (fX - iX)/(iX + 1 - iX)*q22.r; unsigned char b = (iY + 1 - fY)/(iY + 1 - iY)*r1_b + (fY - iY)/(iY + 1 - iY)*r2_b; unsigned char g = (iY + 1 - fY)/(iY + 1 - iY)*r1_g + (fY - iY)/(iY + 1 - iY)*r2_g; unsigned char r = (iY + 1 - fY)/(iY + 1 - iY)*r1_r + (fY - iY)/(iY + 1 - iY)*r2_r; dest->pixels[i*dest->w*3 + j*3 + 0] = b; dest->pixels[i*dest->w*3 + j*3 + 1] = g; dest->pixels[i*dest->w*3 + j*3 + 2] = r; } } } void im_scale(image_t *src, image_t *dest, int h, int w) { std::cerr << "scale to : " << w << " x " << h << "\n"; dest->h = h; dest->w = w; dest->pixels = (unsigned char*)malloc(h * w * 3); float scale_x = w / (float)src->w; float scale_y = h / (float)src->h; im_scale_factor(src, dest, scale_y, scale_x); } int main(int argc, char *argv[]) { if (argc < 3) { std::cerr << "Usage: " << argv[0] << " in out" << std::endl; return -1; } image_t image; read_jpg(argv[1], &image); image_t result; im_scale(&image, &result, 1280, 720); write_jpg(argv[2], &result); return 0; } 

To use will require libjpg:

 g++ main.cpp -ljpeg