bmp_output.cpp

#include "bmp_output.h"
#include <cmath>
#include <vector>
#include <string>
#include <fstream>
#include "vector.h"

void saveBMP(const std::string& filename, int width, int height, const std::vector<Vector3>& framebuffer) {
    // BMP requires row padding to a multiple of 4 bytes
    int rowStride = width * 3;
    int padding = (4 - (rowStride % 4)) % 4;
    int fileSize = 54 + (rowStride + padding) * height;

    unsigned char header[54] = {
        // Bitmap file header (14 bytes)
        'B', 'M',                // Signature
        0,0,0,0,                 // File size (will fill below)
        0,0, 0,0,                // Reserved
        54,0,0,0,                // Pixel data offset

        // DIB header (BITMAPINFOHEADER — 40 bytes)
        40,0,0,0,                // Header size
        0,0,0,0,                 // Width (will fill below)
        0,0,0,0,                 // Height (will fill below)
        1,0,                     // Planes
        24,0,                    // 24 bits per pixel
        0,0,0,0,                 // Compression (0 = none)
        0,0,0,0,                 // Image size (ignored for uncompressed)
        0,0,0,0,                 // X pixels per meter
        0,0,0,0,                 // Y pixels per meter
        0,0,0,0,                 // Colors in color table
        0,0,0,0                  // Important colors
    };

    // Fill dynamic parts of header
    header[ 2] = (unsigned char)(fileSize      );
    header[ 3] = (unsigned char)(fileSize >>  8);
    header[ 4] = (unsigned char)(fileSize >> 16);
    header[ 5] = (unsigned char)(fileSize >> 24);

    header[18] = (unsigned char)(width      );
    header[19] = (unsigned char)(width >>  8);
    header[20] = (unsigned char)(width >> 16);
    header[21] = (unsigned char)(width >> 24);

    header[22] = (unsigned char)(height      );
    header[23] = (unsigned char)(height >>  8);
    header[24] = (unsigned char)(height >> 16);
    header[25] = (unsigned char)(height >> 24);

    std::ofstream out(filename, std::ios::binary);
    out.write((char*)header, 54);

    unsigned char pad[3] = {0, 0, 0};

    // BMP stores scanlines bottom → top
    for (int y = height - 1; y >= 0; --y) {
        for (int x = 0; x < width; ++x) {
            const Vector3& c = framebuffer[y * width + x];

            unsigned char r = (unsigned char)(std::min(1.0f, c.x) * 255.0f);
            unsigned char g = (unsigned char)(std::min(1.0f, c.y) * 255.0f);
            unsigned char b = (unsigned char)(std::min(1.0f, c.z) * 255.0f);

            unsigned char pixel[3] = { b, g, r }; // BMP is BGR
            out.write((char*)pixel, 3);
        }
        out.write((char*)pad, padding); // Row padding
    }
}