How to turn an argb pixel array into an SDL surface? - c

Question

Using C and SDL2, I have a pixel array with an ARGB8888 format.
Uint32 *pixels = (Uint32 *) malloc (sizeof(Uint32)*(Uint32)windowWidth*(Uint32)windowHeight);
I want to put all that pixel information into a new SDL_Surface ready to save as a .bmp. How do I do this?
I'm unsure because new surfaces have RGBA8888 format and sdl convert functions require an existing surface to convert to a new surface. And there is no function which simply passes all pixel array values to a surface, so I know it will involve some sort of loop which assigns the pixels one by one.

Answer 1

You can manually save bitmap bits to a file. Bitmap files are usually 24bit or less, and bitmap viewers tend to ignore alpha. The code below assumes bits is 32-bit pixels. If you need 24-bit format, the input has to be converted from ARGB to RGB, and you must also account for padding.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#pragma pack(push, 1)
typedef struct my_BITMAPFILEHEADER {
uint16_t bfType;
uint32_t bfSize;
uint16_t bfReserved1;
uint16_t bfReserved2;
uint32_t bfOffBits;
}my_BITMAPFILEHEADER;
typedef struct my_BITMAPINFOHEADER {
uint32_t biSize;
int32_t biWidth;
int32_t biHeight;
uint16_t biPlanes;
uint16_t biBitCount;
uint32_t biCompression;
uint32_t biSizeImage;
int32_t biXPelsPerMeter;
int32_t biYPelsPerMeter;
uint32_t biClrUsed;
uint32_t biClrImportant;
}my_BITMAPINFOHEADER;
#pragma pack(pop)
int copy(uint8_t* bits, int width, int height, int bitcount)
{
//compiler test:
if(sizeof(my_BITMAPFILEHEADER) != 14 && sizeof(my_BITMAPINFOHEADER) != 40)
{
printf("bitmap structures not packed properly\n");
return 0;
}
//width_in_bytes is roughly w * bytes_per_pixel, it takes padding in to account
int width_in_bytes = ((width * bitcount + 31) / 32) * 4;
uint32_t imagesize = width_in_bytes * height;
my_BITMAPFILEHEADER filehdr = { 0 };
my_BITMAPINFOHEADER infohdr = { 0 };
memcpy(&filehdr, "BM", 2);
filehdr.bfSize = 54 + imagesize;
filehdr.bfOffBits = 54;
infohdr.biSize = 40;
infohdr.biPlanes = 1;
infohdr.biWidth = width;
infohdr.biHeight = height;
infohdr.biBitCount = bitcount;
infohdr.biSizeImage = imagesize;
FILE *fout = fopen("test.bmp", "wb");
fwrite(&filehdr, sizeof(filehdr), 1, fout);
fwrite(&infohdr, sizeof(infohdr), 1, fout);
fwrite((char*)bits, 1, imagesize, fout);
fclose(fout);
return 1;
}
Usage:
copy(bits, width, height, 32);
//or
copy(bits, width, height, 24);