copy paste it to dump float rgba textures (*data = the pointer I'd give in TexImage2D)

dump_float_rgba_tex.c

static bool
dump_float_image_rgba(const char *fname,
		      int w, int h,
		      float *data)
{
	FILE *fp;
	int i;
	float *p;

	if (!(fp = fopen(fname, "wb"))) {
		fprintf(stderr, "Failed to open file: %s.\n", fname);
		return false;
	}

	p = data;
	fprintf(fp, "P6\n%d %d\n255\n", w, h);
	for (i = 0; i < w * h; i++) {
		fputc(p[0] * 255, fp);
		fputc(p[1] * 255, fp);
		fputc(p[2] * 255, fp);

		p += 4;
	}

	fclose(fp);
	return true;
}