Add basic bitmap output

src/bmp.c

@@ -0,0 +1,93 @@
+#include "bmp.h"
+#include "ff.h"
+
+typedef struct
+{
+    uint8_t b;
+    uint8_t m;
+    uint32_t bitmap_file_size;
+    uint32_t reserved;
+    uint32_t bitmap_data_offset;
+} __attribute__((packed)) BMP_FileHeader_t;
+
+typedef struct
+{
+    uint32_t header_size;
+    uint16_t bitmap_width;
+    uint16_t bitmap_height;
+    uint16_t colour_planes;
+    uint16_t bits_per_pixel;
+} __attribute((packed)) BMP_CoreHeader_t;
+
+void BMP_Save(uint8_t *data, int width, int height)
+{
+    FATFS fs;
+    FIL fp;
+    FRESULT rc;
+    unsigned int bw;
+
+    rc = f_mount(&fs, "", 0);
+    if(rc)
+    {
+        return;
+    }
+
+    rc = f_open(&fp, "IMAGE.BMP", FA_WRITE | FA_CREATE_ALWAYS);
+    if(rc)
+    {
+        return;
+    }
+
+    // Rows must be padded to a multiple if 4 bytes
+    int row_size = width / 8;
+    int padding_length = (4 - (row_size % 4)) % 4;
+    uint8_t padding_bytes[4] = {0};
+    row_size += padding_length;
+
+    BMP_FileHeader_t file_header =
+    {
+        .b = 'B',
+        .m = 'M',
+        .bitmap_file_size = 14 + 12 + row_size * height,
+        .bitmap_data_offset = 14 + 12 + 6
+    };
+
+    // Use the simplest possible header
+    BMP_CoreHeader_t core_header =
+    {
+        .header_size = 12,
+        .bitmap_width = width,
+        .bitmap_height = height,
+        .colour_planes = 1,
+        .bits_per_pixel = 1
+    };
+
+    // At one bit per pixel, we need a colour table
+    uint8_t colour_table[6] =
+    {
+        0xff, 0xff, 0xff,
+        0x00, 0x00, 0x00
+    };
+
+    if(f_write(&fp, &file_header, sizeof(file_header), &bw) != FR_OK)
+        return;
+    if(f_write(&fp, &core_header, sizeof(core_header), &bw) != FR_OK)
+        return;
+    if(f_write(&fp, &colour_table, sizeof(colour_table), &bw) != FR_OK)
+        return;
+
+    // BMPs are stored with their rows from the bottom upwards
+    for(int y = height - 1; y >= 0; y--)
+    {
+        uint8_t *row = data + y * (width / 8);
+        if(f_write(&fp, row, width / 8, &bw) != FR_OK)
+            return;
+        if(padding_length != 0)
+        {
+            if(f_write(&fp, padding_bytes, padding_length, &bw) != FR_OK)
+                return;
+        }
+    }
+
+    f_close(&fp);
+}