iff-ilbm-on-pc-vga/iffilbm.c

/*
 * Read an IFF ILBM image and display it in VGA 320x200.
 *
 * Copyright 2024 John Bintz. Licensed under the MIT
 * License. Visit https://theindustriousrabbit.com
 * for more!
 */

#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <dos.h>
#include <conio.h>

char buffer[8];
long current_chunk_size;

char far *VGA = (char*)0xA0000;

typedef struct BMHDHeader {
    unsigned short int width;   
    unsigned short int height;   
    unsigned short int _a1;
    unsigned short int _a2;
    unsigned char planes;
    unsigned char _a3;
    unsigned char compression;
    unsigned char _a4;
    unsigned short int _a5;
    unsigned char _a6;
    unsigned char _a7;
    unsigned short int _a8;
    unsigned short int _a9;   
};

typedef struct ILBMColor {
    unsigned char red;
    unsigned char green;
    unsigned char blue;
};

typedef struct ImageReadData {
    short int width;
    short int height;
    char compressed;
    char planes;
    FILE *fh;
    char *row_data;
};

/**
 * Turn a big endian long (68k) into little endian (x86)
 */
long ntohl(long input) {
    return input >> 24 & 0xFF |
    input >> 8  & 0xFF00 |
    input << 8  & 0xFF0000 |
    input << 24 & 0xFF000000;
}

/**
 * Turn a big endian word (68k) into little endian (x86)
 */
unsigned short int ntohw(unsigned short int input) {
    return input >> 8 & 0xFF |
    input << 8  & 0xFF00;
}

/**
 * Read the next IFF chunk header.
 */
void read_chunk_header(FILE *fh) {
    int read_count;
    
    read_count = fread(buffer, sizeof(char), 8, fh);
    current_chunk_size = ntohl(*((long*)(buffer + 4)));
}

/**
 * Read the filetype chunk, which is 4 bytes long.
 */
void read_filetype_chunk(FILE *fh) {
    fread(buffer, sizeof(char), 4, fh);
    
    // reset the chunk size to 0 so no future code tries to use it.
    current_chunk_size = 0;
}

/**
 * Check if the current chunk type is of the requested type.
 * This doesn't use strcmp because the buffer only contains the
 * four characters in the IFF header.
 */
int check_chunk_type(char* type) {
    int i;
    
    for (i = 0; i < 4; ++i) {
        if (type[i] != buffer[i]) {
            return 0;
        }
    }
    
    return 1;
}

/**
 * Read the BMHD struct.
 * 
 * This also converts the width and height to little endian.
 */
void read_bmhd_data(FILE *fh, struct BMHDHeader *bmhd) {
    fread(bmhd, sizeof(struct BMHDHeader), 1, fh);
    bmhd->width = ntohw(bmhd->width);
    bmhd->height = ntohw(bmhd->height);    
}

/**
 * Read the CMAP data into the palette.
 */
int read_cmap_data(FILE *fh, struct ILBMColor palette[]) {
    int number_of_colors = current_chunk_size / 3;
    
    fread(palette, sizeof(struct ILBMColor), number_of_colors, fh);
    
    return number_of_colors;
}

/**
 * Read Amiga-specific display info.
 */
void read_camg_data(FILE *fh) {
    long whatever;
    // we throw this out
    fread(&whatever, sizeof(long), 1, fh);
}

/**
 * Turn a bitplane byte of data into indexed color data.
 */
void add_byte_to_row_data(
    unsigned char data,
    int plane,
    int x,
    unsigned char *row_data
) {
    int bit;
    
    for (bit = 7; bit >= 0; --bit) {
        row_data[x + bit] |= ((data & 1) << plane);
        data >>= 1;
    }
}

/**
 * Read BODY data and write to the VGA buffer.
 */
void read_body_data(struct ImageReadData *image_read_data) {
    int y, x, literal, plane;
    unsigned char data;
    
    char far *current_vga_pos;
    
    unsigned char *row_data = malloc(image_read_data->width);
    int pos = 0;
    int run_count;
    
    for (y = 0; y < image_read_data->height; ++y) {
        for (x = 0; x < image_read_data->width; ++x) {
            row_data[x] = 0;
        }
        
        for (plane = 0; plane < image_read_data->planes; ++plane) {
            for (x = 0; x < image_read_data->width;) {  
                // ByteRun1 run encoding?
                if (image_read_data->compressed) {
                    fread(&data, 1, 1, image_read_data->fh);
                    if (data < 128) {
                        // literal: copy the next bytes from the data stream
                        for (run_count = data + 1; run_count > 0; --run_count) {
                            fread(&data, 1, 1, image_read_data->fh);
                            pos++;
                            
                            // we can't handle more than 320 columns of data
                            if (x < 320) {
                                add_byte_to_row_data(data, plane, x, row_data);
                            }
                            x += 8;
                        }
                    } else {
                        // repeat: copy the next byte this many times into the data stream
                        run_count = 129 - (data - 128);
                        fread(&data, 1, 1, image_read_data->fh);
                        
                        for (; run_count > 0; --run_count) {
                            pos++;
                            
                            // we can't handle more than 320 columns of data
                            if (x < 320) {
                                add_byte_to_row_data(data, plane, x, row_data);
                            }
                            x += 8;
                        }                        
                    }
                } else {
                    // uncompressed, copy the next byte into the data stream
                    fread(&data, 1, 1, image_read_data->fh);
                    pos++;
                    
                    // we can't handle more than 320 columns of data
                    if (x < 320) {
                        add_byte_to_row_data(data, plane, x, row_data);
                    }
                    x += 8;
                }
            }
            
            // data is written to the stream word-padded
            if (pos & 1) {
                fread(&data, 1, 1, image_read_data->fh);
                pos++;
            }
        }
        
        // write row data to VGA buffer
        current_vga_pos = (char far *)(VGA + y * 320);
        for (x = 0; x < image_read_data->width; ++x) {
            *(current_vga_pos++) = row_data[x];   
        }
        
        // we can't handle more than 200 lines of data
        if (y >= 200) break;
    }
    
    free(row_data);
}

int main(int argc, char *argv[]) {
    char *filename = argv[1];
    int keep_reading = 1;

    FILE *image;
    
    struct BMHDHeader bmhd_header; 
    struct ILBMColor palette[32];
    int i, number_of_colors;
    
    struct ImageReadData image_read_data;
    
    union REGS regs;
    
    if (argc < 2) {
        printf("Usage: %s filename.iff\n", argv[0]);
        return 1;
    }
    
    // open for binary reading
    // char 26 was not being read properly when I didn't do this
    image = fopen(filename, "rb");
    if (!image) {
        printf("Can't open %s for reading.\n", filename);
        return 1;
    }
    
    read_chunk_header(image);
    if (!check_chunk_type("FORM")) {
        printf("Not an IFF file");
        return 1;
    }
    
    read_filetype_chunk(image);
    if (!check_chunk_type("ILBM")) {
        printf("Not an ILBM file");
        return 1;
    }
    
    /* next chunks can be in any order, but once we hit BODY, we stop */
    while (keep_reading) {
        keep_reading = 0;
        read_chunk_header(image);
        
        if (check_chunk_type("BMHD")) {
            read_bmhd_data(image, &bmhd_header);
            
            keep_reading = 1;            
        }
        
        if (check_chunk_type("CMAP")) {
            number_of_colors = read_cmap_data(image, palette);
            
            keep_reading = 1;
        }
        
        if (check_chunk_type("CAMG")) {
            read_camg_data(image);
            
            keep_reading = 1;
        }
        
        if (check_chunk_type("BODY")) {
            // we made it, let's activate VGA mode and set up the
            // palette now
            regs.h.ah = 0x00;
            regs.h.al = 0x13;
            int386(0x10, &regs, &regs);    

            // if we send 0 to 0x3c8, we can dump all the colors in
            // one after the other
            outp(0x3c8, 0);
            for (i = 0; i < number_of_colors; ++i) {
                outp(0x3c9, (palette[i].red >> 2));
                outp(0x3c9, (palette[i].green >> 2));
                outp(0x3c9, (palette[i].blue >> 2));                
            }
            
            image_read_data.width = bmhd_header.width;
            image_read_data.height = bmhd_header.height;
            image_read_data.compressed = bmhd_header.compression;
            image_read_data.planes = bmhd_header.planes;
            image_read_data.fh = image;
            
            read_body_data(&image_read_data);
            // stop reading here
        } else {
            if (!keep_reading) {
                // if we're out of chunks, stop reading
                if (!feof(image)) {
                    // otherwise, it's an unknown chunk, skip it and keep reading
                    fseek(image, current_chunk_size, SEEK_CUR);
                    keep_reading = 1;
                }
            }
        }
    }
    
    // wait 10 seconds
    delay(10000);
    
    // back to 80x25 text mode
    regs.h.ah = 0x00;
    regs.h.al = 0x03;
    int386(0x10, &regs, &regs);
    
    if (fclose(image)) {
        printf("Can't close %s for reading.\n", filename);
        return 1;
    }
    
    return 0;
}