PNG Decoder

Overview

PNG decoder implemented in C, utilizing zlib for decompression and SDL2 for visualization. The program reads a PNG file, processes its chunks, decompresses the image data, applies filtering, and displays the image.

Usage

The program will read and decode basn6a08.png by default. You can modify the filename in the main function:

FILE* png_file;
fopen_s(&png_file, "basn6a08.png", "rb");
if (!png_file) {
    printf("Error opening file\n");
    return 1;
}

Key Points

PNG chunks structure

Defines the basic PNG chunk format, storing length, type, and data.

typedef struct chunk {
    unsigned int length;
    char* type;
    char* data;
} Chunk;

IHDR

Represents and parsing the PNG header chunk containing metadata about the image.

typedef struct ihdr {
    unsigned int width, height;
    unsigned char bitDepth, color, compression, filter, interlace;
} IHDR;


void ParseIHDR(const char* data, IHDR* ihdr) {
    ihdr->width = (data[0] << 24) | (data[1] << 16) | (data[2] << 8) | data[3];
    ihdr->height = (data[4] << 24) | (data[5] << 16) | (data[6] << 8) | data[7];
}

Reading chunks

Reads a chunk from the PNG file and extracts type and length information.

void ReadChunk(FILE* file, Chunk* chunk) {
    if (feof(file)) {
        chunk->type = NULL;
        chunk->data = NULL;
        return;
    }
    fread(&chunk->length, sizeof(unsigned int), 1, file);
    chunk->length = ToLittleEndian(chunk->length);
    chunk->type = (char*)malloc(5);
    fread(chunk->type, sizeof(char), 4, file);
    chunk->type[4] = '\0';
}

Concatenating IDAT Data

Combines multiple chunks into a single data block for decompression.

void ConcatenateIDATData(const Chunk* chunks, size_t chunks_number, unsigned char** idat_data, unsigned long* idat_length) {
    size_t total_idat_length = 0;
    for (size_t i = 0; i < chunks_number; i++) {
        if (strcmp(chunks[i].type, "IDAT") == 0) {
            total_idat_length += chunks[i].length;
        }
    }
    *idat_data = (unsigned char*)malloc(total_idat_length + 1);
}

Notes

• The image is displayed in a SDL2 window and closes when the user exits

• All allocated memory is freed before terminating