[Need Help] iOS png some question

[dong136279559]

Hello, I used this code to compress PNG image in IOS project, but the data I got failed to pass a transparency judgment. I wonder if the compressed PNG is not a true color image. How can I modify the code,thank you!

my Code:

int progress_callback_function(float progress_percent, void* user_info) {
    return 1;
}
+ (NSData *)compressToPNGWithImage:(UIImage *)comImg {
    
    unsigned int width, height;

    CFDataRef pixelData = CGDataProviderCopyData(CGImageGetDataProvider(comImg.CGImage));
    const uint8_t* data = CFDataGetBytePtr(pixelData);

    width = (int)comImg.size.width;
    height = (int)comImg.size.height;

    liq_attr *handle = liq_attr_create();
    liq_attr_set_progress_callback(handle, progress_callback_function, NULL);
    liq_set_quality(handle, 75, 90);//图片质量范围 > 0 < 100;
    liq_set_speed(handle, 3);//压缩速度对压缩比有一定影响的
    liq_image *input_image = liq_image_create_rgba(handle, data, width, height, 0);
        // You could set more options here, like liq_set_quality
    liq_result *quantization_result;
    if (liq_image_quantize(input_image, handle, &quantization_result) != LIQ_OK) {
        fprintf(stderr, "Quantization failed\n");
        return nil;
    }

    // Use libimagequant to make new image pixels from the palette

    size_t pixels_size = width * height;
    unsigned char *raw_8bit_pixels = malloc(pixels_size);
    liq_set_dithering_level(quantization_result, 1.0);

    liq_write_remapped_image(quantization_result, input_image, raw_8bit_pixels, pixels_size);
    const liq_palette *palette = liq_get_palette(quantization_result);

        // Save converted pixels as a PNG file
        // This uses lodepng library for PNG writing (not part of libimagequant)

    LodePNGState state;
    lodepng_state_init(&state);
    state.info_raw.colortype = LCT_PALETTE;
    state.info_raw.bitdepth = 8;
    state.info_png.color.colortype = LCT_PALETTE;
    state.info_png.color.bitdepth = 8;

    for(int i=0; i < palette->count; i++) {
        lodepng_palette_add(&state.info_png.color, palette->entries[i].r, palette->entries[i].g, palette->entries[i].b, palette->entries[i].a);
        lodepng_palette_add(&state.info_raw, palette->entries[i].r, palette->entries[i].g, palette->entries[i].b, palette->entries[i].a);
    }

    unsigned char *output_file_data;
    size_t output_file_size;
    unsigned int out_status = lodepng_encode(&output_file_data, &output_file_size, raw_8bit_pixels, width, height, &state);
    if (out_status) {
        fprintf(stderr, "Can't encode image: %s\n", lodepng_error_text(out_status));
        return nil;
    }
    NSData *output_data = [NSData dataWithBytes:output_file_data length:output_file_size];

    liq_result_destroy(quantization_result); // Must be freed only after you're done using the palette
    liq_image_destroy(input_image);
    liq_attr_destroy(handle);

    free(raw_8bit_pixels);
    lodepng_state_cleanup(&state);
    return output_data;
}

judge:

       uint8_t c;
       [imageData getBytes:&c range:NSMakeRange(25, 1)];
       if (c == 0x03) {
           NSLog(@"It's not what I want!");
           return;
       }

[ofirkris]

Hi @dong136279559 , have you managed to solve this issue?

[dong136279559]

@ofirkris I didn't solve the problem. I need some help

[kornelski]

iOS uses premultiplied alpha (R*A, G*A, B*A, A), not a uncorrelated alpha (R, G, B, A), so any pixels you get from iOS you have to convert to RGBA first by dividing RGB values by their alpha.

[kornelski]

Have a look at this:

https://github.com/kornelski/pngquant/blob/bc4702cd55772878cc6fa8668cf9ecf019f06aaa/rwpng_cocoa.m

[dong136279559]

Do you mean that? But it didn't work. Did I understand that?

`
if (palette->entries[i].a) {
lodepng_palette_add(&state.info_png.color, palette->entries[i].r255/palette->entries[i].a, palette->entries[i].g255/palette->entries[i].a, palette->entries[i].b255/palette->entries[i].a, palette->entries[i].a);
lodepng_palette_add(&state.info_raw, palette->entries[i].r255/palette->entries[i].a, palette->entries[i].g255/palette->entries[i].a, palette->entries[i].b255/palette->entries[i].a, palette->entries[i].a);
}else {
lodepng_palette_add(&state.info_png.color, 0, 0, 1, 0);
lodepng_palette_add(&state.info_raw, 1, 1, 1, 0);
}

`

[kornelski]

No, totally not. The palette is never premultiplied! You must adjust the input pixels. See the file I linked to.