stbir_resize_uint8 crashing on memory access

Question

I'm using stb_image to upload an image to the GPU. If I just upload the image with stbi_load I can confirm (nvidia Nsight) that the image is correctly stored in the GPU memory. However, some images I like to resize before I upload the to the GPU. In this case, I get a crash. This is the code:

    int      textureWidth;
    int      textureHeight;
    int      textureChannelCount;
    stbi_uc* pixels = stbi_load(fullPath.string().c_str(), &textureWidth, &textureHeight, &textureChannelCount, STBI_rgb_alpha);

    if (!pixels) {
        char error[512];
        sprintf_s(error, "Failed to load image %s!", pathToTexture);
        throw std::runtime_error(error);
    }

    stbi_uc* resizedPixels = nullptr;
    uint32_t imageSize     = 0;
    if (scale > 1.0001f || scale < 0.9999f) {
        stbir_resize_uint8(pixels, textureWidth, textureHeight, 0, resizedPixels, textureWidth * scale, textureHeight * scale, 0, textureChannelCount);
        stbi_image_free(pixels);
        textureWidth *= scale;
        textureHeight *= scale;
        imageSize = textureWidth * textureHeight * textureChannelCount;
    } else {
        resizedPixels = pixels;
        imageSize     = textureWidth * textureHeight * textureChannelCount;
    }

    // Upload the image to the gpu

When this code is run with scale set to 1.0f, it works fine. However, when I set the scale to 0.25f, the program crashes in method stbir_resize_uint8. The image I'm providing in both cases is a 1920x1080 RGBA PNG. Alpha channel is set to 1.0f across the whole image.

Which function do I have to use to resize the image?

EDIT: If I allocate the memory myself, the function no longer crashes and works fine. But I though stb handles all memory allocation internally. Was I wrong?

Answer 1

I see you found and solved the problem in your edit but here's some useful advice anyway:

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It seems like the comments in the source (which is also the documentation) don't explicitly mention that you have to allocate memory for the resized image, but it becomes clear when you take a closer look at the function's signature:

STBIRDEF int stbir_resize_uint8(     const unsigned char *input_pixels , int input_w , int input_h , int input_stride_in_bytes,
                                           unsigned char *output_pixels, int output_w, int output_h, int output_stride_in_bytes,
                                     int num_channels);

Think about how you yourself would return the address of a memory chunk that you allocated in a function. The easiest would be to return the pointer directly like so:

unsigned char* allocate_memory( int size )
{ return (unsigned char*) malloc(size); }

However the return seems to be reserved for error codes, so your only option would be to manipulate the pointer as a side-effect. To do that, you'd need to pass a pointer to it (pointer to pointer):

int allocate_memory( unsigned char** pointer_to_array, int size )
{
    *pointer_to_array = (unsigned char*) malloc(size);
    /* Check if allocation was successful and do other stuff... */
    return 0;
}

If you take a closer look at the resize function's signature, you'll notice that there's no such parameter passed, so there's no way for it to return the address of internally allocated memory. (unsigned char* output_pixels instead of unsigned char** output_pixels). As a result, you have to allocate the memory for the resized image yourself.

I hope this helps you in the future.

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There is a mention of memory allocation in the docs but as far as I understand, it's about allocations required to perform the resizing, which is unrelated to the output.