Size of an image imported with FreeImage

Question

I'm having abit of a brainfart and I can't quite grasp what I'm doing wrong. It's quite simple, I am importing an image with FreeImage (http://freeimage.sourceforge.net/) which has a method FreeImage_GetBits that returns a pointer to the first byte of the image data. I then try to load all the data into memory using (bitsperpixel / 8) * pixelsWidth ' pixelHeight, like this:

uint32_t bitsPerPixel           = FreeImage_GetBPP(bitmap);   // resolves to 24
uint32_t widthInPixels          = FreeImage_GetWidth(bitmap);  // resolves to 1024
uint32_t heightInPixels         = FreeImage_GetHeight(bitmap);  // resolves to 1024

// container is a std::vector<uint8_t>
pkgMaterial.mTextureData.insert(pkgMaterial.mTextureData.begin(), FreeImage_GetBits(bitmap), FreeImage_GetBits(bitmap) + ((bitsPerPixel/8) * widthInPixels * heightInPixels));

I have a jpg which is 31 kilobytes in size on disc. Yet when I load it using the above formula, I see the vector is then filled with 3145728 bytes, which is approx 3145 kilobytes. What am I doing wrong?

Answer 1

You are doing nothing wrong.
FreeImage simply decompresses the image, so you can easily read from it and/or modify it.
You cannot render an image or read any pixel values from it without decompressing it first.
And obviously the memory consumption increases if you decompress the image.

You can try to compare the file size of a BMP image before and after loading it. It should roughly be the same, because BMP images are not compressed.