Should I vertically flip the lines of an image loaded with stb_image to use in OpenGL?

Question

I'm working on an OpenGL-powered 2d engine.

I'm using stb_image to load image data so I can create OpenGL textures. I know that the UV origin for OpenGL is bottom-left and I also intend to work in that space for my screen-space 2d vertices i.e. I'm using glm::ortho( 0, width, 0, height, -1, 1 ), not inverting 0 and height.

You probably guessed it, my texturing is vertically flipped but I'm 100% sure that my UV are specified correctly.

So: is this caused by stbi_load's storage of pixel data? I'm currently loading PNG files only so I don't know if it would cause this problem if I was using another file format. Would it? (I can't test right now, I'm not at home).

I really want to keep the screen coords in the "standard" OpenGL space... I know I could just invert the orthogonal projection to fix it but I would really rather not.

I can see two sane options:

1- If this is caused by stbi_load storage of pixel data, I could invert it at loading time. I'm a little worried about that for performance reason and because I'm using texture arrays (glTexture3d) for sprite animations meaning I would need to invert texture tiles individually which seems painful and not a general solution.

2- I could use a texture coordinate transformation to vertically flip the UVs on the GPU (in my GLSL shaders).

A possible 3rd option would be to use glPixelStore to specify the input data... but I can't find a way to tell it that the incoming pixels are vertically flipped.

What are your recommendations for handling my problem? I figured I can't be the only one using stbi_load + OpenGL and having that problem.

Finally, my target platforms are PC, Android and iOS :)

EDIT: I answered my own question... see below.

Answer 1

I know this question's pretty old, but it's one of the first results on google when trying to solve this problem, so I thought I'd offer an updated solution.

Sometime after this question was originally asked stb_image.h added a function called "stbi_set_flip_vertically_on_load", simply passing true to this function will cause it to output images the way OpenGL expects - thus removing the need for manual flipping/texture-coordinate flipping.

Also, for those who don't know where to get the latest version, for whatever reason, you can find it at github being actively worked on: https://github.com/nothings/stb

It's also worth noting that in stb_image's current implementation they flip the image pixel-by-pixel, which isn't exactly performant. This may change at a later date as they've already flagged it for optimsation. Edit: It appears that they've swapped to memcpy, which should be a good bit faster.

Answer 2

Ok, I will answer my own question... I went thru the documentation for both libs (stb_image and OpenGL).

Here are the appropriate bits with reference:

glTexImage2D says the following about the data pointer parameter: "The first element corresponds to the lower left corner of the texture image. Subsequent elements progress left-to-right through the remaining texels in the lowest row of the texture image, and then in successively higher rows of the texture image. The final element corresponds to the upper right corner of the texture image." From http://www.opengl.org/sdk/docs/man/xhtml/glTexImage2D.xml

The stb_image lib says this about the loaded image pixel: "The return value from an image loader is an 'unsigned char *' which points to the pixel data. The pixel data consists of *y scanlines of *x pixels, with each pixel consisting of N interleaved 8-bit components; the first pixel pointed to is top-left-most in the image." From http://nothings.org/stb_image.c‎

So, the issue is related the pixel storage difference between the image loading lib and OpenGL. It wouldn't matter if I loaded other file formats than PNG because stb_image returns the same data pointer for all formats it loads.

So I decided I'll just swap in place the pixel data returned by stb_image in my OglTextureFactory. This way, I keep my approach platform-independent. If load time becomes an issue down the road, I'll remove the flipping at load time and do something on the GPU instead.

Hope this helps someone else in the future.

Answer 3

Yes, you should. This can be easily accomplished by simply calling this STBI function before loading the image:

stbi_set_flip_vertically_on_load(true);