Opencv Image Loading for Opengl Texture

OpenCV image loading for OpenGL Texture

From only looking at your texture loading code you are ignoring many considerations about how OpenCV lays out images in memory. I've already explained that for the opposite direction (glGetTexImage into OpenCV image) in this answer, but will recapitulate it here for the CV-GL direction:

First of all OpenCV doesn't neccessarily store image rows tightly packed but might align them to certain byte boundaries (don't know how much, at least 4, but maybe 8 or more?). If you're lucky it will use 4-byte alignment and the GL is set to the default pixel storage mode of 4-byte alignment, too. But it's best to manually fiddle with the pixel storage modes in order to be on the safe side:

//use fast 4-byte alignment (default anyway) if possible
glPixelStorei(GL_UNPACK_ALIGNMENT, (image.step & 3) ? 1 : 4);

//set length of one complete row in data (doesn't need to equal image.cols)
glPixelStorei(GL_UNPACK_ROW_LENGTH, image.step/image.elemSize());

Then you have to account for the fact that OpenCV stores images from top to bottom, while the GL uses bottom to top. This could be taken care of by just mirroring the t-texture coordinate appropriately (maybe directly in the shader), but you could also just flip the image before uploading:

cv::flip(image, flipped, 0);
image = flipped;              //maybe just cv::flip(image, image, 0)?

Last but not least OpenCV stores color images in BGR format so uploading it as RGB would distort the colors. So use GL_BGR (requires OpenGL 1.2, but who doesn't have that?) in glTexImage2D.

These might not be the complete solution to your problem (since I think those errors should rather result in a distorted rather than a black image), but they are definitely problems to take care of.

EDIT: Does your fragment shader actually compile successfully (in the complete version using the texture)? I'm asking because in the GLSL 3.30 you're using the word texture is also the name of a builtin function (which should actually be used instead of the deprecated texture2D function), so maybe the compiler has some name resolution problems (and maybe this error is ignored in your simplified shaders, given that the whole uniform will be optimized away and many GLSL compilers are known to be anything else than strictly standard compliant). So just try to give that sampler uniform a different name.

Loading texture for OpenGL with OpenCV

Your error appears there, right?

if( texture_cv = imread("stones.jpg"))  {

because in if(expr) expr must be bool or can be casted to bool. But there is no way to convert cv::Mat into boolean implicitly. But you can check the result of imread like that:

texture_cv = imread("stones.jpg");
if (texture_cv.empty()) {
  // handle was an error
} else {
  // do right job
} 

See: cv::Mat::empty(), cv::imread

Hope that helped you.

Passing grayscale OpenCV image to an OpenGL texture

input format

I'd use GL_RED, since the GL_LUMINANCE format has been deprecated

internalFormat

depends on what you want to do in your shader, although you should always specify a sized internal format, e.g. GL_RGBA8 which gives you 8 bits per channel. Although, with GL_RGBA8, the green, blue and alpha channels will be zero anyway since your input data only has a single channel, so you should probably use the GL_R8 format instead. Also, you can use texture swizzling:

GLint swizzleMask[] = {GL_RED, GL_RED, GL_RED, GL_RED};
glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_RGBA, swizzleMask);

which will cause all channels to 'mirror' the red channel when you access the texture in the shader.

OpenCV IplImage *Image to OpenGL GLuint texture

You need to set the pixel unpack/pack alignment to 1 when you send/read RGB image to/from GL. By default GL is going to read your image data as if each row were aligned to a 4-byte boundary, and clearly with tightly packed (8-bit) RGB that is not the case.

This should fix at least part of your problem:

glPixelStorei (GL_UNPACK_ALIGNMENT, 1);
glTexImage2D  (GL_TEXTURE_2D, 0, 3, Image->width, Image->height, 0, GL_BGR, GL_UNSIGNED_BYTE, Image->imageData);

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Update:

The IplImage data structure provides all of the fields that you need in order to do this portably:

glPixelStorei (GL_UNPACK_ALIGNMENT,  Image->align);
glPixelStorei (GL_UNPACK_ROW_LENGTH, Image->widthStep / Image->nChannels);
glTexImage2D  (GL_TEXTURE_2D, 0, 3,  Image->width, Image->height, 0, GL_BGR, GL_UNSIGNED_BYTE, Image->imageData);

The fundamental issue is the same, however. It comes down to differences in image data alignment between the two machines.

As for your image being upside down, have a look at the origin field of the IplImage struct. You need to compensate for that yourself. GL does not have the ability to flip images during pixel transfer, unfortunately.

OpenGL render to a OpenCV mat as texture and reuse it in OpenCV

So I solved my problem, which I should have posted more code to make an answer easier, by adding a VertexBufferObject which I totally ignored from the tutorial. Also and this is very important, I use SFML lib at the same time to display my frames. SFML also uses OpenGL internally. So I need to save and restore the OpenGL states which interfere with eachother.

I drop this link in case somebody has similar problems:
using-opengl-together-with-the-graphics-module

This added state save/restore before I use SFML together with the missing VAO code and a reset of OpenGL bindings after my Draw() method made it work. This might not be the most performant solution but for now it is a way to achieve what I needed.

Texture isn't loading properly OpenGL

The association of the texture coordinates to the vertices is wrong. Change to:

float vertices[] =
{
    _min.x, _min.y,   0.0f, 1.0f,
    _max.x, _min.y,   1.0f, 1.0f,
    _max.x, _max.y,   1.0f, 0.0f,

    _min.x, _min.y,   0.0f, 1.0f,
    _max.x, _max.y,   1.0f, 0.0f,
    _min.x, _max.y,   0.0f, 0.0f,
};

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By default OpenGL assumes that the start of each row of an image is aligned to 4 bytes.

This is because the GL_UNPACK_ALIGNMENT parameter by default is 4. When a RGB image with 3 color channels is loaded to a texture object and 3*width is not divisible by 4 this may cause a misalignment.

Change the alignment by setting the GL_UNPACK_ALIGNMENT to 1, before specifying the texture image with glTexImage2D:

glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glTexImage2D(GL_TEXTURE_2D, 0, format, width, height, 0, format, GL_UNSIGNED_BYTE, data);

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When you remove glGenerateMipmap(GL_TEXTURE_2D);, then you have to change the minifying function (GL_TEXTURE_MIN_FILTER) as well. Since the filter is GL_LINEAR_MIPMAP_LINEAR, the texture would be "Mipmap Incomplete" if you do not change the minimize function to GL_NEAREST or GL_LINEAR.

OpenCV webcam frames to OpenGL texture

Drawing a rectangle bounding the GLControl solved it:

            GL.Begin(PrimitiveType.Quads);
                GL.TexCoord2(0, 0); GL.Vertex2(0, 0);
                GL.TexCoord2(0, 1); GL.Vertex2(0, 1);
                GL.TexCoord2(1, 1); GL.Vertex2(1, 1);
                GL.TexCoord2(1, 0); GL.Vertex2(1, 0);
            GL.End();
            m.Dispose();

Be sure to dispose the object after drawing the frame so you will not run out of memory.

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