c++ - OpenCV image loading for OpenGL Texture

Question

I want to load an image (jpg and png) with OpenCV as OpenGL Texture.

Here is how I load the image to OpenGL:

glEnable(GL_TEXTURE_2D);
  textureData = loadTextureData("textures/trashbin.png");
  cv::Mat image = cv::imread("textures/trashbin.png");
  if(image.empty()){
      std::cout << "image empty" << std::endl;
  }else{
      glGenTextures( 1, &textureTrash );
      glBindTexture( GL_TEXTURE_2D, textureTrash );
      glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
      glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
      glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S , GL_REPEAT );
      glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT );
      glTexImage2D(GL_TEXTURE_2D,0,3,image.cols, image.rows,0,GL_RGB,GL_UNSIGNED_BYTE, image.data);
  }

The image is loaded, as "image.empty" always returns false

Here is how I render the scene using the created texture:

  glActiveTexture(GL_TEXTURE0);
  glBindTexture(GL_TEXTURE_2D, textureTrash);
  glm_ModelViewMatrix.top() = glm::translate(glm_ModelViewMatrix.top(),0.0f,-13.0f,-10.0f);
  glUniformMatrix4fv(uniformLocations["modelview"], 1, false, glm::value_ptr(glm_ModelViewMatrix.top()));

  std::cout << "textureShaderID: " << glGetUniformLocation(shaderProgram,"texture") << std::endl;

  glUniform1i(glGetUniformLocation(shaderProgram,"texture"), 0);
  objLoader->getMeshObj("trashbin")->render();

And finally the fragmentShader where I want to apply the texture to my geometry

#version 330
in vec2 tCoord;

// texture //
// TODO: set up a texture uniform //
uniform sampler2D texture;

// this defines the fragment output //
out vec4 color;

void main() {
  // TODO: get the texel value from your texture at the position of the passed texture coordinate //
  color = texture2D(texture, tCoord);
}

The texture coordinates are comeing from a Vertex Buffer Object and are correctly set from the .obj file. Also I can see the Object in my scene when I set the color to e.g. red in the fragment shader, or to vec4(tCoord,0,1); then the object is shaded in different color.

Unfortunately the screen stays black when I want to apply the texture... Can someone help me and tell me why is stays black?

Answer 1

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.