[OpenGL learning notes 8] texture

Catalog

To paint a texture

1. Create texture objects

2. Bind texture object

3. Attach a texture image to the texture object

4. Define the sampler in the clip shader and use

5. Relate texture objects to shaders

6. Pass texture objects to texture units , And call glDrawElements mapping

Texture coordinates

Texture units

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Texture is a 2D picture （ There is even 1D and 3D The texture of ）, Generally speaking, painting texture is using OpenGL Drawing pictures .

In addition to images , Textures can also be used to store large amounts of data , This data can be sent to the shader .

To paint a texture

1. Create texture objects

// Generated OpenGL object , Texture is also used ID Refer to the 
unsigned int texture;
glGenTextures(1, &texture);
//glGenTextures The function first needs to input the number of generated textures , Then store them in the... Of the second parameter unsigned int Array 

2. Bind texture object

After binding, any texture instruction can configure the currently bound texture ：

glBindTexture(GL_TEXTURE_2D, texture);

3. Attach a texture image to the texture object

// adopt glTexImage2D Generate a texture on the current texture object 
// When calling glTexImage2D when , The currently bound texture object will be attached with a texture image 
QImage image1 = QGLWidget::convertToGLFormat(QImage(":/image/image/container.jpg"));
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, image1.width(), image1.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, image1.bits());
glGenerateMipmap(GL_TEXTURE_2D);

When calling glTexImage2D when , The currently bound texture object will be attached with a texture image .

4. Define the sampler in the clip shader and use

GLSL There is a built-in data type for texture objects , It's called a sampler (Sampler), It uses texture type as suffix , such as sampler1D、sampler3D, Or in our case sampler2D. We can simply declare a uniform sampler2D Add a texture to the clip shader , We'll assign the texture to this later uniform.

#version 330
out vec4 fragColor;

in vec2 TexCord;
uniform sampler2D texture1;
void main()
{
    fragColor = texture(texture1, TexCord);
# Use GLSL The built-in texture Function to sample the color of the texture , Its first parameter is the texture sampler ,
 The second parameter is the texture coordinates .texture The function samples the corresponding color values using the texture parameters previously set .
 The output of this fragment shader is texture （ interpolation ） In texture coordinates ( Filtered ) Color .
}

5. Relate texture objects to shaders

Texture data of texture objects cannot be directly passed to shaders , Texture objects can be passed to texture units .

The sampler can pass glUniform1i To specify which texture unit the sampler corresponds to .

sharedprogram.bind();
// Set the texture unit corresponding to the sampler 
sharedprogram.setUniformValue("texture1",0);// Designated sampler texture1 For texture units 0
//sharedprogram.setUniformValue("texture2",1);// Designated sampler texture2 For texture units 1
sharedprogram.release();

6. Pass texture objects to texture units , And call glDrawElements mapping

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture1);// Transfer texture objects to texture cells 
//glActiveTexture(GL_TEXTURE1);
//glBindTexture(GL_TEXTURE_2D, texture2);

glBindVertexArray(VAO);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

/********************************************************************/
#version 330
out vec4 fragColor;

in vec2 TexCord;
uniform sampler2D texture1;
uniform sampler2D texture2;
void main()
{
    fragColor = mix(texture(texture1, TexCord), texture(texture2, TexCord), 0.2) ;
}
/*********************************************************************/   
 // Create a texture object 
    glGenTextures(1, &texture_container);
    // Bind texture object , After any texture instruction can configure the currently bound texture 
    glBindTexture(GL_TEXTURE_2D, texture_container);
    // adopt glTexImage2D Generate a texture on the current texture object 
    // When calling glTexImage2D when , The currently bound texture object will be attached with a texture image 
    QImage image1 = QGLWidget::convertToGLFormat(QImage(":/image/image/container.jpg"));
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, image1.width(), image1.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, image1.bits());
    glGenerateMipmap(GL_TEXTURE_2D);

    glGenTextures(1, &texture_awesomeface);
    glBindTexture(GL_TEXTURE_2D, texture_awesomeface);

    QImage image = QGLWidget::convertToGLFormat(QImage(":/image/image/awesomeface.png"));
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, image.width(), image.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, image.bits());
    glGenerateMipmap(GL_TEXTURE_2D);

/*********************************************************************/
    sharedprogram.bind();
    // Set the texture unit corresponding to the sampler 
    sharedprogram.setUniformValue("texture1",0);// Designated sampler texture1 For texture units 0
    sharedprogram.setUniformValue("texture2",1);// Designated sampler texture2 For texture units 1
    sharedprogram.release();

/*********************************************************************/   
    sharedprogram.bind();


    glActiveTexture(GL_TEXTURE0);//  Activate the texture unit before binding the texture 
    glBindTexture(GL_TEXTURE_2D, texture_container);// Pass texture to texture unit 

    glActiveTexture(GL_TEXTURE1);
    glBindTexture(GL_TEXTURE_2D, texture_awesomeface);

    glBindVertexArray(VAO);

    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
    sharedprogram.release();

Texture coordinates

Texture coordinates in x and y On the shaft , The scope is 0 To 1 Between （ Note that we are using 2D texture image ）. Texture coordinates start at (0, 0), The lower left corner of the texture image , Always at (1, 1), The upper right corner of the texture image .

The texture coordinates in the above figure are as follows

float texCoords[] = {
    0.0f, 0.0f, //  The lower left corner 
    1.0f, 0.0f, //  The lower right corner 
    0.5f, 1.0f //  upper-middle 
};

Texture units

The position value of a texture is usually called a texture unit (Texture Unit). The default texture unit for a texture is 0, It is the default active texture unit , So we didn't assign a location value in the previous part of the tutorial .

The main purpose of texture unit is to let us use more than one texture in shader . By assigning texture units to the sampler , We can bind multiple textures at once , As long as we first activate the corresponding texture unit . It's like glBindTexture equally , We can use glActiveTexture Activate texture unit , Pass in the texture unit we need to use ：

glActiveTexture(GL_TEXTURE0); //  Activate the texture unit before binding the texture 
glBindTexture(GL_TEXTURE_2D, texture);

After activating the texture unit , Next glBindTexture The function call binds the texture to the currently active texture unit , Texture units GL_TEXTURE0 Default is always activated , So in the previous example, when we use glBindTexture When , No texture units need to be activated .

OpenGL At least guarantee 16 Texture units for you to use , That is to say you can activate from GL_TEXTURE0 To GL_TEXTRUE15. They are all defined in order , So we can also pass GL_TEXTURE0 + 8 The way to get GL_TEXTURE8, This is useful when we need to loop some texture elements .