Update a QImage using a shader in QOpenGLWidget - c++

Question

I have a simple vertex shader
static const char *vertexShader=
"attribute vec4 vPosition; \n"
"void main(){\n"
"gl_Position = vPosition;\n"
"}";
Also I have a shader which creates a "Billboard" effect on an image.
static const char *fragmentShader=
"uniform float grid;\n"
"uniform float dividerValue;\n"
"uniform float step_x;\n"
"uniform float step_y;\n"
"uniform sampler2D source;\n"
"uniform lowp float qt_Opacity;\n"
"uniform vec2 qt_TexCoord0;\n"
"void main(){\n"
"vec2 uv = qt_TexCoord0.xy;\n"
"float offx = floor(uv.x / (grid * step_x));\n"
"float offy = floor(uv.y / (grid * step_y));\n"
"vec3 res = texture2D(source, vec2(offx * grid * step_x , offy * grid * step_y)).rgb;\n"
"vec2 prc = fract(uv / vec2(grid * step_x, grid * step_y));\n"
"vec2 pw = pow(abs(prc - 0.5), vec2(2.0));\n"
"float rs = pow(0.45, 2.0);\n"
"float gr = smoothstep(rs - 0.1, rs + 0.1, pw.x + pw.y);\n"
"float y = (res.r + res.g + res.b) / 3.0;\n"
"vec3 ra = res / y;\n"
"float ls = 0.3;\n"
"float lb = ceil(y / ls);\n"
"float lf = ls * lb + 0.3;\n"
"res = lf * res;\n"
"vec3 col = mix(res, vec3(0.1, 0.1, 0.1), gr);\n"
"if (uv.x < dividerValue)\n"
"gl_FragColor = qt_Opacity * vec4(col, 1.0);\n"
"else\n"
"gl_FragColor = qt_Opacity * texture2D(source, uv);\n"
"}";
What I'd like to do is to use this shader to apply this effect on an image in QtOpenGlWidget. But I dont get how to set my image as a texture and pass it to a shader and then to return it modified with a shader effect. What is I want to achieve is: https://imgur.com/a/NSY0u But my shader doesn't affect image https://imgur.com/a/dgSfq . My GLWidget class:
GLWidget::GLWidget(Helper *helper, QWidget *parent)
: QOpenGLWidget(parent), helper(helper)
{
QImage img("E:\\pictures\\0151.jpg");
image = img;
image = image.convertToFormat(QImage::Format_RGBA8888);
setFixedSize(512, 512);
setAutoFillBackground(false);
targetWidth = width();
targetHeight = height();
qDebug() << "targetWidth="<<targetWidth;
qDebug() << "targetHeight ="<<targetHeight ;
//this values i am trying to pass to my fragment shader
grid = 5.0;//grid on image
dividerValue = 0.5;
step_x = 0.0015625;
step_y = height() ? (2.5 * step_x * targetWidth / targetHeight) : 0.0;
}
void GLWidget::initializeGL()
{
initializeOpenGLFunctions();
m_program = new QOpenGLShaderProgram;
m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShader);
m_program->addShaderFromSourceCode(QOpenGLShader::Fragment,fragmentShader);//?
m_program->link();
m_program->bind();
m_program->release();
}
//we can use paintEvent to display our image with opengl
void GLWidget::paintEvent(QPaintEvent *event)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
m_program->bind();
QPainter painter;
painter.begin(this);
painter.drawImage(0,0,image);
QOpenGLTexture texture(image); //I dont know how to setUniformValue(m_program->uniformLocation("source"),texture) to my shader
GLuint m_texture;
glGenTextures(1, &m_texture);
glBindTexture(GL_TEXTURE_2D, m_texture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, image.width(), image.height(), 0, GL_BGRA, GL_UNSIGNED_BYTE, image.bits());
glGenerateMipmap(GL_TEXTURE_2D);
glEnable(GL_TEXTURE_2D);
//open an image
m_program->setUniformValue("grid", grid);
m_program->setUniformValue("dividerValue",dividerValue);
m_program->setUniformValue("step_x", step_x);
m_program->setUniformValue("step_y", step_y);
m_program->setUniformValue(m_program->uniformLocation("source"),m_texture);
painter.end();
m_program->release();
}

Answer 1

When you bind a texture, it is bound to the currently active texture image unit (See Binding textures to samplers).
The active texture unit can be selected by glActiveTexture. The default texture unit is GL_TEXTURE0.
The value which you have to provide to the texture sampler uniform is not the name of a texture, it is the texture unit (number), where the texture is bound to:
int texture_unit = 0; // <----- e.g. texture unit 0
glActiveTexture( GL_TEXTURE0 + texture_unit );
glBindTexture( GL_TEXTURE_2D, m_texture );
.....
m_program->bind();
m_program->setUniformValue( "source", texture_unit ); // <----- texture unit
For a QOpenGLTexture object the texture unit can be selected by QOpenGLTexture::bind:
int texture_unit = 1; // <----- e.g. texture unit 1
QOpenGLTexture texture(image);
texture.bind( texture_unit );
m_program->bind();
m_program->setUniformValue( "source", texture_unit ); // <----- texture unit
Note, since OpenGL 4.2 the texture unit can be initialized within the shader, by a Binding point:
layout(binding = 0) uniform sampler2D source; // binding = 0 -> texture unit 0
Extension to the answer:
The following code will draw the a image to the entire widget with processing it by your shader. Finally the rendered image is read back from the GPU:
class GLWidget : public QOpenGLWidget
{
.....
QOpenGLShaderProgram * m_program = nullptr;
QOpenGLTexture * m_texture = nullptr;
};
void GLWidget::initializeGL()
{
initializeOpenGLFunctions();
QImage img("E:\\pictures\\0151.jpg");
m_texture = new QOpenGLTexture( img );
m_program = new QOpenGLShaderProgram;
m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShader);
m_program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShader);
m_program->bindAttributeLocation("vPosition", 0);
m_program->link();
}
void GLWidget::paintEvent(QPaintEvent *event)
{
// celar the framebuffer
glClearColor(0, 0, 0, 1);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// bind the texture
uint texture_unit = 1;
m_texture->bind( texture_unit );
// activate the shader
m_program->bind();
m_program->setUniformValue( "source", texture_unit );
m_program->setUniformValue( "grid", grid );
m_program->setUniformValue( "dividerValue", dividerValue );
m_program->setUniformValue( "step_x", step_x );
m_program->setUniformValue( "step_y", step_y );
// draw a quad over the entire widget
GLfloat vertices[]{ -1.0f, -1.0f, 1.0f, -1.0f, -1.0f, 1.0f, 1.0f, 1.0f };
m_program->enableAttributeArray(0);
m_program->setAttributeArray(0, GL_FLOAT, vertices, 2);
glDrawArrays( GL_TRIANGLE_STRIP, 0, 4 );
m_program->disableAttributeArray(0);
// release the shader
m_program->release();
// read the rendered image
int width = ....;
int height = ....;
unsigned char *pixels = new unsigned char[width * height * 4];
glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
QImage *img = new QImage( pixels, width, height, QImage::Format_RGBA8888 );
.....
}
Further you have to do some changes to the vertex shader and fragment shader. In the vertex shader you have to pass the vertex position to the fragment shader:
attribute vec2 vPosition;
varying vec2 v_pos;
void main()
{
v_pos = vPosition.xy;
gl_Position = vec4(vPosition.xy, 0.0, 1.0);
}
In the fragment shader you have to calcualte the texute coordinate fromt the vertex position:
varying vec2 v_pos;
void main()
{
vec2 uv = v_pos.xy * 0.5 + 0.5;
....
}
See also glwidget.cpp Example File.