Draw text in a QOpenGLWidget

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Hi,

I have a QOpenGLWidget with a context core profile (3.2). I want to draw a text but I didn't found how do that.
Is it possible to blit a QImage of my text over the QOpenGLWidget or I should used OpenGL GUI ?

Best regards,
Robin

SGaist

Hi,

AFAIK, you can use QPainter for that in your QOpenGLWidget subclass. It's mentioned here

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Thank you for your replay. I made some test but QPainter seem not be usable with OpenGL Core Profile.
This is a sample code test:

#include <iostream>

#include <GL/glew.h>

#include <QApplication>
#include <QOpenGLWidget>
#include <QPainter>

class OpenglWidget : public QOpenGLWidget {
public:
	void paintEvent(QPaintEvent *e) {
		QPainter painter;
		painter.begin(this);
		paintGL();

		painter.setPen(Qt::black);
		painter.drawRect(100, 100, 300, 300);
		printf("error: %d\n\n", glGetError()); //0x0502

		painter.end();
	}

	void paintGL() {
		glClearColor(0.3f, 0.0f, 0.0f, 1.0f);
		glClear(GL_COLOR_BUFFER_BIT);
	}

	void initializeGL() {
		glewExperimental = GL_TRUE;
		if (glewInit() != GLEW_OK) {
			exit(-1);
		}
		glGetError();

		glViewport(0, 0, 600, 600);  
	}
};

int main(int argc, char** argv) {
	QApplication app(argc, argv);

	QSurfaceFormat format;
	format.setMajorVersion(3);
	format.setMinorVersion(2);
	format.setProfile(QSurfaceFormat::CoreProfile);

	OpenglWidget opengl;
	opengl.setFormat(format);
	opengl.show();

	return app.exec();
}

EDIT: painter.draw[...](...) generate an OpenGL error (0x0502)

Regards ,
Robin

SGaist

You shouldn't implement paintEvent, but use QPainter in paintGL

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It's the same thing :(
I try with and without the vao.

#include <iostream>

#include <GL/glew.h>

#include <QApplication>
#include <QOpenGLWidget>
#include <QPainter>

class OpenglWidget : public QOpenGLWidget {
public:

	void paintGL() {
		glClearColor(0.3f, 0.0f, 0.0f, 1.0f);
		glClear(GL_COLOR_BUFFER_BIT);

		GLuint vao;
		glGenVertexArrays(1, &vao);
		glBindVertexArray(vao);

		QPainter painter;
		painter.begin(this);

		painter.setPen(Qt::black);
		painter.drawRect(100, 100, 300, 300);
		printf("error: %d\n\n", glGetError()); //0x0502

		painter.end();

		glBindVertexArray(0);
		glDeleteVertexArrays(1, &vao);
	}

	void initializeGL() {
		glewExperimental = GL_TRUE;
		if (glewInit() != GLEW_OK) {
			exit(-1);
		}
		glGetError();

		glViewport(0, 0, 600, 600);  
	}
};

int main(int argc, char** argv) {
	QApplication app(argc, argv);

	QSurfaceFormat format;
	format.setMajorVersion(3);
	format.setMinorVersion(2);
	format.setProfile(QSurfaceFormat::CoreProfile);

	OpenglWidget opengl;
	opengl.setFormat(format);
	opengl.show();

	return app.exec();
}

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Hi dig into the Qt code and I have found the problem. The shader used are in GLSL 120.

The vertex shader:

    void setPosition(); 
    void main(void) 
    { 
        setPosition(); 
    }

    attribute highp   vec2      vertexCoordsArray; 
    attribute highp   vec3      pmvMatrix1; 
    attribute highp   vec3      pmvMatrix2; 
    attribute highp   vec3      pmvMatrix3; 
    void setPosition(void) 
    { 
        highp mat3 pmvMatrix = mat3(pmvMatrix1, pmvMatrix2, pmvMatrix3); 
        vec3 transformedPos = pmvMatrix * vec3(vertexCoordsArray.xy, 1.0); 
        gl_Position = vec4(transformedPos.xy, 0.0, transformedPos.z); 
    }

and the fragement shader:


    lowp vec4 srcPixel(); 
    void main() 
    { 
        gl_FragColor = srcPixel(); 
    }

    uniform   lowp    vec4      fragmentColor; 
    lowp vec4 srcPixel() 
    { 
        return fragmentColor; 
    }

Is it possible to use shader in GLSL greather than 130 ?

SGaist

Yes, you have to ensure that you request the correct context (and that you get it)

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I'm in the correct OpenGL context scope. The solution I choose is to convert a QImage into OpenGL texture and draw it in blend mode with a pass through shader.

I think it really impossible to used a QPainter into a Core Profile context.

Thank for you're time :)