iOS 上 OpenGLES 上的冲突绘图 [英] Conflict drawing on OpenGLES on iOS

问题描述

我正在编写一个允许绘制自由样式(使用手指)和绘制图像的应用程序.我在 .

I'm writing an app allows drawing free style (using finger) and drawing image. I posted one of my problems at OpenGL ES glFragColor depend on if condition on Fragment shader in iOS. Thanks for many suggestions, I solved that. Now I still have another new issue.

I have 2 programs which have id PROGRAM_POINT (drawing free style) and PROGRAM_POINT_0 (draw image). Those are initialized. Each program has a pair of shader files. PROGRAM_POINT has shader files named point.vsh and point.fsh. For PROGRAM_POINT_0 is texture.vsh and texture.fsh.

Here are these files

---- point.vsh ------

attribute vec4 inVertex;

uniform mat4 MVP;
uniform float pointSize;
uniform lowp vec4 vertexColor;

varying lowp vec4 color;

void main()
{
    gl_Position = MVP * inVertex;
    gl_PointSize = pointSize;
    color = vertexColor;
}

---- point.fsh -------

precision mediump float;

uniform sampler2D texture;
varying lowp vec4 color;

void main()
{
    //Drawing point
    gl_FragColor = color * texture2D(texture, gl_PointCoord);
}

---- texture.vsh --------

attribute vec4 a_Position;
attribute vec2 a_TextureCoordinates;

varying vec2 v_TextureCoordinates;

void main()
{
    v_TextureCoordinates = a_TextureCoordinates;
    gl_Position = a_Position;
}

---- texture.fsh ------

precision mediump float;

uniform sampler2D u_TextureUnit;
varying vec2 v_TextureCoordinates;

void main()
{
    //Draw texture
    gl_FragColor = texture2D(u_TextureUnit, v_TextureCoordinates);
}

Besides, here are the drawing free style and image

--- Drawing free style -----

    glBindBuffer(GL_ARRAY_BUFFER, vboId);
    glBufferData(GL_ARRAY_BUFFER, vertexCount*2*sizeof(GLfloat), vertexBuffer, GL_DYNAMIC_DRAW);

    glEnableVertexAttribArray(ATTRIB_VERTEX);
    glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, GL_FALSE, 0, 0);

    // the brush texture will be bound to texture unit 0
    glUniform1i(program[PROGRAM_POINT].uniform[UNIFORM_TEXTURE], 0);

    glUseProgram(program[PROGRAM_POINT].id);
    glDrawArrays(GL_POINTS, 0, (int)vertexCount);

--- Drawing image -----

    GLuint a_position_location = glGetAttribLocation(program[PROGRAM_POINT_0].id, "a_Position");
    GLuint a_texture_coordinates_location = glGetAttribLocation(program[PROGRAM_POINT_0].id, "a_TextureCoordinates");
    GLuint u_texture_unit_location = glGetUniformLocation(program[PROGRAM_POINT_0].id, "u_TextureUnit");

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, texName);
    glUniform1i(u_texture_unit_location, 0);

    const float textrect[] = {-1.0f, -1.0f, 0.0f, 0.0f,
        -1.0f,  1.0f, 0.0f, 1.0f,
        1.0f, -1.0f, 1.0f, 0.0f,
        1.0f,  1.0f, 1.0f, 1.0f};

    glBindBuffer(GL_ARRAY_BUFFER, vboId_1);
    glBufferData(GL_ARRAY_BUFFER, sizeof(textrect), textrect, GL_STATIC_DRAW);

    glVertexAttribPointer(a_position_location, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(float), (void*)(0));
    glVertexAttribPointer(a_texture_coordinates_location, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(float), (void*)(2 * sizeof(float)));

    glEnableVertexAttribArray(a_position_location);
    glEnableVertexAttribArray(a_texture_coordinates_location);

    glUseProgram(program[PROGRAM_POINT_0].id);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    [context presentRenderbuffer:GL_RENDERBUFFER];

And now the issue happens if I do the following steps:

• Drawing free style (still fine)

• Drawing image (still fine)

• Drawing free style again (PROBLEM HAPPEN!!!)

The colors of the number 3 and 4 are changed and seems terrible. I suppose does it merged or conflicted after drawing image. Do I need clear the buffer after drawing image

解决方案

point.fsh fragment shader uses a texture, but your "Drawing free style" GL code doesn't have glBindTexture. So it will use the last glBindTexture-ed texture.

Also, "Drawing free style" enables one vertex attribute array.

glEnableVertexAttribArray(ATTRIB_VERTEX)

However "Drawing image" enables two attribute arrays.

glEnableVertexAttribArray(a_position_location);
glEnableVertexAttribArray(a_texture_coordinates_location);

You should call glDisableVertexAttribArray before another glDrawArrays.

Or you can use VAO. Please take a look at the Apple Document.

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