OpenGL ES glFragColor depends on whether condition on Fragment shader in iOS


I'm writing an app on iOS allows drawing free style (using finger) and drawing image on screen. I use OpenGL ES to implement. I have 2 functions, one is drawing free style, one is drawing texture

--- Code drawing free style

- (void)drawFreeStyle:(NSMutableArray *)pointArray {

        //Prepare vertex data

        // Load data to the Vertex Buffer Object
        glBindBuffer(GL_ARRAY_BUFFER, vboId);
        glBufferData(GL_ARRAY_BUFFER, vertexCount*2*sizeof(GLfloat), vertexBuffer, GL_DYNAMIC_DRAW);

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

        **GLuint a_ver_flag_drawing_type = glGetAttribLocation(program[PROGRAM_POINT].id, "a_drawingType");
        glVertexAttrib1f(a_ver_flag_drawing_type, 0.0f);

        GLuint u_fra_flag_drawing_type = glGetUniformLocation(program[PROGRAM_POINT].id, "v_drawing_type");
        glUniform1f(u_fra_flag_drawing_type, 0.0);**

        glDrawArrays(GL_POINTS, 0, (int)vertexCount);

        // Display the buffer
        glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
        [context presentRenderbuffer:GL_RENDERBUFFER];

--- Code drawing texture

- (void)drawTexture:(UIImage *)image atRect:(CGRect)rect {

    GLuint a_ver_flag_drawing_type = glGetAttribLocation(program[PROGRAM_POINT].id, "a_drawingType");

    GLuint u_fra_flag_drawing_type = glGetUniformLocation(program[PROGRAM_POINT].id, "v_drawing_type");

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

    glBindTexture(GL_TEXTURE_2D, texName);
    glUniform1i(u_texture_unit_location, 0);
    glUniform1f(u_fra_flag_drawing_type, 1.0);

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

    glVertexAttrib1f(a_ver_flag_drawing_type, 1.0f);

    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)));


    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
    [context presentRenderbuffer:GL_RENDERBUFFER];

Notice 2 variables a_ver_flag_drawing_type (attribute) and u_fra_flag_drawing_type (uniform). They're use for setting flags on vertex shader and fragment shader to determine drawing free style or texture on both files

--- Vertex shader

attribute lowp float a_drawingType;

//For drawing
attribute vec4 inVertex;

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

varying lowp vec4 color;

//For texture
attribute vec4 a_Position;
attribute vec2 a_TextureCoordinates;

varying vec2 v_TextureCoordinates;

void main()
    if (abs(a_drawingType - 1.0) < 0.0001) {
        //Draw texture
        v_TextureCoordinates = a_TextureCoordinates;
        gl_Position = a_Position;
    } else {
        //Draw free style
        gl_Position = MVP * inVertex;
        gl_PointSize = pointSize;
        color = vertexColor;


--- Fragment shader

precision mediump float;

uniform sampler2D texture;
varying lowp vec4 color;

uniform sampler2D u_TextureUnit;
varying vec2 v_TextureCoordinates;

uniform lowp float v_drawing_type;

void main()
    if (abs(v_drawing_type - 1.0) < 0.0001) {
        //Draw texture
        gl_FragColor = texture2D(u_TextureUnit, v_TextureCoordinates);
    } else {
        //Drawing free style
        gl_FragColor = color * texture2D(texture, gl_PointCoord);


My idea is setting these flags from drawing code at drawing time. Attribute a_drawingType is used for vertex shader and Uniform v_drawing_type is used for fragment shader. Depending these flags to know draw free style or texture.

But if I run independently, each time just one type (if run drawing free style, comment code config drawing texture on vertex shader and fragment shader file and vice versa) it can draw as I want. If I combine them, it not only can't draw but also makes app crash as

glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

I'm new for OpenGL ES and GLSL language, so I'm not sure my thinking about setting flags like that is right or wrong. Can anyone help me

So why don't you just build 2 seperate shader programs and useProgram() on one of them, instead of sending flag values to GL and making expensive conditional branch in vertex and especially fragment shader?