OpenGL ES updating a sub-part of a texture without using glPixelStorei

Question

I'm using a open source project for drawing text with OpenGL called fontstash. It can be found here on github: https://github.com/memononen/fontstash

That works fine for desktop but when I tried to build it for android and it failed. This since it is using glPixelStorei with the three paramter named GL_UNPACK_ROW_LENGTH, GL_UNPACK_SKIP_PIXELS, GL_UNPACK_SKIP_ROWS. These are not available in OpenGL ES 1.1 or 2.0 (I think they have been added in 3.0 though). But I found this stackoverflow answer stating that one can do this by yourself by using som pointer math: openGL SubTexturing

But I'm not getting it to work.

This is the original code:

static void glfons__renderUpdate(void* userPtr, int* rect, const unsigned char* data)
{
    struct GLFONScontext* gl = (struct GLFONScontext*)userPtr;
    int w = rect[2] - rect[0];
    int h = rect[3] - rect[1];

    if (gl->tex == 0) return;
    glBindTexture(GL_TEXTURE_2D, gl->tex);
    glPixelStorei(GL_UNPACK_ALIGNMENT,1);
    glPixelStorei(GL_UNPACK_ROW_LENGTH, gl->width);
    glPixelStorei(GL_UNPACK_SKIP_PIXELS, rect[0]);
    glPixelStorei(GL_UNPACK_SKIP_ROWS, rect[1]);
    glTexSubImage2D(GL_TEXTURE_2D, 0, rect[0], rect[1], w, h, GL_ALPHA,GL_UNSIGNED_BYTE, data);
}

And this is what I have come up with so far (not working):

static void glfons__renderUpdate(void* userPtr, int* rect, unsigned char* data)
{
    struct GLFONScontext* gl = (struct GLFONScontext*)userPtr;
    int w = rect[2] - rect[0];
    int h = rect[3] - rect[1];

    if (gl->tex == 0) return;
    glBindTexture(GL_TEXTURE_2D, gl->tex);
    glPixelStorei(GL_UNPACK_ALIGNMENT,1);
    glTexSubImage2D(GL_TEXTURE_2D, 0, rect[0], rect[1], w, h, GL_ALPHA, GL_UNSIGNED_BYTE, NULL);

    int y = 0;
    for(y; y < h; y++)
    {
        char *row = data + ((y + rect[1])*gl->width + rect[0]) * 4;
        glTexSubImage2D(GL_TEXTURE_2D, 0, rect[0], rect[1] + y, w, 1, GL_ALPHA, GL_UNSIGNED_BYTE, row);
   }
}

why are you using 4 for a multiplier when GL_ALPHA + GL_UNSIGNED_BYTE implies that a pixel takes 1 byte? — programmerjake, Aug 21 '14 at 11:36
Yes, why am I doing that? Thanks, works better when I removed the multiplier of 4. — Robert Sjödahl, Aug 21 '14 at 12:39

score 1 · Accepted Answer · answered Jan 19 '18 at 01:47

1

I've found this post while looking how to update part of texture from data in OpenGL ES 1.1/2.0, and found this solution. But it turns out that this method is very slow, so anybody looking for faster method can check this answer.

answered Jan 19 '18 at 01:47

vedranm

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OpenGL ES updating a sub-part of a texture without using glPixelStorei

1 Answers1