Fixed bug 2421 - SDL_RenderCopyEx off by one when rotating by 90 and -90

chasesan

When using SDL_RenderCopyEx, I get a problem on some platforms where the output is offset by +/-1 on other platforms and not on others. I tried it with a center of both 0,0 (and offsetting by width/height) and NULL (for centered).

The rotation involved is 90, and/or -90 rotation. The rotation was a constant, no arithmetic was involved when inputting it into SDL_RenderCopyEx.

This occurred with 32x32, 24x24, and 16x16 texture sizes. I apologize that I don't have more precise information, as I received the information as a bug report myself. But I have tracked the problem down to here.

My program requires pixel perfect alignment on several different platforms, so this is something of a showstopper for me.

--

Sylvain

It appears the RenderCopyEx is done as expected,
this is the red rectangle which is not correctly positionned !

So, here's patch with a 0.5 float increment, like for opengles2, for DrawLines, and also Draw Points.

src/render/opengles/SDL_render_gles.c

@@ -814,12 +814,24 @@ GLES_RenderDrawPoints(SDL_Renderer * renderer, const SDL_FPoint * points,
                       int count)
 {
     GLES_RenderData *data = (GLES_RenderData *) renderer->driverdata;
+    GLfloat *vertices;
+    int idx;
 
     GLES_SetDrawingState(renderer);
 
-    data->glVertexPointer(2, GL_FLOAT, 0, points);
+    /* Emit the specified vertices as points */
-    data->glDrawArrays(GL_POINTS, 0, count);
+    vertices = SDL_stack_alloc(GLfloat, count * 2);
+    for (idx = 0; idx < count; ++idx) {
+        GLfloat x = points[idx].x + 0.5f;
+        GLfloat y = points[idx].y + 0.5f;
 
+        vertices[idx * 2] = x;
+        vertices[(idx * 2) + 1] = y;
+    }
+
+    data->glVertexPointer(2, GL_FLOAT, 0, vertices);
+    data->glDrawArrays(GL_POINTS, 0, count);
+    SDL_stack_free(vertices);
     return 0;
 }
 
@@ -828,10 +840,22 @@ GLES_RenderDrawLines(SDL_Renderer * renderer, const SDL_FPoint * points,
                      int count)
 {
     GLES_RenderData *data = (GLES_RenderData *) renderer->driverdata;
+    GLfloat *vertices;
+    int idx;
 
     GLES_SetDrawingState(renderer);
 
-    data->glVertexPointer(2, GL_FLOAT, 0, points);
+    /* Emit a line strip including the specified vertices */
+    vertices = SDL_stack_alloc(GLfloat, count * 2);
+    for (idx = 0; idx < count; ++idx) {
+        GLfloat x = points[idx].x + 0.5f;
+        GLfloat y = points[idx].y + 0.5f;
+
+        vertices[idx * 2] = x;
+        vertices[(idx * 2) + 1] = y;
+    }
+
+    data->glVertexPointer(2, GL_FLOAT, 0, vertices);
     if (count > 2 &&
         points[0].x == points[count-1].x && points[0].y == points[count-1].y) {
         /* GL_LINE_LOOP takes care of the final segment */
@@ -842,6 +866,7 @@ GLES_RenderDrawLines(SDL_Renderer * renderer, const SDL_FPoint * points,
         /* We need to close the endpoint of the line */
         data->glDrawArrays(GL_POINTS, count-1, 1);
     }
+    SDL_stack_free(vertices);
 
     return 0;
 }